Matching technology to value creation: Drones in agriculture

‘Drones in agriculture’ has been a topic hot lately, but is it a real business?

Drones may someday perform all kinds of “work” in the field, like spraying or seeding.  However, today the product of the drone industry is not drones, it’s data. Two ideas persuaded me that there is a business here.   The first is that variability information has real value to growers — it is production control data.  The second is that there are places where drone technology allows real data to replace heuristics and estimation in order to drive action.

Agriculture is a very sophisticated business.  Even in specialty crops with less automation, the growers are extremely sophisticated and as scientifically inclined as one would expect of managers of multi-million dollar production facilities.  Putting production facilities out of doors and using production implements derived from nature only increases the complexity of these operations.   In particular, it introduces uncertainty, variation, and risk into the production process.  The managers of these operations, who  have responsibility for maximizing production, are the customers for this data.

When these managers are trying to squeeze the most production out of a given set of inputs, particularly land and labor, there is a natural cycle of input opportunities.  Data only has value if the manager can act on it, and this is where agriculture becomes extremely heterogeneous.  At one end of the spectrum you have dry farming of corn, where there might be three or four viable input opportunities (plant, a spray or two, then harvest), most of which are already done automatically — or robotically — on a combine.  In high value per acre specialty crops (like vegetables, grapes, or nuts), managers might make over 50 discrete, pre-planned input decisions each year, especially if the crop is deficit irrigated.  Even though these crops are in some ways “lower tech” in that they don’t use as much machine automation (such as combines), they do have a much more sophisticated ability to react to variation data.  These are where the drone’s fundamental advantages are most likely to be valued.

The tradeoff in data collection is a tradeoff between the cost of collection per geographic unit and “responsiveness.”  Responsiveness is the degree to which the system can deliver exactly the data needed to make better decisions, exactly when needed, and is a function of many factors including system reliability, resolution, revisit rate, sensor selection, controllability, and ease of deployment.

Drones and other overhead networked systems provide a continuum of data options.  At one end you have LANDSAT satellite data, which provides free, 30-meter pixels, un-interpreted data once every 16 days (if you are lucky enough not have clouds).  At the other end of the continuum, there are hand-launched drones, which have a high real cost per acre because of their short lifespan and the amount of labor required, but can be launched at anytime and are cheap enough to keep in the back of your truck.  In between you have a variety of commercial solutions.


There will always be a tradeoff between cost and responsiveness, but the idea of drones is that electronics will move the efficient frontier in this tradeoff towards more responsiveness at a given cost.  The challenge with this access to better data is not to overshoot the mark.  It is very expensive to overshoot the required responsiveness for the crop.  The trick is match responsiveness to the needs of the manager.  This is the task for the commercial drone industry: match responsiveness to the needs of the decision-maker.

As time goes on, every category of platform across the continuum will have to offer more responsiveness for the same money.  However, the customer responsiveness needs will not increase unless the managers are also given new mechanisms for taking action. Beyond being used to control fast-spreading crop diseases, I do not know what new markets will open up for drones — perhaps they aren’t in agriculture; the film industry certainly requires even more responsiveness than anything in this field  — but it is going to be fun to discover them.

One of the challenges that I would like to lay down to the ground robotics and manipulation communities is to reduce the cost of making an intervention.  Until it makes production sense to go out into a cornfield 20 or 30 times per season and take an action to improve growing, there won’t be a need for more responsive information.  As interventions become more profitable and crop managers decide to increase the number of interventions, there will be a greater need to automate data collection.  Then we can kick off a virtuous cycle of robotic technology.

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Crop responses to climatic variation

Crop responses to climatic variability and extremes

The yield productivity of many crops has risen over the past 40 years. Rates of yield increase in Europe have ranged from 0.8% (oats; Avena sativa) to 2.6% (triticale; Triticosecale) per year (Ewert et al. 2005). Rates of increase in wheat yields differ between European countries, but regional variation about the linear trend is less clear. More southerly European countries have lower rates of wheat yield increase than more northern ones, suggesting that weather factors such as temperature and precipitation play a more determining role in yield than in the north.. The fact that there have been lower rates of increase in yields in areas of Europe with more extreme conditions and that deviations from a linear trend have increased, points to the conclusion that warming since the start of the 1990s (Schär et al. 2004) has started to affect European wheat yields.

For crops, both changes in the mean and variability of temperature can affect crop processes, but not necessarily the same processes. Some crop processes, mostly related to growth such as photosynthesis and respiration, show continuous and mainly nonlinear changes in their rates as temperature increases. Rates of development and progression through a crop life cycle more often show linear responses to temperature. Both growth and developmental processes show temperature optima, whereby process rates increase over a range but thereafter flatten and decrease. For example, the light-saturated photosynthesis rate of C3 crops such as wheat and rice is at a maximum for temperatures from about 20–32 °C; total crop respiration, the sum of the growth and maintenance components, shows a steep nonlinear increase for temperatures from 15–40 °C followed by a rapid and nearly linear decline. The threshold developmental responses of crops to temperature are often well defined, changing direction over a narrow temperature range, as will be seen later.

An experimental study of climatic variability and wheat

Physiological responses to temperature changes in plants may occur at short or long time-scales (Wollenweber et al. 2003). Rapid changes in enzymatic reactions caused by differential thermosensitivity of various enzymes can deplete or result in accumulation of key metabolites. In addition, short-term effects involving altered gene expression, such as heat-shock protein synthesis, are likely to occur. Longer-term responses include alterations in the rate of carbon dioxide (CO2) assimilation and electron transport per unit leaf area, and impaired cell anaplerotic carbon metabolism, sucrose synthesis and carbon (C) and nitrogen (N) partitioning within and between organs (Jagtap et al. 1998). Altered carbon availability brought about by these events will affect uptake, transport and assimilation of other nutrients, disturb lipid metabolism and injure cell membranes (Maheswari et al. 1999), resulting in changes in growth rates and grain yield (Al Khatib & Paulsen 1999). However, temperature responses for specific physiological processes do not always relate directly to growth, because the latter is an integration of the effects of temperature on total metabolism (Bowes 1991).

The developmental stage of the crop exposed to increased temperatures has an important effect on the damage experienced by the plant (Slafer & Rawson 1995), but experimental studies of the effects of temperature variability on crop productivity are rare. This is mainly because of the difficulties in establishing and maintaining a temperature regime where a mean climatic value can be held constant between treatments that vary the amplitude of temperature (Moot et al. 1996). A solution to this is to examine the effects of extreme conditions at particular developmental stages (Ferris et al. 1998), in which the extreme conditions are defined with reference to literature (Porter & Gawith 1999). Wollenweber et al. (2003) tested the null hypothesis that wheat plants react to two separate periods of high temperature as if they were independent of each other. The chosen stages were the double-ridge stage of the apical meristem, which is close in time to the transition from vegetative to reproductive development of the apical meristem, and anthesis when extreme temperature events interfere with the development of fertile grains, as meiosis and pollen growth are affected (Wallwork et al. 1998). The experimental design, and the extreme temperature conditions were defined as a heat period of eight days of 25 °C at the double-ridge stage and/or a heat event of 35 °C at anthesis. Biomass accumulation, photosynthesis and the components of grain yield were analysed. While a high temperature event of 25 °C at the double-ridge stage is not a stress event sensu strictu for wheat, reproductive spikelet initiation can be impaired (Porter & Gawith 1999) and 25 °C is 13 °C higher than mean daily temperatures measured over 30 years at the experimental site in Denmark.

Grain yields were significantly lower in the treatments with high temperatures at anthesis and at both developmental stages. The major yield component reduced by the treatments was the harvest index; that is, the proportion of total dry matter invested in grain. The harvest index was lower in plants experiencing heat periods because their grain number per plant was reduced by 60% . However, there was no significant difference in the grain yield of plants as between those warmed at anthesis and those at double ridges and anthesis, meaning that the plants experienced the warming periods as independent and that critical temperatures of 35 °C for a short-period around anthesis had severe yield reducing effects. The conclusions from such results for climate change are that yield damaging weather signals for cereals such as wheat are in the form of absolute temperature thresholds, are linked to particular developmental stages and can be effective over short time-periods. This means that yield damage estimates of coupled crop–climate models need to have a maximum temporal resolution of a few days and incorporate models of crop phenology to deal with the overlap between such extreme weather events and crop sensitivity to them.

In contrast to the effects on developmentally linked processes, no significant differences were seen in the relation between light-saturated photosynthesis and leaf internal CO2 concentration for the heat treatments. A heat episode during DR increased the rates of light-saturated photosynthesis (Asat) in green leaves slightly. There were no significant differences in Asat and carboxylation efficiency, reinforcing the conclusion that the principal effects of high temperatures are on developmental processes, such as flowering and the formation of sinks for assimilated carbon, which in itself either is stimulated or is little affected by short-term warming. An extreme heat episode during vegetative development does not seem subsequently to affect the growth and developmental response of wheat to a second heat event at anthesis, and high-temperature episodes seem to operate independently of each other.

Crop temperature thresholds

In addition to the linear and nonlinear responses of crop growth and development processes described above, short-term extreme temperatures can have large yield-reducing effects on major crops. These effects were reviewed for wheat by Porter & Gawith (1999) and, for annual crops in general, by Wheeler et al. (2000). A general point arising from these reviews were that temperature thresholds are well defined as absolute threshold temperatures above which particularly the formation of reproductive sinks, such as seeds and fruits, are adversely affected, as seen in the experiment described above.

The largest standard error found was 5.0 °C for the maximum temperature for root growth, followed by 3.7 °C for the optimum temperature of root growth. Others, such as the base and optimum temperatures for shoot growth, the optimum temperature for leaf initiation and base temperature for anthesis have standard errors of less than 0.5 °C. Thus, the consensus is that functionally important temperatures for wheat are conservative when compared between different studies.

A crop that is important in the developing world is groundnut (Arachis hypogaea L.). This is an important food crop of the semi-arid tropics, including Africa, and can experience temperatures above 40 °C for periods during the growing season (Vara Prasad et al. 2000). The harvestable seeds of groundnut are formed following flowering and fruiting periods. When exposed for short-periods at high temperatures of up to 42 °C just after flowering, a clear relationship between fruit set and mean floral temperature was found (Vara Prasad et al. 2000). From between 32 and 36 °C and up to 42 °C, the percentage fruit set fell from 50% of flowers to zero and the decline in rate was linear, illustrating once more the sharpness of response of crop plants to temperatures between 30 and 35 °C during the flowering and fruiting periods.

Various literature sources have identified similar patterns for other important food crops such as maize and rice. For example, maize exhibits reduced pollen viability for temperatures above 36 °C; rice grain sterility is brought on by temperatures in the mid-30s °C and similar temperatures can lead to a reverse of the vernalizing effects of cold temperatures in wheat. What is perhaps more surprising than the consistent damaging effects of high temperatures in food crops is that cold-blooded animals also exhibit threshold temperature responses for various activities. As with plants, the lethal limits are the widest, followed by activity limits, development and growth with the reproductive limits being the narrowest from 24 to 30 °C, the upper value interestingly close to the limits seen for many crop plants, but this is presumably a coincidence. It would be very useful to have equivalent diagrams for the major crop plants in the world and thereby provide specific quantitative information on the probability and consequences, in other words the risk, from crop damaging climate change at the regional or country level. This would further be the linkage between crop physiology, crop agronomy and climate science (Porter 2005).

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Cover crops: Buckweat (Fagopyrum esculentum)

Type: summer or cool-season annual broadleaf grain

Roles: quick soil cover, weed suppressor, nectar for pollinators and beneficial insects, topsoil loosener, rejuvenator for low-fertility soils

Buckwheat is the speedy short-season cover crop. It establishes, blooms and reaches maturity in just 70 to 90 days and its residue breaks down quickly. Buckwheat suppresses weeds and attracts beneficial insects and pollinators with its abundant blossoms. It is easy to kill, and reportedly extracts soil phosphorus from soil better than most grain-type cover crops.

Buckwheat thrives in cool, moist conditions but it is not frost tolerant. Even in the South, it is not grown as a winter annual. Buckwheat is not particularly drought tolerant, and readily wilts under hot, dry conditions. Its short growing season may allow it to avoid droughts, however.


Quick cover. Few cover crops establish as rapidly and as easily as buckwheat. Its rounded pyramid- shaped seeds germinate in just three to five days. Leaves up to 3 inches wide can develop within two weeks to create a relatively dense, soil shading canopy. Buckwheat typically produces only 2 to 3 tons of dry matter per acre, but it does so quickly—in just six to eight weeks. Buckwheat residue also decomposes quickly, releasing nutrients to the next crop.

Weed suppressor. Buckwheat’s strong weed suppressing ability makes it ideal for smothering warm-season annual weeds. It’s also planted after intensive, weed-weakening tillage to crowd out perennials. A mix of tillage and successive dense seedings of buckwheat can effectively suppress Canada thistle, sowthistle, creeping jenny, leafy spurge, Russian knapweed and perennial peppergrass. While living buckwheat may have an allelopathic weed-suppressing effect, its primary impact on weeds is through shading and competition.

Phosphorus scavenger. Buckwheat takes up phosphorus and some minor nutrients (possibly including calcium) that are otherwise unavailable to crops, then releasing these nutrients to later crops as the residue breaks down. The roots of the plants produce mild acids that release nutrients from the soil. These acids also activate slow-releasing organic fertilizers, such as rock phosphate. Buckwheat’s dense, fibrous roots cluster in the top 10 inches of soil, providing an extensive root surface area for nutrient uptake.

Thrives in poor soils. Buckwheat performs better than cereal grains on low-fertility soils and soils with high levels of decaying organic matter. That’s why it was often the first crop planted on cleared land during the settlement of woodland areas and is still a good first crop for rejuvenating over-farmed soils. However, buckwheat does not do well in compacted, droughty or excessively wet soils.

Quick regrowth. Buckwheat will regrow after mowing if cut before it reaches 25 percent bloom. It also can be lightly tilled after the midpoint of its long flowering period to reseed a second crop. Some growers bring new land into production by raising three successive buckwheat crops this way.

Soil conditioner. Buckwheat’s abundant, fine roots leave topsoil loose and friable after only minimal tillage, making it a great mid-summer soil conditioner preceding fall crops in temperate areas.

Nectar source. Buckwheat’s shallow white blossoms attract beneficial insects that attack or parasitize aphids, mites and other pests. These beneficials include hover flies (Syrphidae), predatory wasps, minute pirate bugs, insidious flower bugs, tachinid flies and lady beetles. Flowering may start within three weeks of planting and continue for up to 10 weeks.

Nurse crop. Due to its quick, aggressive start, buckwheat is rarely used as a nurse crop, although it can be used anytime you want quick cover. It is sometimes used to protect late-fall plantings of slow-starting, winter-hardy legumes wherever freezing temperatures are sure to kill the buckwheat.


Buckwheat prefers light to medium, well-drained soils—sandy loams, loams, and silt loams. It performs poorly on heavy, wet soils or soils with high levels of limestone. Buckwheat grows best in cool, moist conditions, but is not frost-tolerant. It is also not drought tolerant. Extreme afternoon heat will cause wilting, but plants bounce back overnight.

Plant buckwheat after all danger of frost. In untilled, minimally tilled or clean-tilled soils, drill 50 to 60 lb./A at 1/2 to 11/2 inches deep in 6 to 8 inch rows. Use heavier rates for quicker canopy development. For a fast smother crop, broadcast up to 96 lb./A (2 bu./A) onto a firm seedbed and incorporate with a harrow, tine weeder, disk or field cultivator. Overall vigor is usually better in drilled seedings. As a nurse-crop for slow growing, winter annual legumes planted in late summer or fall, seed at one-quarter to one-third of the normal rate.

Buckwheat compensates for lower seeding rates by developing more branches per plant and more seeds per blossom. However, skimping too much on seed makes stands more vulnerable to early weed competition until the canopy fills in. Using cleaned, bin-run or even birdseed-grade seed can lower establishment costs, but increases the risk of weeds. As denser stands mature, stalks become spindly and are more likely to lodge from wind or heavy rain.

Buckwheat is used most commonly as a mid-summer cover crop to suppress weeds and replace bare fallow. In the Northeast and Midwest, it is often planted after harvest of early vegetable crops, then followed by a fall vegetable, winter grain, or cool-season cover crop. Planted later, winterkilled residue provides decent soil cover and is easy to no-till into. In many areas, it can be planted following harvest of winter wheat or canola.

In parts of California, buckwheat grows and flowers between the killing of winter annual legume cover crops in spring and their re-establishment in fall. Some California vineyard managers seed 3-foot strips of buckwheat in row middles, alternating it and another summer cover crop, such as sorghum-sudangrass.

Buckwheat is sensitive to herbicide residues from previous crops, especially in no-till seedbeds. Residue from trifluralin and from triazine and sulfonylurea herbicides have damaged or killed buckwheat seedlings. When in doubt, sow and water a small test plot of the fast germinating seed to detect stunting or mortality.

Pest Management
Few pests or diseases bother buckwheat. Its most serious weed competitors are often small grains from preceding crops, which only add to the cover crop biomass. Other grass weeds can be a problem, especially in thin stands. Weeds also can increase after seed set and leaf drop. Diseases include a leaf spot caused by the fungus Ramularia and Rhizoctonia root rot.

Other Options
Plant buckwheat as an emergency cover crop to protect soil and suppress weeds when your main crop fails or cannot be planted in time due to unfavorable conditions.

To assure its role as habitat for beneficial insects, allow buckwheat to flower for at least 20 days—the time needed for minute pirate bugs to produce another generation.

Buckwheat can be double cropped for grain after harvesting early crops if planted by mid-July in northern states or by early August in the South. It requires a two-month period of relatively cool, moist conditions to prevent blasting of the blossoms. There is modest demand for organic and specially raised food-grade buckwheat in domestic and overseas markets. Exporters usually specify variety, so investigate before planting buckwheat for grain.

Management Cautions
Buckwheat can become a weed. Kill within 7 to 10 days after flowering begins, before the first seeds begin to harden and turn brown. Earliest maturing seed can shatter before plants finish blooming. Some seed may overwinter in milder regions.

Buckwheat can harbor insect pests including Lygus bugs, tarnished plant bugs and Pratylynchus penetrans root lesion nematodes.


 Buckwheat has only about half the root mass as a percent of total biomass as small grains. Its succulent stems break down quickly, leaving soils loose and vulnerable to erosion, particularly after tillage. Plant a soil-holding crop as soon as possible.
 Buckwheat is nearly three times as effective as barley in extracting phosphorus, and more than 10 times more effective than rye—the poorest P scavenger of the cereal grains.
 As a cash crop, buckwheat uses only half as much soil moisture as soybeans.

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Cover crops survey analysis

Cover crops can slow erosion, improve soil, smother weeds, enhance nutrient and moisture availability, help control many pests, and bring a host of other benefits to farms across the country. For more than 20 years, NCR-SARE has supported projects by researchers, producers, and educators who are using this time-tested method of revitalizing soil, curbing erosion, and managing pests.

Cover crop adoption has been increasing rapidly in the last 5 years, with an estimated 1.5 to 2.0 million acres of cover crops planted in the U.S. in 2012. During the winter of 2012-13, the NCR-SARE program contracted with the Conservation Technology Information Center (CTIC) to carry out a survey of farmers who have grown cover crops.  A short survey instrument of a dozen questions was developed with help from steering committee members of the Midwest Cover Crops Council.  The survey was distributed at several farmer conferences in the Midwest over the winter, and was also sent out in an online format to individuals across the U.S.

A total of 759 farmers completed the survey.  The farmers who completed the survey used cover crops on about 218,000 acres in 2012, and expected to increase that to over 300,000 acres in 2013.

Questions on cover crop adoption, benefits, challenges, and yield impacts were included in the survey.  Key findings included the following:

  • During the fall of 2012, corn planted after cover crops had a 9.6% increase in yield compared to side-by-side fields with no cover crops.  Likewise, soybean yields were improved 11.6% following cover crops.
  • In the hardest hit drought areas of the Corn Belt, yield differences were even larger, with an 11.0% yield increase for corn and a 14.3% increase for soybeans.
  • Surveyed farmers are rapidly increasing acreage of cover crops used, with an average of 303 acres of cover crops per farm planted in 2012 and farmers intending to plant an average of 421 acres of cover crops in 2013.  Total acreage of cover crops among farmers surveyed increased 350% from 2008 to 2012.
  • Farmers identified improved soil health as a key overall benefit from cover crops.  Reduction in soil compaction, improved nutrient management, and reduced soil erosion were other key benefits cited for cover crops.  As one of the surveyed farmers commented, “Cover crops are just part of a systems approach that builds a healthy soil, higher yields, and cleaner water.”
  • Farmers are willing to pay an average (median) amount of $25 per acre for cover crop seed and an additional $15 per acre for establishment costs (either for their own cost of planting or to hire a contractor to do the seeding of the cover crop).

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Biological Control and Natural Enemies

Biological control is the beneficial action of predators, parasites, pathogens, and competitors in controlling pests and their damage. Biocontrol provided by these living organisms (collectively called “natural enemies”) is especially important for reducing the numbers of pest insects and mites. Natural enemies also control certain rangeland and wildland weeds, such as Klamath weed (St. Johnswort). Plant pathogens, nematodes, and vertebrates also have many natural enemies, but this biological control is often harder to recognize, less-well understood, or more difficult to manage. Conservation, augmentation, and classical biological control (also called importation) are tactics for harnessing the effects of natural enemies.


Predators, parasites, and pathogens are the primary groups used in biological control of insects. Most parasites and pathogens, and many predators, are highly specialized and attack only one or several closely related pest species.


Pathogens are microorganisms including certain bacteria, fungi, nematodes, protozoa, and viruses that can infect and kill the host. Populations of some aphids, caterpillars, mites, and other invertebrates are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pest populations. In addition to naturally occurring disease outbreaks, some beneficial pathogens are commercially available as biological or microbial pesticides. These include Bacillus thuringiensis or Bt, entomopathogenic nematodes, and granulosis viruses. Additionally, some microorganism by-products such as avermectins and spinosyns are used in certain insecticides, but applying these products is not considered to be biological control.


A parasite is an organism that lives and feeds in or on a larger host. Insect parasites (more precisely called parasitoids) are smaller than their host and develop inside, or attach to the outside, of the host’s body. Often only the immature stage of the parasite feeds on the host, and it kills only one host individual during its development. However, adult females of certain parasites (such as many wasps that attack scales and whiteflies) feed on their hosts, providing an easily overlooked but important source of biological control in addition to the host mortality caused by parasitism.

Most parasitic insects are either flies (Diptera) or wasps (Hymenoptera). Parasitic Hymenoptera occur in over three dozen families. For example, Aphidiinae (a subfamily of Braconidae) attack aphids. Trichogrammatidae parasitize insect eggs. Aphelinidae, Encyrtidae, Eulophidae, and Ichneumonidae are other groups of tiny size to medium-sized wasps that parasitize pests but do not sting people. The most common parasitic flies are Tachinidae. Adult tachinids often resemble house flies. Their larvae are maggots that feed inside the host.


Insects are important food for many amphibians, birds, mammals, and reptiles. Many beetles, true bugs (Hemiptera or Heteroptera), flies, and lacewings are predators of various pest mites and insects. Most spiders feed entirely on insects. Predatory mites that prey primarily on spider mites include Amblyseius spp., Neoseiulus spp., and thewestern predatory mite (Galendromus occidentalis).

Recognizing Natural Enemies

Proper identification of pests, and distinguishing pests from their natural enemies, are essential to effectively using biological control. For example, some people may mistake aphid-eating syrphid fly larvae for caterpillars. The adult syrphid, commonly also called a flower fly or hover fly, is sometimes mistaken for a honey bee. Consult publications such as the UC Statewide Integrated Pest Management Program Pest Notes series listed in Suggested Reading to learn more about the specific pests and their natural enemies in your gardens and landscapes. Take unfamiliar organisms you find to your Cooperative Extension office or county agriculture commissioner for an expert identification. Carefully observe the creatures on your plants to help discern their activity. For example, to distinguish plant-feeding mites from predaceous mites, observe them on your plants with a good hand lens. Predaceous species appear more active than plant-feeding species. In comparison with pest mites, predaceous mites are often larger and do not occur in large groups.


Preserve naturally occurring beneficial organisms whenever possible. Most pests are attacked by several different types and species of natural enemies, and their conservation is the primary way to successfully use biological control in gardens and landscapes. Ant control, habitat manipulation, and selective pesticide use are key conservation strategies.

Pesticide Management

Broad-spectrum pesticides often kill a higher proportion of predators and parasites than of the pest species they are applied to control. In addition to immediately killing natural enemies that are present (contact toxicity), many pesticides are persistent materials that leave residues that kill natural enemies that migrate in after spraying (residual toxicity). Residues often are toxic to natural enemies long after pests are no longer affected. Even if beneficials survive an application, low levels of pesticide residues can interfere with natural enemies’ reproduction and their ability to locate and kill pests.

Biological control’s importance often becomes apparent when broad-spectrum, persistent pesticides cause secondary pest outbreaks or pest resurgence. A secondary outbreak of a different species occurs when pesticides applied against a target pest kill natural enemies of other species, causing the formerly innocuous species to become pests. An example is the dramatic increase in spider mite populations that sometimes results after applying a carbamate (e.g., carbaryl or Sevin) or organophosphate (malathion) to control caterpillars or other pests.

Eliminate or reduce the use of broad-spectrum, persistent pesticides whenever possible. Carbamates, organophosphates, and pyrethroids are especially toxic to natural enemies. When pesticides are used, apply them in a selective manner. Treat only heavily infested spots instead of entire plants. Choose insecticides that are more specific in the types of invertebrates they kill, such as Bacillus thuringiensis (Bt) that kills only caterpillars that eat treated foliage. Rely on insecticides with little or no persistence, including insecticidal soap, horticultural or narrow-range oil, and pyrethrins.

A less-persistent pesticide can result in longer control of the pest in situations where biological control is important because the softer pesticide will not keep killing natural enemies. One soft pesticide spray plus natural enemies can be effective for longer than the application of one hard spray.

Ant Control and Honeydew Producers

Ants are beneficial as consumers of weed seeds, predators of many insect pests, soil builders, and nutrient cyclers. Ants may attack people and pets or are direct pests of crops, feeding on nuts or fruit (See Pest Notes: Red Imported Fire Ants). The Argentine ant and certain other species are pests primarily because they feed on honeydew produced by Homopteran insects such as aphids, mealybugs, soft scales, and whiteflies. Ants protect honeydew producers from predators and parasites that might otherwise control them. Ants sometimes move these honeydew-producing insects from plant to plant. Where natural enemies are present, if ants are controlled, populations of many pests will gradually (over several generations of pests) be reduced as natural enemies become more abundant. Control methods include cultivating soil around ant nests, encircling trunks with ant barriers, and applying insecticide baits near plants. See Pest Notes: Ants for more information.

Habitat Manipulation

Manage gardens and landscapes by using cultural and mechanical methods that enhance natural enemy effectiveness. Grow diverse plant species and tolerate low populations of plant-feeding insects and mites so that some food is always available to retain predators and parasites. Plant a variety of sequentially flowering species to provide natural enemies with nectar, pollen, and shelter throughout the growing season. The adult stage of many insects with predaceous larvae (such as green lacewings and syrphid flies) and many adult parasites feed only on pollen and nectar. Even if pests are abundant for the predaceous and parasitic stages, many beneficials will do poorly unless flowering and nectar-producing plants are available to adult natural enemies. Reduce dust, for example, by planting ground covers and windbreaks. Dust can interfere with natural enemies and may cause outbreaks of pests such as spider mites. Avoid excess fertilization and irrigation, which can cause phloem-feeding pests such as aphids to reproduce more rapidly than natural enemies can provide control.


When resident natural enemies are insufficient, their populations can sometimes be increased (augmented) through the purchase and release of commercially available beneficial species. However, there has been relatively little research on releasing natural enemies in gardens and landscapes. Releases are unlikely to provide satisfactory pest control in most situations. Some marketed natural enemies are not effective. Praying mantids, often sold as egg cases, make fascinating pets. But mantids are cannibalistic and feed indiscriminately on pest and beneficial species. Releasing mantids does not control pests.

Only a few natural enemies can be effectively augmented in gardens and landscapes. These include entomophagous nematodes, predatory mites, and perhaps a few other species. For example, convergent lady beetles (Hippodamia convergens) purchased in bulk through mail order and released in very large numbers at intervals can temporarily control aphids; however, lady beetles purchased through retail outlets are unlikely to be sufficient in numbers and quality to provide control.

Successful augmentation generally requires advanced planning, biological expertise, careful monitoring, optimal release timing, patience, and situations where certain levels of pests and damage can be tolerated. Desperate problems where pests or damage are already abundant are not good opportunities for augmentation.


Classical biological control, also called importation, is primarily used against exotic pests that have inadvertently been introduced from elsewhere. Many organisms that are not pests in their native habitat become unusually abundant after colonizing new locations without their natural controls. Researchers go to the pest’s native habitat, study and collect the natural enemies that kill the pest there, and then ship promising natural enemies back for testing and possible release. Many insects and some weeds that were widespread pests in California are now partially or completely controlled by introduced natural enemies, except where these natural enemies are disrupted, such as by pesticide applications or honeydew-seeking ants.

Natural enemy importation by law must be done only by qualified scientists with government permits. Natural enemies are held and studied in an approved quarantine facility to prevent their escape until research confirms that the natural enemy will have minimal negative impact in the new country of release. Because classical biological control can provide long-term benefits over a large area and is funded through taxes, public support is critical for continued success. Consult Natural Enemies Handbook and Pests of Landscape Trees and Shrubs to learn about situations where imported natural enemies are important and conserve them whenever possible.

Is Biological Control “Safe”?

One of the great benefits of biological control is its relative safety for human health and the environment. Most negative impacts from exotic species have been caused by undesirable organisms contaminating imported goods, by travelers carrying in pest-infested fruit, by introduced ornamentals that escape cultivation and become weeds, and by poorly conceived importations of predatory vertebrates like mongooses. These ill-advised or illegal importations are not part of biological control. To avoid these problems, biological control researchers follow regulations and work with relatively host-specific insects.

Help preserve our environment and avoid introducing exotic new pests.

Do not bring uncertified fruit, plants, or soil into California. Take unfamiliar pests to your county agricultural commissioner or Cooperative Extension office for identification.


Although many animals prey on pest insects or mites, not all can be relied upon to reduce a pest population enough to protect plants. The most effective natural enemies are often relatively host specific, feeding on a single pest species or a group of similar pests such as aphids or scales. Good examples include predatory mites, most parasitic wasps, and syrphid flies. Very general predators such as praying mantids are often likely to kill as many beneficials as pests and thus rarely provide effective control.

Synchronization of the life cycle and environmental requirements of the pest and natural enemy also determine the effectiveness of biological control. Natural enemies that do not arrive or become abundant until after pests are very abundant may not prevent serious damage to plants. Conversely, a parasite or predator with multiple annual generations, that can attack a broad range of life stages of the pest and can feed and reproduce when pest populations are low or moderate, will likely be a more effective natural enemy.

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Erosion: soil at risk

While soil is a resource that can re-create itself, it is a very slow process. Unfortunately, soils have been and continue to be degraded at an alarming rate. Soil erosion is still the dominant cause of soil degradation. Other causes of soil degradation include: soil compaction, soil acidification, soil pollution, and salinization. Dramatic increases in the use of no-till systems by American farmers have led to many benefits, including reductions in erosion, and savings of time, labor, fuel, and machinery. Between 1990 and 2000, no-till farming acreage rose from 16 million acres to 52 million acres, an increase of 300 percent. Now that some fields have been under no-till production systems for many years, farmers and researchers have begun to notice additional. The erosion rate is often greater than the soil formation rate. For instance, the average soil erosion rate in Pennsylvania was 5.1 tons/acre in 1997, whereas the tolerable soil loss level is 3-4 tons /acre per year for most of the soils of this state. With the average loss of 5.1 tons/acre, you can see that the tolerable soil loss level was far exceeded on many fields. That means that our current rate of erosion is a threat to the future productivity of the soil.

Soil erosion removes the best portion of the soil—the part that contains most of the plant nutrients and soil organic matter. In many cases, the topsoil has more favorable soil texture for crop growth than the subsoil. When the topsoil is gone, the farmer is left with less productive subsoil. In addition, eroded soil becomes an environmental threat; polluting streams, lakes, and estuaries. In Pennsylvania, sediment is still the number one pollutant of streams and other bodies of water.


The process of planting, growing and harvesting brings about a certain amount of expected erosion that is considered acceptable to bring a crop to the table. The tolerable soil loss level is called “T” by soil conservationists. The major soil management practice that causes soil erosion is tillage, the process of preparing a field for seeding. Erosion due to tillage can be kept in check through methods such as contour farming, contour stripcropping, conservation buffers, grassed waterways, terraces and diversions to meet soil loss tolerance levels.  You will find that soil can still move within a field––for example, in a strip cropping system where sediment from unprotected soil is trapped by a down-slope strip with high residue or permanent cover. In fact, average soil loss on this entire field or system may be at, or below T, where it exceeds T on the tilled strips. But, if soil can be kept covered, erosion can be stopped before it starts and T can be met on the entire field every year.

The way to dramatically reduce soil erosion is the no-till systems approach. This method keeps the soil covered with crop residue, reduces soil disturbance to almost zero, and attempts to maximize the number of days in the year when living roots grow in the soil.

Farmers and researchers have demonstrated that there are many other benefits to the no-till system besides soil savings. For example, a farmer can save significant amounts of time not working the fields prior to planting. That can result in more timely planting as well as increased acreage that can be managed with the same equipment and labor force. The efficiency of field operations will also increase because the farmer can often meet soil conservation requirements in a no-till system without adding as many conservation practices. Finally, the costs of producing a crop are ,decreased by excluding tillage machinery expenses.

Soil will improve over time in a no-till system through increased organic matter. Soil structure and water infiltration will improve in a no-till system through the slow, but continuous decomposition of crop residue and roots and the high activity of living organisms creating a permanent macro-pore system in the soil. Due to this high biological activity in  no-till, soil compaction can be minimized.Finally, there are other environmental benefits of a no-till system that extend beyond the farm––cleaner air and streams and increased groundwater recharge.

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Cover and Green Manure Crop Benefits to Soil Quality

Soil Quality and Resource Management

Soil is one of the five resources—soil, water, air, plants, and animals—that NRCS deals with in resource planning. Soil is intimately related to the other four resources, and its condition can either negatively or positively impact the other resources. For example, if the soil surface is functioning adequately, the soil will allow water to infiltrate, thus reducing the potential for erosion and increasing the amount of water stored for plant use. This function of soil affects water quality, plant growth, and the health of animals. In addition, protection of the surface layer resists wind erosion, thus protecting the air resource. Soil Quality is a critical factor in the management of natural resources, and the protection or enhancement of soil quality is the key component of all resource management assistance activities in the NRCS.

What is Soil Quality?

Soil quality is the capacity of a specific kind of soil to function within natural or managed ecosystem boundaries to:

* sustain plant and animal productivity

* maintain or enhance water and  air quality

* support human health and habitation.

As defined, the terms soil quality, soil health, and soil condition are interchangeable.

Effects of Conservation Practices

One of the goals of conservation planning is to consider the effects of conservation practices and systems on soil quality. This is the first technical note in a series on how conservation practices affect soil quality. This technical note is designed to compliment local or regional information on the specific nature of cover crops. Cover and Green Manure Crop Benefits to Soil Quality

1. EROSION – Cover crops increase vegetative and residue cover during periods when erosion energy is high, especially when main crops do not furnish adequate cover. Innovative planting methods such as aerial seeding, interseeding with cyclone seeder, or other equipment may be needed, when main crop harvest, delays conventional planting of cover crops during recommended planting dates.

2. DEPOSITION OF SEDIMENT – Increase of cover reduces upland erosion, which in turn reduces sediment from floodwaters and wind.

3. COMPACTION – Increased biomass, when decomposed, increases organic matter promoting increased microbial activity and aggregation of soil particles. This increases soil porosity and reduces bulk density. Caution: plant cover crops when soils are not wet, or use other methods such as aerial seeding.

4. SOIL AGGREGATION AT THE SURFACE – Aggregate stability will increase with the addition of and the decomposition of organic material by microorganisms.

5. INFILTRATION – Surface cover reduces erosion and run-off. Cover crop root channels and animal activities, such as earthworms, form macropores that increase aggregate stability and improve infiltration. Caution: Macropores can result in an increase in leaching of highly soluble pesticides if a heavy rain occurs immediately after application. However, if only sufficient rainfall occurs to move the pesticide into the surface soil after application, the risks for preferential flow are minimal. Cover crops, especially small grains, utilize excess nitrogen.

6. SOIL CRUSTING – Cover crops will provide cover prior to planting the main crop. If conservation tillage is used, benefits will continue after planting of main crop. Increases of organic matter, improved infiltration, and increased aggregate stability reduce soil crusting.

7. NUTRIENT LOSS OR IMBALANCE – Decomposition of increased biomass provides a slow release of nutrients to the root zone. Legume cover crops fix atmospheric nitrogen and provide nitrogen for the main crop. Legumes utilize a higher amount of phosphorus than grass or small grains. This is useful in animal waste utilization and management. Small grains are useful as catch crops to utilize excess nitrogen, which reduces the potential for nitrogen leaching. Caution: To prevent nutrient tie ups, cover crops should be killed 2-3 weeks prior to planting main crop. Tillage tools are used to kill and bury cover crops in conventional tillage systems. However, with conservation tillage systems, cover crops are killed with chemicals and left on or partially incorporated in the soil.

Caution: Research has shown that incorporation of legume cover crops results in more rapid mineralization. However, due to delay in availability of nitrogen from legume cover crop in conservation tillage, a starter fertilizer should be applied at planting. (Reeves, 1994). An ARS study done in Morris, Minnesota reported dramatically higher carbon losses through C02 remissions under moldboard plow plots as compared to no-till. It was reported that carbon was lost as C02 in 19 days following moldboard plowing of wheat stubble that was equal to the total amount of carbon synthesized into crop residues and roots during the growing season. Long-term studies indicate that up to 2 percent of the residual organic matter in soils are oxidized per year by moldboard plowing” (Schertz and Kemper, 1994).

8. PESTICIDE CARRYOVER – Cover crops reduce run-off resulting in reduced nutrient and pesticide losses from surface runoff and erosion. Increased organic matter improves the environment for soil biological activity that will increase the breakdown of pesticides.

9. ORGANIC MATTER – Decomposition of increased biomass results in more organic matter. Research shows cover crops killed 2-3 weeks prior to planting main crop, results in adequate biomass and reduces the risk of crop losses from soil moisture depletion and tie up of nutrients.

10. BIOLOGICAL ACTIVITY – Cover and green manure crops increase the available food supply for microorganisms resulting in increased biological activity.

11.WEEDS AND PATHOGENS – Increased cover will reduce weeds. Caution: Research has shown reductions in yield are possible in conservation tillage cotton systems following winter cover crops. Reductions are attributed to interference from residue (poor seed/soil contact), cool soil temperatures at planting, increased soil borne pathogens, and increased insects and other pests. Harmful effects from the release of chemical compounds of one plant to another plant (allelopathic) are possible with crops like cotton, but losses can be reduced by killing the cover crop 2-3 weeks prior to planting main crop, and achieving good seed/soil contact with proper seed placement. Cover crops have shown some allelopathic effects on weeds reducing weed populations in conservation tillage (Reeves, 1994).

12. EXCESSIVE WETNESS – Cover and green manure crops may remove excess moisture from wet soils, resulting in reduction of “waterlogging” in poorly drained soils. Caution: transpiration of water can be a detriment in dry climates. Planners should adjust the kill date of cover crops to manage soil water.


Cover and Green Manure Crops as a conservation practice can improve soil health. Soil quality benefits such as increased organic matter, biological activity, aggregate stability, infiltration, and nutrient cycling accrue much faster under no-till than other tillage practices that partially incorporate the residue.

One example comes from the Jim Kinsella farming operation near Lexington, Illinois. He reports that organic matter levels have increased from 1.9 percent 6.2 percent after 19 years of continuous no-till (Schertz and Kemper, 1994). Future technical notes will deal with other conservation practice effects on soil quality. The goal of the Soil Quality Institute is to provide this information to field offices to enable them to assist landusers in making wise decisions when managing their natural resources.

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Precision Farming Tools

GPS: Its Uses and Potential Are Growing

Global positioning systems (GPS) are widely available in the agricultural community. Farm uses include:

•mapping yields(GPS + combine yield monitor),

• variable rate planting (GPS + variable rate planting system),

•variable rate lime and fertilizer application (GPS + variable rate controller),

•field mapping for records and insurance purposes (GPS + mapping software), and

•parallel swathing (GPS + navigation tool).

For a review of the principles of GPS to locate specific field points, refer to this GPS Tutorial (Trimble Navigation Limited, 2008). GPS and associated navigation systems are used in many types of agricultural operations. These systems are useful particularly in applying pesticides, lime, and fertilizers and in tracking wide planters/drills or large grain-harvesting platforms.

GPS navigation tools can replace foam for sprayers and planter/drill-disk markers for making parallel swaths across a field. Navigation systems help operators reduce skips and overlaps, especially when using methods that rely on visual estimation of swath distance and/ or counting rows. This technology reduces the chance  of misapplication of agrochemicals and has the potential to safeguard water quality. Also, GPS navigation can be used to keep implements in the same traffic pattern year-to-year (controlled traffic), thus minimizing adverse effects of implement traffic.

Use of GPS navigation in agrochemical application with ground equipment has grown rapidly, and commercial applicators are quickly adopting the tool. According to a 2007 survey of those who offered custom application (Whipker and Akridge, 2007), 82 percent applied at least some of the fertilizer/chemicals using a GPS navigation system with a manual-control/light bar  guidance system. Twenty-nine percent said they used a  GPS navigation system with an auto-control/auto-steer guidance system for at least some of their custom application. On average, for all custom applied materials, percent was applied with GPS light bar, and 12 percent was applied with auto-steer GPS. GPS navigation has  become standard practice for U.S. aerial applicators. Crop producers are also starting to adopt these systems, because GPS navigation is an excellent way to improve accuracy, speed, and uniformity of application.

Why are navigation systems important to field operations?

Automated guidance of agricultural vehicles (tractors, combines, sprayers, spreaders) has been motivated by a number of factors—most important is to relieve the operator from continuously making steering adjustments while striving to maintain field equipment or implement performance at an acceptable level. This is not surprising, considering the many functions an operator must monitor, perform, and/or control while operating the vehicle.

The requirements placed on farm-equipment operators have changed drastically with increases in equipment size, power, multiple equipment functions, and speed—as well as monitors reporting on specific system performance. These increasing demands on the operator can result in increased errors in function, costs, environmental problems, and operator fatigue.

Foam markers, a widely used technology

Foam markers are the most common form of navigation aid used during fertilizer and pesticide application. The foam is dropped and used to align the applicator during the return pass. Foam markers utilize an air pump to pressurize a tank containing the foaming agent. The pressurized fluid causes the foaming agent to flow into an accumulating chamber. The foam collects in this chamber until the accumulated mass overcomes surface tension, causing a foam blob to fall to the ground.

Most often the foam accumulators are placed at the ends of the applicator boom or, alternately, at the center of the applicator when booms were not utilized, as in the case with spinner-disk granular applicators. Equipment operators use the foam blobs left on the field surface as a navigation aid to know where the applicator has passed.

GPS + Navigation Aids

Relatively inexpensive navigation aids known as parallel-tracking devices assist the operators to visualize their position with respect to previous passes and to  recognize the need for steering adjustments. These aids are commercially available in several configurations. One system is a light bar, which consists of a horizontal series of Light Emitting Diodes (LEDs) in a plastic case 12 inches to 18 inches long. This system is linked to a GPS receiver and a microprocessor. The light bar is usually positioned in front of the operator, so he or she can see the accuracy indicator display without taking their eyes off the field. The light bar can be mounted inside or outside of the cab, and the operator watches the “bar of light.” If the light is on the centerline, the machine is on target. If a bar of light extends to the left, the machine is off the path to the left and needs to be corrected. In like manner, if a bar of light extends to the right, the machine is off to the right.

Software allows the operator to specify the sensitivity to and distance between the swaths. Similar GPS  navigation systems have been used for aerial application since the early 1990s. Also, the GPS system gives the current location of the implement and, with past traffic patterns, the computer interface provides the operator directions to maintain proper swath width to  match adjacent traffic paths. If an operator leaves the field to refill the applicator or is forced out of the field due to weather, upon return, the operator can resume and maintain accurate swath widths, and over-spraying on previous sprayed areas is eliminated. More advanced systems have a screen showing the swath of the machine a sit moves through the field (Figure 2). Early models only allowed straight-line parallel swaths, but current models are available for any contour traffic pattern. Areas covered with previous swaths are indicated on the screen. The advanced navigation system coupled with a variable rate spreader drive and  software has the capacity to generate “as-applied” maps showing previous coverage and the application  pattern. This provides an excellent record of the pattern and timing while operating in the field. Portions of the field that are not treated, such as wet areas, can be marked in the computer and stored for later operations when conditions permit application. All of this is done without having to physically mark the field area with flags.

GPS + Auto-Steer Navigation

More advanced navigation systems (auto-steer systems) possess similar capabilities as the navigation aids and also have the additional option to automatically steer the vehicle. Auto-steering is accomplished with a device mounted to the steering column or through the electro-hydraulic steering system. The accuracy level of these system is based on the quality of differential correction and internal data processing: as the accuracy improves, the corresponding cost increases. These navigation systems are classified in three categories:

Submeter accuracy usually means approximately  two feet to four feet year-to-year, and less than one foot pass-to-pass errors. The differential correction source could be from a Coast Guard beacon, WAAS (Wide Area Augmentation System), or satellite providers. These systems are relatively inexpensive (about  $6,000 to $15,000) and can be used while performing tillage, some types of fertilizer and chemical applications, seeding, and harvesting. These devices can be easily transferred between vehicles, so the same steering system can be used on different vehicles. However, operations requiring highly accurate guidance are not feasible with submeter level equipment.

Decimeter accuracy provides approximately four inches to eight inches year-to-year and three inches to  five inches pass-to-pass errors. This can be achieved using either a local base station or dual frequency receivers with private satellite differential correction subscription. With the increased performance, operators can use auto-steering during most of the conventional field practices. Prices range from $15,000 to $25,000 plus the satellite subscription (up to $1,500 annually).

Centimeter accuracy can be obtained by using a local  base station with the real-time kinematic (RTK) differential correction. Both long-term and short-term errors (of approximately one inch) have been reported for these systems. Vehicles equipped with this high level equipment can be used to conduct strip tilling, drip-tape placement, land leveling and other operations requiring superior performance—as well as virtually any other task. In addition to the ability to accurately determine geographic location, auto-guidance systems usually measure vehicle orientation in space and compensate for unusual attitude, including roll, pitch, and yaw (see “Additional Features” for definitions). The price ranges from $40,000 to $50,000 with no annual subscription fees.

GPS Navigation vs. Foam Markers

Potential advantages of GPS Navigation (GN) relative to foam markers for agricultural applications include:

• GN is more reliable and more accurate than foam  markers. Using foam markers could cause about 10 percent of the field to be skipped or overlapped. With the GN, the skip and overlap rate drops to about 5 percent. Some tests have shown that with an experienced operator, the skip and overlap rate can be as low as 1.5 percent.

• GN allows accuracy at higher speeds. GPS navigation can attain a 13 percent to 20 percent higher speed than a foam marker (Buick and White, 1999). Naturally, an increase of speed is terrain dependent. If field conditions limit speed, then a GN benefit is unlikely.

• GN is a possibility with spinner spreaders. Foam markers are not generally used with spinner spreaders. The spreaders have no boom on  which the foam equipment can be installed. Due to the spread width, a foam marker in the center of the machine path is difficult to see from the next swath, and the driver would still be using a visual estimate  for the spreader swath.

• GN is easy to use. Anybody can learn to use GPS navigation systems, regardless of computer skills. The systems require only a little practice—typically about 30 minutes.

• GN provides effective guidance over growing crops. With solid-seeded crops, foam tends to fall through the canopy to the ground where it is almost invisible, contributing to skipping or overlapping. Crop height does not affect GPS.

•  GN allows operation when visibility is poor. GN works at night, in dust, or in fog. This lengthens working time during critical planting and spraying periods. In many areas, nighttime is best for spraying because of low wind speed. Leaf orientation changes at night and herbicide may or may not be as  effective as during daylight(check with local Extension specialists). Nighttime spraying of insecticides can protect sensitive bee colonies.

• GN is less affected by weather. In certain conditions such as low humidity, heat, and large fields, foam markers can be rendered ineffective. The foam can evaporate before the operator makes the return swath. GN works at any temperature, including low temperatures when foam systems freeze.

• GN has lower recurring costs. GN has no moving  parts or tubes to clog. Depending on the manufacturer, software updates are usually free to system owners. Foam-marker systems require foam, dye, and tank cleaner.

The primary recurring cost for GPS navigation is satellite differential correction. Typically, the subscription costs $600 to $1,500 per year for each GPS unit. Many producers already have GPS for yield monitoring and may subscribe for differential correction. For them, GN has almost no recurring costs. In many areas of Virginia, it is possible to use the Coast Guard beacon or WAAS for GPS differential correction. These services are free and may be adequate for some applications, including dry fertilizer with a spinner spreader.

Accuracy for spraying should be within six inches(10 centimeters); the GPS equipment supplier can provide information regarding the differential correction needs for a specific location. For more on GPS systems, review Precision Farming Tools: Global Positioning  System (GPS), Virginia Cooperative Extension (VCE).

•  GN reduces operator fatigue and eye strain. With  the light bar mounted directly in front, GN operators do not need to look backward or sideways. They can drive accurate swaths while looking straight ahead.

• GN has lower setup time. Foam markers require that tanks be filled and dyes be changed. GN begins workingapproximately30secondsafterthemachine is switched on.

• GN is not affected by wind or boom bounce.  Blowing foam or a foam system bouncing over rough ground at the end of a long boom may introduce substantial error. GPS systems are not affected by rough terrain or wind.

• GN reduces pesticide use by reducing overlaps.  If a 10 percent overlap is reduced to 5 percent, pesticide use also is reduced by 5 percent. The same is true for fertilizer and seed, so using GN is better for the environment and good for the bottom line.

• GN reduces the need for people to enter already  sprayed areas. The operator can mark where spraying stopped without dismounting.

Additional Features

Decisions about adapting GPS navigation should be based on particular farm needs, farm operation procedures, and understanding of positioning errors. Such issues as vehicle dynamics, tracking of trailed implements, and field terrain also need to be considered. Proper alignment and installation of the GPS navigation system is required for effective field operation. Poor quality of the steering-control systems, a sloped terrain, or misaligned implements will cause the field performance of GPS navigation to suffer. An important feature of GPS navigation systems is the ability to follow particular traffic patterns and provide effective feedback so that the operator or auto-steer system can appropriately respond. Most systems can  effectively perform straight-line patterns(linear swathing), and many can follow contours and other field features.

Although most GPS navigation systems are designed specifically for the task of vehicle steering, some systems have features to collect spatial data (such as application maps and yield maps) or to operate variable-rate  controllers. Additional features spread the cost of the system among several tasks while increasing efficiency in multiple areas of the crop production program. The documentation provided by a GPS system could be  used to show that the product or field work was applied in specific locations at a specific time. Setbacks specified by regulatory agencies and other sensitive areas (such as ornamental crops, nurseries, gardens, schools, and residential areas) may be defined as “no-apply zones.” The “as-applied” map includes areas in which  application occurred and rates can be recorded. It is possible to include not only position, but time and date information in an application record as well. Such documentation could be used for accurate recordkeeping for agrochemical application and machine operation and, potentially, for regulatory and legal defense purposes.

GPS navigation products are distinguished by the compactness of their components and user interfaces. Some systems cause technical challenges during installation  and calibration, while others may be functional in less than an hour. User interfaces range from intuitive, colorful, graphic touch-screen displays to hard-key menus with limited graphics feedback.

A sloped terrain makes control of vehicle dynamics challenging. Roll (tilt from side-to-side), pitch (movement from front-to-rear), and yaw (rotation around the vertical axis) alter the GPS antenna location with the  projected center of the vehicle . For example, when driving across a slope, the horizontal position of the GPS antenna is downhill with respect of the center of the vehicle, and the guidance will be in error down the slope. To compensate, some systems include gyroscopes, accelerometers, or additional GPS antennas. Less advanced terrain-compensation modules deal with only roll and pitch angles of the vehicle, while others can measure total dynamic attitude in six  degrees of freedom and enable the system to compensate for variable terrain.

Several manufacturers have add-on implement steering systems for proper implement tracking. Compensating for known geometry of the tracking from implements can be adjusted with the vehicle’s trajectory to keep the implement aligned. Optical and crop-based tracking systems can be useful when positioning implements with respect to established rows.

Bottom Line

GPS navigation has many advantages over conventional marking devices such as foam markers, and especially over the visual-estimation method for spinner spreaders. With an existing GPS being used for yield monitoring or field mapping and soil sampling, the GPS navigation system can increase the efficiency of the farm or agribusiness while minimizing adverse environmental impacts associated with overlapping  applications. The system can also reduce operator  fatigue and anxiety regarding fertilizer and pesticide application. Finally, use of this technology can demonstrate to the nonagricultural community that advanced  technology is being used to farm efficiently and safely.

The advantage of “as-applied” maps, provided by some systems, is documentation that applications were made at the appropriate location and rate.

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Potential monitoring of crop production using a satellite-based Climate-Variability Impact Index

Crop monitoring and early yield assessment are important for agriculture planning and policy making atregional and national scales. Numerous crop growth simulation models are generated using crop state variables and climate variables at the crop/soil/atmosphere interfaces to get the pre-harvest information on crop yields. However, most of these models are limited to specific regions/periods due to significant spatial–temporal variations of those variables. Furthermore, the limited network of stations and incomplete climate data make crop monitoring and yield assessment a daunting task. In addition, the meteorological data may miss important variability in vegetation production, which highlights the need for quantification of vegetation changes directly when monitoring climate impacts upon vegetation. In this sense, remotely sensed metrics of vegetation activity have the following advantages: a unique vantage point, synoptic view, cost effectiveness, and a regular, repetitive view of nearly the entire Earth’s surface, thereby making them potentially better suited for crop monitoring and yield estimation than conventional weather data. For instance, it has been shown that the application of remotely sensed data can provide more accurate crop acreage estimates at national/continental scales. Furthermore, numerous field measurements and theoretical studies have demonstrated the utility of remotely sensed data in studies on crop growth and production. These two applications suggest the feasibility of large-scale operational crop monitoring and yield estimation.

Empirical relationships between the remotely sensed data and crop production estimates have been developed for monitoring and forecasting purposes since the early 1980s. For instance, Colwell found a strong correlation between winter wheat grain yield and Landsat spectral data. However, these relationships did not hold when extended in space and time (Barnett and Thompson, 1983). Later, various other vegetation indices generated from Landsat data, such as the ratio of the reflectance at near infrared to red and the normalized difference vegetation index (NDVI) were used in yield estimation of sugarcane, wheat, and rice. The Landsat series have a spatial resolution of 30 m and can provide reflectance data from different spectral bands. However, these highresolution data require enormous processing effort, and  may not be applicable for surveys of large-area general crop conditions.

Vegetation indices derived from data from the Advanced Very High Resolution Radiometer (AVHRR) were also used for crop prediction, environmental monitoring, and drought monitoring/assessment. For example, found that millet yields in northern Burkina Faso are linearly correlated with the AVHRR NDVI integrated over the reproductive period. Similarly, Hochheim and Barber found that the accumulated AVHRR NDVI provided the most consistent estimates of spring wheat yield in western Canada. The Vegetation Condition Index (VCI) derived from AVHRR data is widely applied in real-time drought monitoring and is shown to provide quantitative estimation of drought density, duration, and effect on vegetation. The VCI can separate the short-term weather signals in the NDVI data from the long-term ecological signals. According to Domenikiotis , the empirical relationship between VCI and cotton yield in Greece are sensitive to crop condition well before the harvest and provide an indication of the final yield. Unfortunately, the AVHRR data are not ideally suited for vegetation monitoring.


1. The MODIS land-cover classification product identifies 17 classes of land cover in the International Geosphere–Biosphere Programme (IGBP) global vegetation classification scheme. This scheme includes 11 classes of natural vegetation, 3 classes of developed land, permanent snow or ice, barren or sparsely vegetated land, and water. The latest version of the IGBP land-cover map is used to distinguish croplands from the other biomes in this research.


The retrieval technique of the MODIS LAI algorithm is as follows. For each land pixel, given red and near infrared reflectance values, along with the sun and sensor-view angles and a biome-type designation, the algorithm uses model-generated look-up tables to identify likely LAI values corresponding to the input parameters. This radioactive transfer-based look-up is done for a suite of canopy structures and soil patterns that represent a range of expected natural conditions for the given biome type. The mean value of the LAI values found within this uncertainty range is taken as the final LAI retrieval value. In certain situations, if the algorithm fails to localize a solution either because of biome misclassification/mixtures, high uncertainties in input reflectance data or algorithm limitations, a backup algorithm is utilized to produce LAI values based upon the empirical relationship between NDVI and LAI (Myneni et al., 1997).

The latest version of MODIS global LAI from February 2000 to December 2004 was taken to characterize the crop activity in this study. The 8-day LAI products are distributed to the public from the Earth Observing System (EOS) Data Gateway Distributed Active Archive Center. The 8-day products also provide quality control variables for each LAI value that indicate its reliability. The monthly global product was composited across the 8-day products using only the LAI values with reliable quality. The monthly global products at 1-km resolution with Sinusoidal (SIN) projection are available at Boston University. In this paper, monthly LAI at 1-km resolution are used to generate our Climate-Variability Impact Index. As these will be compared with estimates of crop production reported at county/state-levels, the vegetation-based CVII fields were aggregated over the corresponding counties/states using the county bound arias 2001 map from the National Atlas of the United States.


AVHRR LAI is used as a substitute for the MODIS LAI to examine the temporal characteristics of vegetation activity over longer time periods. The AVHRR LAI is derived from the Global Inventory Modeling and Mapping Studies (GIMMS) NDVI produced by NASA GIMMS group. Monthly LAI from 1981 to 2002 at 0.258 were derived based on the empirical relationship between NDVI and LAI for different biomes. Literature works show that this empirical relationship might be different for the same biome at different locations. To eliminate this effect, models are generated for each pixel to calculate GIMMS LAI from GIMMS NDVI. The MODIS LAI and GIMMS NDVI overlapped from March 2000 to December 2002, which provides a basis for generating a piecewise linear relationship between these two products. Once the coefficients of the linear model are calculated, the whole range of GIMMS NDVI can be converted into GIMMS LAI, which is consistent with the MODIS products. Our preliminary results indicate a good agreement between GIMMS LAI and MODIS LAI at quarter degree resolution with less than 5% relative difference for each main biome (results not shown).


In this research, we also use model-generated estimates of Net Primary Production (NPP) from Nemani  as a predictor of crop production. This NPP is a monthly product from 1982 to 1999 at a spatial resolution of half degree. This global NPP product was generated as follows. GIMMS NDVI were first used to create LAI and FPAR with a 3D radiative transfer model and a land-cover map as described in Myneni. Then, NPP was estimated from a production efficiency model (PEM) using the following three components: the satellite-derived vegetation properties, daily climate data, and a biome specific look-up table of various model constants and variables. Further details can be found in Nemani et al. (2003).

5. Crop production

Crop production data from several sources are used in this research. We focus upon total production, as opposed to yield, for instance, because although the two are highly correlated with each other, total production is typically the parameter of interest for crop monitoring and yield prediction. In this paper, we will explicitly refer to ‘‘production’’ when discussing quantitative results, however for simple qualitative statements wesometimes retain the generic term ‘‘yield’’ as synonymous for ‘‘production’’. The country-level crop production from 1982 to 2000 in European countries is from FAOSTAT 2004 data set. The county-, district-, and state-level production data in United States are from the National Agricultural Statistics Service (NASS) at United States

Department of Agriculture (USDA) USDA provides two independent sets of county crop data: one is a census of agriculture, which is released every 5 years; the other one is annual county crop data, which is based on reports from samples. We used the annual crop estimates in this study. Due to the processing effort required for the fine resolution remotely sensed data, we studied two crops (corn and spring wheat) in two US states (Illinois and North Dakota) at county- and district-scales. At coarser scales, we expanded the regions to include Illinois (IL), Minnesota (MN), Michigan (MI), Iowa (IA), Indiana (IN), and Wisconsin (WI) for corn; to North Dakota (ND), Montana (MT), Minnesota (MN), and South Dakota (SD) for spring wheat; to Kansas (KS), Oklahoma (OK), Colorado (CO), and Nebraska (NE) for winter wheat. The county- and district-level data of Illinois and North Dakota are from 2000 to 2004; the state-level data are from 1982 to 1999.

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Map- and sensor-based variable-rate application systems: pros and cons

Both map- and sensor-based variable-rate application systems are available to the sitespecific farmer. There are also VRA strategies that incorporate aspects of both sensing and mapping. Each variable-rate application method holds advantages and disadvantages. Strong points of each system are summarized below:

Advantages of Map-Based Variable-Rate Application

• systems are already available for most crop production inputs

• the user has a database that can be useful for a number of management-related activities

• the user can employ multiple sources of information in the process of formulating a  variable-rate application plan

• the user has significant control regarding the function of such systems because of the  involvement in application rate planning

• field travel speeds need not be reduced

Advantages of Sensor-Based Variable Rate Application

• pre-application data analysis time requirements can be eliminated

• sensors produce far higher data resolution than traditional sampling methods

• no time delay between measurement and application with real-time systems

• systems are self-contained

It is important to match the application system with the objectives of the overall site-specific management program in which it will be used. Producers should expect an increasing number of options for both map-based and sensor-based site-specific operations as research and development efforts continue.

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