Using Cover Crops To Improve Nitrogen Availability For Grain Sorghum

Covers crops grown between periods of primary cash crop production can offer many benefits to the sustainability of cropping systems. Improvements in soil quality resulting from cover crops may include increases in soil organic matter, reduced soil compaction, or increased soil microbial activity. Cover crops can be an important management tool to reduce environmental pollution from soil erosion, leaching, and surface runoff. Past research in other regions has found mixed results ranging from increases in corn and sorghum yields after summer legume cover crops to no yield advantage following non-legume or winter legume cover crops.
An ongoing study is being conducted by researchers at K-State to determine how long-term effects of legume and non-legume summer and winter cover crops grown before grain sorghum impact nitrogen (N) availability and the response of sorghum yield to N fertilization.
The crop rotation was wheat-grain sorghum-soybean. Treatments included four different cover crops (see below), double crop soybeans (DSB) as a cash crop alternative, and a chemical fallow (CF) check. These treatments were imposed following wheat harvest so grain sorghum is the crop in the rotation most likely to be affected. The four cover crop treatments included:
  • Summer legume (SL) –  forage soybean
  • Summer non-legume (SNL) – sorghum-sudangrass
  • Winter legume (WL) –crimson clover
  • Winter non-legume (WNL) – daikon radish

Nitrogen fertilizer was applied after grain sorghum planting in a subsurface band at 0, 40, 80, 120, and 160 lb N/acre.

After three cycles of cover crops, yields of grain sorghum with no N fertilizer applied were highest following the summer legume cover crop treatment (Figure 1).  A minimum of about 35 lb N/acre of added N fertilizer would be required for sorghum yields to reach similar yield levels with the CF and other cover crop treatments.
Figure 1.  Sorghum response to 3rd cycle of cover crops (3-year average, 2014 to 2016)
Sorghum yield response to cover crops over an 8-year average is shown in Table 1. Grain sorghum planted after the summer legume and double-crop soybean cover crops produced significantly greater yields than the other treatments when no fertilizer N was added. There is potential to replace a portion of the cash crop N requirement with summer legume cover crops. The summer legume cover crop contributed an average of 33 lb N/acre to the grain sorghum during the growing season. Double-crop soybean contributed an average of 19 lb N/acre.
On the other extreme, sorghum sudangrass, a summer non-legume, removed an average of 47 lb N/acre from the plant-available soil N pool (Table 1). Sorghum sudangrass has a relatively high carbon-to-nitrogen (C:N) ratio which leads to immobilization (tie-up) of available soil N. For cover crops with high C:N ratios, additional N input by the grower may be necessary to maintain sorghum or corn yields.
Table 1. Nitrogen fertilizer replacement value and sorghum yield (8-year average)
Although there was no significant improvement in yield or N supply from winter cover crops, these plants may reduce N loss by leaching over the winter through N uptake before it moves out of the rooting zone as well as a providing residue to protect the soil surface and help reduce erosion.
The agronomically optimum fertilizer rate for sorghum after an 8-year average of yield data was approximately 80 lb N/acre for all treatments except where a summer non-legume cover crop (sorghum-sudangrass) was used. With the summer non-legume cover crop, the agronomically optimum fertilizer rate was 120 lb N/acre.
Appropriate management adjustments will need to be made if you are incorporating cover crops into your wheat-grain sorghum or similar cropping system. Cover crops are likely to affect N availability and uptake by the subsequent crop. Fertilizer N applications should be adjusted according to specific cover crop species and management to maximize yield.
(ByPeter Tomlinson, source –

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