Cover crops use water, impact wheat yields in High Plains
February 20, 2014
Do cover crop mixtures or cocktails use less water than a single-species cover crop? Do cover crops reduce evaporation losses enough to make up for their water use? Do cover crop mixtures improve soil biological activity in a semi-arid environment?
These and other questions are being asked as interest grows across the United States in using cover crops to improve cropping practices.
Research conducted at the UNL High Plains Ag Lab (HPAL) near Sidney and at a USDA research station at Akron, Colo., suggests that farmers should approach the use of cover crops with caution. The research points out several factors for farmers to consider.
Results from the cover crop study were shared with the HPAL advisory board Thursday at the annual research update at Sidney, along with other crop and livestock research conducted at HPAL. The meeting also included updates on faculty positions and building plans.
The cover crop study was conducted by David C. Nielsen, Research Agronomist, USDA-Agricultural Research Station, at the Central Great Plains Research Station at Akron, Colo.; Drew J. Lyon, Professor, Crop & Soil Science Department, Washington State University, Pullman, Wash.; and Gary W. Hergert, Soil & Nutrient Management Specialist, at the UNL Panhandle Research & Extension Center.
Cover cropping has been defined as growing a crop between the regular grain crop production periods in order to protect and improve the soil. It is also referred to as "growing green manure" or (in the semi-arid Great Plains) "practicing green fallow". Cover crops can consist of grasses, legumes, oilseeds, and other broadleaf plant species and can be planted in either fall or spring.
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Until recently cover crops have not been used for any direct economic benefit. Currently, they are sometimes used for grazing, or the biomass produced by the cover crops is sold.
Some of the benefits claimed for cover crops include increased organic matter in the surface soil layer; improved soil structure and infiltration; reduced surface soil water evaporation; protection from wind and water erosion; increased snow catch; production of nitrogen; increased soil biological activity; increased nutrient availability; reduced nutrient loss; alleviating compaction; suppression of weeds, insects, and diseases; and improved water availability for subsequent crops.
Many of these benefits have been reported in areas that receive more precipitation and have greater humidity than the Central Great Plains. But they have not been rigorously verified with scientific trials in the semi-arid environment of the Central Great Plains – in particular, claims that cover crops grown in multi-species plantings of at least eight species (sometimes referred to as cocktail mixes) produce significant amounts of biomass while using almost no water.
This claim was recently tested at HPAL and Akron by growing spring-planted cover crops in proso millet residue in a no-till system under several water availability conditions ranging from rainfed to nearly non-water-stressed conditions. The cover crop treatments consisted of single-species plantings of oats, peas, flax, or rapeseed, and a 10-species mixture of those same four crops plus lentils, vetch, clover, barley, safflower, and phacelia.
Both at HPAL and Akron, cover crops used more water during the growing season than the amount of water that evaporated from the fallow plot. Also, the cover crop mixture did not use less water to grow biomass than the single-species plantings.
The researchers also analyzed soil samples for 16 microbiological populations. As expected, both the numbers of total fatty acids and mychorrhyzal fungi were greater where there was a growing crop with living roots compared with the fallow plot. However, the biological activity was not greater where cover crops were grown in the mixture compared with single-species.
The water use and yields of the wheat following the cover crops were measured under the two water availability regimes. It is clear that all cover crops, whether grown as single species or in a mixture, use water, thus reducing available water and water use for the subsequent wheat crop, resulting in decreased yield compared with wheat following fallow.
Reduction in available soil water at wheat planting reduces subsequent wheat yield by 5.94 bushels per acre for every inch of water used by the cover crop that is not replenished prior to wheat planting. Even when a cover crop is terminated early, yield loss was significant.
The researchers say they are confident that the results in this study apply to previously published data on lower wheat yields due to water use by single-species legume cover crop.
The costs of the cover crop seeds used in this study range from $6 per acre to $40 per acre. The cost of the cover crop mixture was $60.29 per acre. Additional costs would include inoculant for the legumes, planting costs, and loss of income due to yield reduction caused by the cover crop water use.
Some of these costs would, of course, be offset if the cover crop could be grazed or a portion of it harvested and sold. However, grazing would likely reduce some of the cover-crop benefits listed above.
The research points out that dryland farmers in the semi-arid Central Great Plains should approach the use of cover crops with some caution, considering several factors:
All of the beneficial effects of cover crops that are reported from their use in more humid environments may not be observed in this region due to the greater evaporative demand.
Experimental results of studies done in this region indicate that there will likely be a significant yield depression on the following wheat crop from growing a cover crop, even if termination of cover crop growth occurs fairly early (e.g., mid-June)
Any beneficial effects (improved soil health, increased nitrogen availability) of growing the cover crop will need to be weighed against the economic consequences associated with the cover crop seeding costs and the yield drag in the subsequent wheat crop
There appears to be no evidence of superior beneficial effects to recommend the additional expense associated with growing a cover crop mixture vs. a single-species cover crop planting. F