Cover Crops, no simple solution

Planting cover crops is a key tenet of conservation agriculture that involves planting non-cash crops on agricultural fields to provide soil cover between primary crop growing seasons. Cover crops primarily benefit future crops. They do this by reducing soil erosion and nitrogen runoff, crowd out weeds, control pests and diseases, increase biodiversity, and improve soil health soil health by helping to build soil carbon.

Building soil carbon serves also to reduce CO2 in theatmosphere. So, cover cropping was well funded under the United States Department of Agriculture's (USDA's) Environmental Quality Incentives Program that turned all agencies towards climate stewardship and has provided more than $100 million of incentives for cover crop adoption each year since 2016. An additional incentive of reduced insurance premiums was added through the Pandemic Cover Crop Program. 

Under these incentives, the total cropland area in the United States planted with cover crops in 2017 was  nearly 50% higher than reported in 2012 and has continued rising in the past five years. It sounds impressive until you realize that overall, in 2017 only about 5% of cropland  used cover crops.

Cover crops (grasses, legumes and forbs) recommended for seasonal cover and other conservation purposes include annual ryegrass, oilseed radish, winter cereal rye, and oats used for scavenging unused fertilizer and releasing nutrients back into the soil for the next crop to use. Good cover crops to break up compacted soils are forage radish  and forage turnip. Similar to commercial nitrogen (N) fertilizers, legume cover crops like crimson clover, hairy vetch and Austrian winter pea can provide some of the nitrogen needs of the primary crop.

Experimental field trials have often found slight yield losses for primary crops. However, these effects appear to vary considerably depending on many factors, including the agricultural region, the combination of cover and primary crop types, weather conditions, and management practices. Results from  field trial varied widely based on the type of cover crop, the level of fertilization, and the date of cover crop termination.

In a new study from Stanford scientists, examines yield loss by using data from actual farmer fields. They used satellite data to observe both the adoption of cover cropping and the yields of corn and soybeans throughout six states in the heart of the US Corn Belt. These observations, cover more than 90,000 fields, are then used in a algorithm developed by others to measure the incremental yield impact of adopting cover crops.

Using the satellite data they could determine the presence or absence of cover crops each year at field-level resolution. They used the previously published Scalable Crop Yield Mapper (SCYM) algorithm to forecast yield. The SCYM uses region-specific crop model simulations and weather to determine yields from satellite pixel data. Because they were using satellite data, their analysis could only represent the yield impacts of cover cropping as practiced in aggregate across the region.

The algorithm results indicated that fields where cover crops were adopted for 3 or more years experienced an average corn yield loss of 5.5%, compared with fields that did not practice cover cropping. The scientists also found on average, soybean yields were reduced by 3.5% following cover crop adoption. Nearly all locations appeared to experience negative effects. In general, impacts appeared most negative in Iowa and Northern Illinois compared with the rest of the study region. These areas were generally associated with better soil ratings, higher mid-season temperatures.

The scientists found greater yield losses for corn than soybean, which they felt was likely due to soybean's lower need for fertilizer nitrogen. They also found that corn yield impacts were significantly more negative on fields with a high soil productivity index (NCCPI). The scientists reasoned that those fields have higher yield potential, they accordingly have higher nitrogen needs to meet their yield potential.

Based on anecdotal observations in our own Prince William Soil and Water Conservation District.  “Small yield losses may be seen in certain situations,  in certain years, and the longer growers work with integrating covers in their systems the better they get at managing them thus reducing these losses.  The other thing they didn't look at was the economics.  Going no-till and using covers reduce fuel and fertilizer used.  Even though yield may be slightly reduced, profit may actually be better.” (Jay Yankey, former Manager PWSWC and current Farmer.)

There is on the ground research supporting the numerous benefits of introducing cover crops into a system, there are also challenges that growers may face in implementation or management. Cover cropping is different in different agricultural systems. Particularly in arid or drought-prone environments, the water needs by cover crops may cause a reduction in the amount available to the main crop, or require the use of supplemental irrigation.

In addition to potential increases in irrigation, there are other economic costs that must be considered. Expenditures for seed and soil preparations as well as labor requirements will change with the introduction of a cover crop. Because cover crops are left in the field, there is no direct profit to the farmer for harvested crop products. If improperly selected or managed, some cover crops can persist as weeds when the field is transitioned and prepared for subsequent plantings.