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Extending the Growing Season to Get More Payback from Cover Crops
Lay Language Summary

The overall aim of the Extended Cover Cropping Project is to provide information that may help change the mindset about cover crop management from one of doing the minimum to qualify for payments to managing for maximal cover crop benefits for soil health and profitability. Maryland and Delaware have some of the highest proportions in the country of cropland acres cover cropped. However, farmers enrolled in the state cover crop programs typically plant cover crops after cash crop harvest, which our research shows is usually too late to effectively capture the large pool of soluble nitrogen left deep in the soil or provide enough cover to adequately control overwinter erosion. Using aerial or ground-based interseeding into standing crops, choosing earlier-maturing corn and soybean cultivars, and making other adjustments to the farming system may allow earlier cover crop establishment.
Many farmers also cut short cover crop growth potential in spring by terminating cover crops as early as possible, commonly in late March or early April. Such termination is too early to allow the cover crops to optimally promote soil health, water conservation and crop yield. Delaying spring cover crop termination until optimal cash crop planting time, especially planting green instead of killing cover crops two to four weeks ahead of planting, could allow both timely cash crop planting and extended cover crop growth. Potential benefits of greater cover crop biomass growth include short-term benefits such as greater nutrient cycling, better weed-suppression, and more effective water-conservation in summer, in addition to longer-term benefits of increased soil organic matter and biological activity. Preliminary experience suggests that planting into living cover crops may also save time and improve stands and gain additional weed suppression advantages.
The aforementioned cover cropping benefits and concerns are of particular relevancy to soybean production for several reasons. First, soybeans tend to leave a large amount of soluble N in the profile at the end of the season and soybeans tend to be harvested later than corn. These factors combine to make early cover crop establishment in fall especially important for soybean systems. Second, soybeans, unlike corn, do not tend to respond adversely to the early shading and N immobilization that may be associated with planting into certain living high-biomass cover crops after extended growth in spring. Soybeans therefore stand to benefit from water-conservation, nutrient-cycling (P, K, S, Ca, Zn, B) and compaction-alleviation effects of high springtime biomass cover crops.
Our objectives are to 1) document the impacts (benefits and/or problems) of planting earlier and killing later, and 2) develop and test strategies and technologies for (a) getting cover crops established earlier, including airplane seeding, early maturing crop cultivars, and inter-seeding with ground equipment and (b) letting them grow longer in spring (including planting green). Using replicated experiments on coastal plain soils at the University CMREC research farm and collaborating commercial farms in 2016-2019 we found dramatic increases in N capture and reductions in nitrate leaching in both winter and spring from planting cover crops just two weeks earlier in September. Biomass carbon added to soil and N fixed by legumes was four times greater with early May instead of early April termination. There was no drag on yields with either practice when we used a multi-species cover crop that included a brassica, a legume, and a cereal. In fall 2019 we established at two sites at the Beltsville CMREC research farm excellent early-planted rye, radish and rye-radish-clover mix cover crops. In addition, we collaborated with commercial farmers on the Eastern Shore to plant mixed cover crops with two different methods and dates (flown into standing crops or drilled after harvest) on six soybean/corn fields (approximately 300 acres). We installed suction lysimeters to measure the concentration of nitrate in leaching water over the winter, and installed chambers to measure nitrous oxide gas emissions. In addition, we collected samples of cover crop biomass to measure productivity and nutrient contents. In 2020, we plan to follow up by monitoring soil water fluctuations and crop growth during summer and yield in fall. In summary, this project will generate important information on how to better use cover crops for improved soil quality, reduced crop stress, enhanced nutrient cycling and profitability. Overall, from three years (some 18 site-years) of experience, we conclude that flying seed onto a standing crop canopy in September is likely to be advantageous compared to drilling after harvest with regard to biomass fall production and nitrogen capture. However, stands tend to be less even and establishment is more variable with aerial seeding. In all cases we also observed substantial suppression of weed biomass with cover crops established by both methods.
Our data is also transforming the way we as researchers look at the nutrient capture / water quality function of winter cover crops. The new view of how cover crops impact leaching of nitrate during the winter focuses on activity during the fall when the cover crop can utilize the growing degree days remaining before winter to send roots deep into the profile and clean up most of the 150 pounds/acre or more of soluble nitrogen our recent research has shown to be present in the soil towards the end of the summer cropping season. This leaves a relatively nitrate-free soil profile exposed to the leaching water that percolates down through the profile during the winter when precipitation greatly exceeds evapotranspiration. If the cover crops are planted a few weeks later in fall, often after summer crop harvest is completed, there may not be sufficient growing degree days remaining before the onset of winter dormancy to achieve this purpose. The result is that where cover crops are planted late the soil is full of soluble nitrate throughout the profile and that nitrate is leached away with the percolating water during the winter.