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The overall aim of this research was 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-conserving 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. Planting green may also impact the risk of slug damage to soybean seedlings in no-till fields.

The aforementioned cover cropping benefits and concerns are of particular relevance 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 (even more than corn) 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 living high-biomass cover crops after extended growth in spring. Soybeans, therefore, stand to benefit from water conservation, nutrient-cycling (K, S, Ca, Zn, B), and compaction alleviation effects of high springtime biomass cover crops. However, since most soybeans are grown in rotation with corn, this research will examine cover crop interactions with both crops. In summary, this project aims to generate important information on how to better use cover crops for improved soil quality, reduced crop stress, enhanced nutrient cycling, and profitability.

Results for 2021
While this project is ongoing, the results so far have provided some important lessons. We have shown that establishing cover crops early by interseeding into standing crops is practical using a highboy broadcast air-seeder. We also have used a special no-till drill with one out of four drill units removed to provide clearance for a young standing crop. This kind of interseeder drill did an excellent job of establishing three rows of cover crop seedlings between the rows of the cash crop. However, we soon learned that the soybean canopy is so dense that not enough light penetrates to allow these seedlings to survive the summer until soybean senescence. The system using a drill works much better in corn which allows more light to penetrate. However, we found that drilling was possible only during a very brief window and was very dependent on suitable soil moisture conditions during that window. This is essentially the same window as the time for side-dressing corn. When soil conditions were on the wet side and in a fine-textured soil, the drill caused enough disturbance to injure some of the corn crop plants and resulted in poor stands because of improper seed furrow closure. In contrast, using a highboy broadcast air-seeder, there was much more flexibility with timing, especially for corn. Good cover crop stands were established when the seed was broadcast anytime between tasseling and the beginning of grain fill in corn canopies. When conditions were dry the seeds sat and waited for the next rain so early planting increased the likelihood of a seed-germinating rain occurring before corn senescence, thus giving the cover crop a good head start. In soybeans, broadcasting cover crop seeds too early resulted in their germinating in a very dark environment and the seedlings soon died. Broadcasting into a soybean canopy was effective when it was done at the beginning of leaf fall rather than the beginning of leaf yellowing. In 2021 successful cover crop stands were established in both corn and soybean with these methods.
During the summer of 2021, our array of sensors showed that the heavier surface residue from the cover crops planted in the Fall of 2020 served to conserve moisture in the upper foot of soil. Because there was relatively good moisture throughout the season this advantage did not translate into significantly higher yields of either corn or soybeans. We expect that in dryer years yields would be advantaged. In the spring of 2021, we observed a significant slug infestation in the fine-textured, wetter field. We took advantage of this to study the impact of cover crops and cover crop termination date on slug damage to soybeans and corn seedlings. The spring of 2021 was unusually cool so the soybeans, in particular, got off to a slow start. However, we did see some significant and interesting effects. Slug numbers were not affected by the cover crop but seemed to be affected by the type of residue on the surface. The slug damage to soybean seedlings was significantly less severe where the cover crop was terminated more than a week after the soybeans were planted. While this was only one site and one year in unusually cool spring conditions, we believe that this effect is likely to be even more pronounced in a warmer spring when soybeans can get off to a faster start. In the next year, the project will continue to study the interaction between cover crop management and the degree of damage caused by slugs.