Reduce nitrate losses, improve soil health, increase corn yield in dry years, and may reduce slug damage to seedlings in a corn-soybean system
Fields with cover crops; Credit: Ray Weil, University of Maryland
Authors: Ray Weil, Melissa Stefun, and Vivian Owens
What are the issues?
Maryland has the nation’s highest proportion of cropland acres with cover crops and wide adoption of no-till farming. The mid-Atlantic region, in general, also has a relatively high rate of adoption for these practices. However, farmers typically plant a single species of cereal cover crop in late fall after cash crop harvest and cut short the growth potential of cover crops in spring by terminating them as early as possible, commonly in late March or early April. The late start in the fall severely limits the ability of the cover crops to prevent nitrate leaching over winter. The early termination in spring hinders the cover crops' ability to promote soil health, conserve summer water, suppress weeds, and increase crop yields. In Maryland, interest in enhanced spring cover crop management has increased in response to a new provision in the Maryland cover crop program, which now offers an extra incentive payment to postpone termination past May 1st. However, many farmers are concerned that using cover crops, especially if they are allowed to grow longer in spring, might increase the risk of slug damage to young crop seedlings.
What did we find and why does it matter?
Our research explored how planting into living cover crops may also save time, improve stands, reduce erosion and nutrient runoff, increase weed suppression, and even reduce slug damage to crop seedlings.
Delaying spring cover crop termination until optimal cash crop planting time, especially “planting green” into living stands of cover crops instead of killing the cover crops two to four weeks before planting, can allow both timely cash crop planting and extended cover crop growth. Potential benefits of greater cover crop 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. This project measured root biomass as well as aboveground biomass since roots are known to contribute more, pound for pound, than shoots to building soil organic matter.
Enhanced cover cropping included early interseeding into standing cash crops (left), cover crop growth in early fall before cash crop harvest (center), and late termination during or after cash crop planting in spring (right). Photos courtesy of Ray Weil, University of Maryland.
Nitrogen (N) and phosphorus (P) loss to ground and surface water
Nitrate concentrations in leaching water over 3 years were reduced by enhanced cover crops to near the limits of detection. Without cover crops, nitrate was variable with monthly spikes as high as 15-25 parts per million nitrogen (ppm N).
Dissolved nitrate concentrations were significantly lower in surface runoff water from cover crop plots than from no-cover crop controls, but dissolved organic N, which accounted for about 75% of the total dissolved N, was unaffected by cover crop treatment. Dissolved phosphate-P concentrations were lower in runoff from fields covered with soybean residue unless there was no cover crop.
Our results suggest that early-planted cover crops can effectively reduce the most reactive forms of N and P that cause eutrophication of natural waters, such as the Chesapeake Bay.
Slug damage and crop yields
Slug damage to soybean seedlings was scored from 0 to 4 in spring 2021, 2022, and 2023 at the silty-clay field experiment. A score of 0 meant no observable damage and a score of 4 mean fatal damage that would kill the seedling. Neither using a cover crop nor planting green into the living cover crop increased slug damage, as many farmers fear. However, in 2021, terminating the cover crop late (10 days after soybean planting) significantly reduced slug damage. In 2022, later termination slightly increased slug damage, but only for the 3-way mixed species cover crop. Additional work on commercial farms confirmed that using cover crops and letting them grow up to planting time in spring does not increase the risk of damage from slugs.
Slug activity observed during the study. (top) Meadow slugs feed on soybean seedlings. (center) Two garden slugs and one meadow slug on a turned-over shingle used to count slugs. (bottom) A slug feeding on rye cover crop tissue after soybean emergence in a late-terminated plot. Photos courtesy of Sydney Gabitzer and Ray Weil, University of Maryland.
What we did
We conducted three field experiments at the Central Maryland Research and Education Center in Beltsville. Experiments #1 and #2 involved a corn-soybean rotation with three cover crop types and three termination timings. These experiments were identical, except that they were conducted on contrasting soils: one a droughty sandy soil and the other a relatively wet silty to clayey soil. Experiment #3 was on a sloping sandy soil adjacent to woodland consisted of four large plots, two with standard cover cropping practices (drilled after harvest and terminated weeks before planting) and two with enhanced cover cropping practices adapted from the other two more complex field experiments.
Techniques used to collect water samples to evaluate nutrient losses from the field experiments. From top to bottom: suction lysimeter pulls water from the larger soil pores at 90 cm deep; wells were augered and casings were installed under cast iron manhole covers to allow groundwater sampling year-round; a sprinkler infiltrometer makes simulated rain that runs off into buried sample bottles; a mini-erosion weir funnels surface runoff from natural rainstorms into a collection jugs buried in a hole and kept from floating up by the red concrete blocks. Photos courtesy of Ray Weil.
Publications completed for this work
Goralczyk, I., Sedghi, N., & Weil, R. R. (2019). When it comes to nitrogen leaching, not all cover crop practices are the same. Agronomy News, 10(1), 3–4.
Hirsh, S., & Weil, R. R. (2019). Deep soil cores reveal large end-of-season residual mineral nitrogen pool. Agricultural & Environmental Letters, 4(1), Article 180055. https://doi.org/10.2134/ael2018.10.0055
Hirsh, S. M., Duiker, S. W., Graybill, J., Nichols, K., & Weil, R. R. (2021). Scavenging and recycling deep soil nitrogen using cover crops on Mid-Atlantic, USA farms. Agriculture, Ecosystems & Environment, 309, Article 107274. https://doi.org/10.1016/j.agee.2020.107274
Sedghi, N., & Weil, R. R. (2022). Fall cover crop nitrogen uptake drives reductions in winter–spring leaching. Journal of Environmental Quality, 51, 337–351. https://doi.org/10.1002/jeq2.20342
Sedghi, N., Fox, R. J., Sherman, L., Gaudlip, C., & Weil, R. R. (2023). Aerial interseeding and planting green to enhance nitrogen capture and cover crop biomass carbon. Journal of Soil and Water Conservation, 78(5). https://doi.org/10.2489/jswc.2023.00051
Stefun, M., Gabitzer, S., & Weil, R. R. (2021). Slug damage to soybeans—Do cover crops help or hurt? University of Maryland Extension. Retrieved from https://extension.umd.edu/resource/slug-damage-soybeans-do-cover-crops-help-or-hurt
Weil, R., Wang, F., Hirsh, S., & Sedghi, N. (2024). Cover crop management to reduce nitrogen leaching from cropland: A decade of research in the Mid-Atlantic USA. In R. Hatano & E. M. Baggs (Eds.), Nitrogen cycling and soil health. Springer Nature.
