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Project Title:
Broadcast Seeding of Cover Crops into Standing Soybean to Improve Soil Health

Executive summary

In order to reduce soil erosion following soybean, which leaves the soil exposed to the elements, cover crops can be interseeded into standing soybean. The objective of this study was to determine cover crop biomass yield and cover, interseeded at two soybean reproductive stages R4 and R6, as well as any impacts on soybean yield and quality.

This experiment was conducted in Fargo and Prosper, ND, in 2016. The experimental design was a RCBD with four replicates, in a split-plot arrangement where soybean reproductive stage was the main plot and cover crop treatment was the sub-plot. Four cover crops, winter camelina cv. Joelle, Austrian winter pea, winter rye cv. Rymin, and forage radish cv. Daikon, were sown into the ground at the R4 and R6 stage of soybean.

Results indicate that soybean yield, test weight, and quality were not affected by the cover crops interseeded. This indicates cover crops interseeded into soybean at R4 or R6 did not compete with soybean. Austrian winter pea establishment and biomass yield was the highest across sowing dates and locations. Winter peas average soil cover at the end of October was 68.9%. Biomass yield of winter pea, cereal rye, and the cover crop mix was higher when established in R4 growth stage compared with establishment on R6. Oppositely, radish had significantly better establishment and biomass yield when established on R6.

Nitrogen accumulation was greater in Austrian winter pea and radish and lower in cereal rye. Most of the N in the cover crops biomass probably came from the soil. Pea is a good N scavenger. Legumes in general use the N available first before N2 fixation. Soil NO3-N in the fall was significantly greater for check plots without a cover crop. This indicates cover crops scavenged the N between the rows of soybean. This difference was no longer seen in the soil samples taken in the spring. Interseeding winter pea has the potential to add 60-80 lbs of N /acre to the soil, which could reduce the N fertilization of the next cash crop in the rotation.

Light under the soybean canopy was measured from growth stages R4 to R8. The light intercepted by soybean canopy decreased as plants start shedding leaves and the leaf area index decreased. Light reaching the soil under the canopy increased as soybean plant matured and thus was available for the cover crop. At soybean R6 stage, pea and rye started to intercept light; by harvest pea and rye intercepted 31 and 14% of the incident PAR.

Interseeding a cover crop (rye, Austrian winter pea or radish) after R4 in soybean is feasible because soon after the cover crop is planted soybean starts to drop the leaves allowing light into the canopy. Interseeding cover crops at later soybean reproductive stages shows to be a viable solution for North Dakota producers to mitigate soil erosion after soybean harvest when there is not time for a cover crop to get enough growth to cover the soil before the hard frost.