Interseeding of cover crops in standing soybean

In order to reduce nutrient losses and soil erosion in the northern Great Plains following soybean, cover crops should be interseeded into standing soybean, since after soybean harvest there is not enough time for a cover crop to grow. If the cover crops cannot provide adequate growth and cover, the soil is exposed to degradation throughout the winter months, and loss of crop productivity in following years. The objective of this study was to determine the establishment and green cover of interseeded cover crops and their impact on soybean yield and quality. This experiment was conducted at two locations, Fargo and Prosper, ND, in 2016 and 2017. The experimental design was a RCBD with four replicates in a split-plot arrangement. Soybean reproductive stages R4 and R6 were the main plots and cover crops were the sub-plots. Four cover crops, winter camelina cv. Joelle, Austrian winter pea [Pisum sativum ssp. arvense (L.) Poir], winter rye cv. Rymin, and forage radish cv. Daikon, were sown at R4 and R6 stages of soybean. Cover crop biomass yield in the fall was highest in radish and winter pea, 2 Mg/ha and 1.85 Mg/ha respectively. Austrian winter pea provided the greatest soil cover. Cover crop above-ground biomass N accumulation ranged from 28.7 to 73.2 kg/ha. Results indicate interseeding cover crops have no impact on soybean yield and quality. Interseeding cover crops at later soybean reproductive stages has good potential to mitigate soil nitrate losses and soil erosion in areas that grow soybean.

2017

The objective of this study was to determine the effect of seeding cover crops into soybean at two late stages of development, on soybean yield and cover crop establishment at two locations, Fargo and Prosper, ND, in 2016 and 2017. The experiment consisted of six treatments (a) Austrian winter pea (Pisum sativum L.), (b) forage radish cv. Daikon, (c) winter camelina cv. Joelle, (d) winter rye cv. Rymin, (e) a mixture of all four cover crops, and (f) a check treatment with no cover crops.  Cover crops were interseeded at the R4 and R6 reproductive stages of soybean.

The treatment by seeding date by environment interaction was significant for cover crop biomass yield and green cover (Table 10). In 2016, camelina emerged but it did not survive due to lack of rainfall two weeks after planting. A similar response occurred in 2017 for the first date in Fargo. Camelina seedlings growing under the soybean canopy are growing in an environment with about 15% of incident solar radiation.  Once soybean starts dropping the leaves, camelina seedlings are quickly exposed to solar radiation. This combined with lack of desiccated the seedlings in only a few days. In 2017, camelina interseeded at R6 in Fargo and R4 in Prosper had similar biomass yield than the other cover crops, except for the cover crop mixture.

Winter pea provided the most soil cover compared with any other cover crop when interseeded at R4 in 2016 (Table 11). While in 2017, greatest biomass yield was observed when interseeded in R6 at both locations.  The rainfall in the three weeks after interseeding is critical for the establishment of a cover crop. An earlier seeding date can increase biomass production in the fall, if enough rainfall allow the crop to establish. Winter rye and the cover crop mix had greater biomass in the first interseeding date only at one environment (Fargo 2016).

Winter pea provided the most soil cover at all environments compared with any other cover crop. Austrian winter pea provided the most soil cover when established at R4 in 2016 in Fargo and at R6 in 2017, at both locations.

Soybean yield, and protein and oil content were not affected by the interseeded crops across environments (Fig. 10).  In 2017, wheat was seeded following the 2016 trials to determine if the cover crops affected wheat yield and protein content in the following season.  Winter rye reduced wheat yield and increased grain protein but it was significant only at one of the locations.  None of the other cover crops had an effect on wheat yield (data not shown).

Fall Soil residual NO₃-N was significantly higher in the check treatment compared with the plots with cover crops in 2016 averaged across locations. This indicates cover crops scavenge the available NO₃-N from the soil preventing it from leaching in the fall and spring. Soil water content was not different among treatments.

This experiment has been completed, all data analyzed and a publication is in preparation.

Table 10. Biomass of cover crops interseeding into standing soybean at two seeding dates, R4 (SD1) and R6 (SD2) and four environments, Fargo and Prosper, in 2016 and 2017.

Table 11. Green cover (%) of cover crops interseeding into standing soybean at two seeding dates, R4 (SD1) and R6 (SD2) and four environments, Fargo and Prosper in 2016 and 2017.

Table 12. Soybean yield, protein and oil content averaged across seeding dates (R4 and R6) and environments.