Jamie Taraila, Graduate Research Assistant; Jarrod O. Miller, Assistant Professor and Extension Specialist, Agronomy, email@example.com; Amy L. Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality, firstname.lastname@example.org
As part of a Northeast SARE sponsored project, we evaluated cover crop growth response (biomass) to various seeding rates. We also followed cover crop growth through the winter using drone imagery. We planted NRCS recommended rates of rye (100 lbs/acre), clover (15 lbs/acre), and a rye/clover mix (40 lbs/acre rye, 10 lbs/acre clover) at the UD Carvel Research and Education Center in Georgetown following corn harvest in late September 2020. We also planted the same cover crops at reduced rates of 25, 50, and 75% the NRCS recommendation. By the time we sampled cover crop biomass in April, there was very little difference in biomass from the 25 to 100% (full) rates. In fact, the biomass response curves show that there was little increase in biomass above the 25% seeding rate (0.25) for each cover crop type, as the curve is relatively flat above this rate (Figure 1).
One potential takeaway is that planting cover crops at a significantly reduced seeding rate may not limit biomass accumulation when cover crops are planted early. In other words, lower seeding rates may not limit the potential benefits (e.g., weed suppression or nutrient scavenging with rye or nitrogen additions from clover) of cover crops. However, this study was only from one season in a field where saturated low spots may have caused a lot of variability in the desired planting rates and we may not have reached a maximum potential biomass.
Drone imagery collected 12 times between planting and harvest helps to explain cover crop conditions over the winter. Photos taken by a drone were analyzed for their reflectance of red and near-infrared light with vegetation index known as NDVI, which can range in value from -1 to +1. This index is very good at measuring leaf area (or plant biomass). Theoretically, higher cover crop planting rates should result in greater NDVI if they result in more biomass.
Normal (bare) soil NDVI background levels are about 0.2, which we observed in September to October as well as in the plots with zero rate plots (Figure 2). The NDVI levels in the rye plots (yellow points, Figure 2a) show and effect of seeding rate by December, as points appear to spread out on the graph. After December, winter temperatures or soil saturation decreased NDVI in plots with the highest rye (NRCS recommended) seeding rate; NDVI of all treatments decreased by mid-February. This may not occur in every field every winter, but it does explain why plots planted with 100 lbs/acre of rye ended up with similar plant growth (biomass) as plots planted at a seeding rate of25 lbs/acre (25% of the NRCS recommended rate). In this case it appears that winter weather resulted in the death of individual plants or a significant reduction in tillers. Most cover crop fields do not receive additional N, which may explain why tillers did not survive through the winter.
Clover growth (green points, Figure 2b) tells a different story. There was very little clover growth from fall through winter, and almost no seeding rate response until late March or early April. The NDVI for the 25% clover seeding rate sits almost evenly between 0 and the other rates (50, 75, and 100%) by the time we reach mid-April. It seems possible that planting clover at a rate of 7 lbs/acre would produce similar biomass to a cover crop planted at a seeding rate of 15 lbs/acre; however, several years or fields need to be evaluated to verify these results.
The inclusion of rye (at a lower rate seeding rate of 40 lbs/ace) in the rye-clover cover crop mixture resulted in separation of NDVI between rates in the fall. In the case of the cover crop mixes, most of the winter kill occurred around February (Figure 2c).
Our study raises interesting questions about whether we can achieve desired biomass production and when cover crops are planted at lower seeding rates. And in the case of rye, would application of a starter N fertilizer be beneficial to control winter kill if high biomass is desired when following corn. While these questions cannot be answered based on this study alone, these are questions that could be answered with additional cover crop research within the Delmarva climate.