Cover crops have been rapidly adopted in Delaware, with cereal rye being a popular option for soybean production. The benefits of a rye mulch is weed suppression and soil moisture conservation, but may also cause increased pest presence and disrupt the release of nitrogen (N) to cash crops. While soybeans may not be as affected by the N cycle as corn, the mineralization and release of N in rye may also provide supplemental N to the plant mid-season. These fields may also include corn fodder from the previous cropping year, which will continue to breakdown through the soybean growing season, providing some carbon the soil surface. What is not currently known is now soybean populations and row spacing may affect the decomposition of residues on the soil surface. Earlier canopy may preserve soil moisture, allowing for increased residue decomposition, or may increase evapotranspiration reducing overall soil moisture. This study will take the first steps in measuring decomposition of residues under soybean planting densities.

1) Plant full season soybeans into a rye cover crop at five populations and two row spacings.

2) Use decomposition bags to measure breakdown of corn fodder and rye biomass under different soybean management.

3) Track soil moisture and temperature under the soybean canopy

Soybeans will be planted at the Carvel Research and Education Center in Georgetown, DE into a rye cover crop. The cover crop will be terminated two weeks prior to planting, like most field practices. Soybeans will be planted at five densities (80, 100, 120, 140, 160 thousand seeds per acre) and two row spacings (15 and 30 inch) and irrigated throughout the season. Ten subsamples across a gradient will be compared to drone imagery to estimate total field rye biomass. Terminated rye biomass will be collected from outside the plot boundaries and corn fodder will be collected from fields at the research center. Biomass will be separated into decomposition bags for each plot (30 rye and 30 corn fodder), weighed, and placed back into the planted plots in the center of a row. Three subsamples of each will be dried and saved to determine the initial carbon (C), N, and moisture content of the biomass. At the end of the season decomposition bags will be collected from the plots, dried, weighed, and analyzed for C, N, and the biomass loss. In three population plots (80, 120, 160), logging sensors will be placed in the 15 and 30” rows to measure EC, moisture, and temperature throughout the season. Yields will be collected with a plot combine in the late fall.

Data will be analyzed in SAS as a randomized complete block design structured by a factorial including biomass loss, changes in C and N, as well as yield. Yield will also be correlated to various predictors from the study.

Update:
A rye cover crop field was managed over the winter months an burned down in May 2023. Between cover crop planting and termination, a drone was used to monitor growth by creating NDVI maps. Prior to burndown, rye biomass was sampled across the field to correlate growth with the NDVI maps produced. Soybeans were then planted into the terminated rye at populations between 60 to 180,000 seeds per acre and in 15 and 30 inch row spacings. Following emergence, soil moisture and temperature sensors were placed in three 15 inch and three 30 inch row spacing plots to monitor conditions through the season. Drone flights have continued through the summer season and plots were monitored for emergence and deer damage, which has been minimal. Irrigation and herbicide applications have been performed as necessary.

Update:
For the soybean spacing project, plots were maintained through early fall with irrigation as necessary as well as hand weeding for palmer and other weeds. Weekly checks of the moisture sensors as well as drone flights were also performed up until harvest. A heavy storm in October causes earlier defoliation, with final harvest occurring in early November. Prior to harvest, decomposition bags were removed, dried, and weighed before being shipped to the University of Delaware Soil Testing Lab. An initial analyses will be presented at the 2024 Delaware Ag Week.

Update:
Final project report is attached. Results from this study were presented at Delaware Ag Week in January 2024.

View uploaded report

Cover crops have been rapidly adopted in Delaware, with cereal rye being a popular option for soybean production. The benefits of a rye mulch is weed suppression and soil moisture conservation, but may also cause increased pest presence and disrupt the release of nitrogen (N) to cash crops. While soybeans may not be as affected by the N cycle as corn, the mineralization and release of N in rye may also provide supplemental N to the plant mid-season. These fields may also include corn fodder from the previous cropping year, which will continue to breakdown through the soybean growing season, providing some carbon the soil surface. What is not currently known is now soybean populations and row spacing may affect the decomposition of residues on the soil surface. Earlier canopy may preserve soil moisture, allowing for increased residue decomposition, or may increase evapotranspiration reducing overall soil moisture. This study will take the first steps in measuring decomposition of residues under soybean planting densities.

For this study, soybeans were planted into a terminated rye cover crop at the Carvel Research and Education Center in Georgetown, DE on 05/25/23 into plots 10’ wide by 60’ long. Soybeans were planted at five densities (80, 100, 120, 140, 160 thousand seeds per acre) and two row spacing's (15 and 30 inch) under irrigated conditions. Soil temperature and moisture sensors were installed in 120 and 180 seeding rate plots at both 15 and 30” in June 2023. Rye and corn fodder were collected and placed into mesh bags, measuring the beginning weight, final weight, and final C and N % of the residues.

The results of this study revleaed similar yield responses to population and row spacing as 2021 and 2022, where no differences were observed with populations (60-180k) for full season beans, but 15” rows provide at least a 7-bushel increase. Although we hypothesized that greater canopy coverage would lead to ideal conditions for residue decomposition, we found the opposite results. Lower populations increased rye breakdown while wider rows increased corn breakdown. Based on soil temperatures, the major differences in planting management appears to occur in July, at least when soybeans are planted in late May. So weather during this period may also explain differences between plots, which were all irrigated.

While no differences were observed in rye or corn fodder %N or %C by planting management, there were relationships with yield and other residue characteristics. Breakdown of residues increases in plots with higher yields, so there is a underlying mechanism that supports both yield and residue decomposition. This includes rye C:N ratios, which were lower in higher yielding plots, although the relationship was weaker. Additionally, there was a moderately strong positive relationship between corn and rye residue decomposition, indicating that there was some similarity if processed that induced breakdown. This project should be expanded to rainfed conditions, and residue may need to be pre-ground to produce consistent (but not field approximate) results.