Quick summary: When available soil N is lower, rye cover crops may occasionally reduce yields while clover cover crops may occasionally improve yields. At adequate fertilizer levels, yields are not affected by cover crops on sandy, Delaware soils.

Figure 1: Corn nitrogen rate trials following cover crops in Georgetown, DE in the summer of 2023.

As part of the Precision Sustainable Agriculture network (https://www.precisionsustainableag.org/), a study was deployed across multiple states to examine the nitrogen (N) cycling that occurs with cover crops. Plots of rye, clover, and a rye-clover mix were seeded each fall over three years (2020-2023). In the spring, plots were terminated two weeks prior to corn planting and then sidedressed to reach total N rates between zero to 320 lbs N/acre. The visual response of the variable N-rates can be observed in Figure 1.

During the first year of the project, the multi-state PSA network observed that when no fertilizer was applied (0 lbs N/acre), the corn yielded less following mix or rye cover crops, but yield was similar across all cover crops with high N rates. At our study site in Georgetown, DE, results varied each year (Figure 2), as our soils are sandy with 1% organic matter and are often lacking residual N. Under irrigation, individual plot yields could be as high as 250 bushels, but the highest N-rates would only average around 200 bushels across all plots (Figure 2).

On our sandy soils in 2021, we had no differences among cover crop types, but were also missing our 0 N-rate treatment (Figure 2a). Although not statistically significant, the no-cover control and rye plots did trend lower in yield when fertilizer rates were less than 250 lbs N. Applying 320 lbs of N produced the highest yields in control and rye plots, while clover and mixed plots only need 240 lbs of N. This is based on treatment values, and not agronomic or economic maximum N derived from calculated plateaus.

Following expected annual variability in weather and fields, 2022 had different trends (Figure 2b). We did observe differences by cover crop types, with clover and mixes producing greater yields between 0 to 80 lbs of N/acre. Above that rate, cover crop type didn’t matter. Only the control plots needed the maximum rate of 340 lbs, while all cover crop plots were similar at 240 lbs N/acre.

Figure 2: Corn yields based on N-rates and cover crop type in a) 2021, b) 2022, and c) 2023. Control = no cover crop.

In 2023 (Figure 2c) we observed no differences by cover crop type, although the control plots again trended lower in yields (grey points). For all plots, a rate of 240 lbs of N would be sufficient to reach maximum yields.

As is typical in crop production, response to management varies each season. While rye may contribute to a tying up N, it didn’t occur every year and was more of an issue with very low N-rates. Similarly, clover can contribute to N, but this may be most beneficial when N is lacking in the soil or leaching has removed starter or sidedressed N. Nationwide recommendations from the PSA network should be forthcoming for both corn and cotton.

This research was funded by a USDA-AFRI Sustainable Agricultural Systems grant.

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