Delaware Agronomy Blog

University of Delaware Cooperative Extension

Category: Cover Crops (page 1 of 2)

Cover Crop Termination and Growing Degree Days

Cover crops respond to planting conditions and temperatures like other crops, and their growth will certainly follow winter temperatures as they accumulate growing degree days (GDD). Rather than set a solid burn down date based on a month or corn/soybean planting dates, you should scout fields to check on the current stage and biomass present. If your goal is to reduce interference with cash crop planting, then warmer winters and excessive growth may occur earlier in the spring than usual. If your goal is to build biomass for nitrogen, weed suppression, or a moisture retaining mulch layer, your potential burn down date can still vary from year to year. Additionally, our wet spring has caused variable conditions our fields, with saturated conditions killing off some cover crop growth (Figure 1).

Figure 1: A rye cover crop with variable growth. The brown colors are ponding and bare soil where the cover crop did not survive the winter.
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Effects of Cover Crops and Nitrogen Rates on Corn Yields

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 (, 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.

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2022-2023 Cover Crop Growth and Tillage

For all crops, initial establishment is as important as fertility in maximum yield. The same principles adhere to cover crop growth, with earlier establishment and good seed to soil contact necessary for good coverage.

Figure 1: Fields in March 2023 of a) turbo tilled rye cover crop after rainfed corn, b) no-till broadcast rye after irrigated corn, c) flown on after full-season + double crop soybeans and d) turbo-tilled rye after irrigated corn.
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Cover Crop Biomass and Termination Considerations

Jarrod Miller, Amy Shober, & Mark VanGessel

The benefits of cover crops to the following corn crop can include additional nitrogen (N) or weed suppression, but maximizing these benefits requires later termination to build greater biomass. These N and weed control characteristics are especially appealing this season as input costs are relatively high while supplies are relatively low. However, growers should take the time to estimate the additional costs of allowing a more robust cover crop to accumulate this spring, as surface residues reduce proper seed placement as well as limit seed to soil contact. This article will discuss the management of cover crops for both maximizing N benefits as well as weed suppression in the following corn crop.

Aerial picture of cover crop plots

Cover crop plots at the Carvel Research and Education Center.

Maximizing Cover Crop Biomass

Estimating the amount of N that could be available requires knowledge about 1) total cover crop biomass and 2) cover crop C:N ratio, both of which are affected by termination timing. The longer a cover crop is allowed to grow, the greater the amount of cover crop biomass will accumulate. Continue reading

Estimation of Cover Crop Nitrogen with Drones

Jarrod O. Miller, Assistant Professor and Extension Specialist, Agronomy, jarrod@udel.eduAmy L. Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality,,  Jamie Taraila, Graduate Research Assistant; 

As part of a Northeastern SARE graduate student grant, we used a drone to predict the nitrogen (N) that may be present in cover crops prior to burndown. We flew fields in Laurel, Georgetown, and Harbeson with a readily available consumer drone (Phantom4) equipped with a standard (RGB) camera. Each of the fields were flown prior to cover crop burndown (late April to late May) resulting in 25-100 images per field that had to be stitched together into one image (Figure 1a). The camera captured different wavelengths of light (i.e., red, green, blue) that were reflected by plants which were transformed into the Visible Atmospherically Resistant Index (VARI). This allowed us to estimate plant biomass by comparing VARI values to cover crop biomass that was collected in the field. We collected 10 samples per field, which were dried, weighted, and then analyzed for N content by the UD soil testing lab.

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