Delaware Agronomy Blog

University of Delaware Cooperative Extension

Category: Corn (page 1 of 6)

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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Average Delaware Growing Degree Days (2019-2021)

The standard method used to follow and predict corn growth stages is using growing degree days (GDD). This is a calculation that uses average daily temperatures measure accumulated heat over the growing season. Using GDD works better than days from planting because cool spring temperatures slow early planted corn, while corn planted later in May can have a more linear growth pattern.

For the past three seasons in Georgetown we have followed our research plots and have these values as the average GDD for our area (Table 1). They will be similar to those found in other states, but represent averages and ranges for our region. You may find GDD values on our regional mesonet (DEOS) or through the Climate Smart Ag page at Cornell (edit the site location).

Growing Degree Days (GDD) Average Accumulation to Reach Corn Vegetative and Reproductive Stages.

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

Keep Stress Down During Reproductive Stages


Corn silks have pollinated kernels as they move into the R2 stage

A corn ear that has pollinated and is in the R2 ( blister) stage.

As we enter the grain fill period, with some of our fields already at R2 (blister stage) or R3 (milk stage), it is important to keep stress down. Fields still at R1 or R2 (mid-May planting dates) may have the most to lose as kernels are being pollinated and start to fill. In past field trials, we have observed that yield correlates the best between about R2-R4 (from drone imagery, see below), so while there isn’t much you can do to improve yield, you can certainly do your best to maintain it by managing irrigation wherever it is needed.

Vegetation indexes and their correlation to yield over the corn growing season.

Vegetation indexes and their correlation to yield over the corn growing season.

July Temperatures and Pollination

According to data from the last two years (, silking and pollination (R1) has occurred between 1320 and 1594 growing degree days, while VT (tasseling) occurs between 1200-1300. In many of our research fields, VT also began to occur in this range last week, followed quickly by silking. Stress during pollination can determine initial pollination and kernel set, so temperature becomes very important. Planting date is an important part of this, as corn growth will follow heat accumulation, so cooler springs will take longer to reach R1 and vice versa. Continue reading

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