Delaware Agronomy Blog

University of Delaware Cooperative Extension

Category: Irrigation

2023 Interactions Between Nitrogen, Planting Population, and Irrigation for Corn

Jarrod Miller, James Adkins

Quick summary: Irrigation boosted yields in southern Delaware by 30 bushels. Under irrigation, N-rates of 200 lbs averaged 247 bushels of corn, while maximum yield occurred at populations of 36,000 seeds acre-1. Rainfed conditions suppressed K uptake in the plant, while the opposite effect was observed with Mg.

Figure1: Research plots at the UD Warrington Irrigation Research Farm. Interactions include planting rates, nitrogen rates, and rainfed versus irrigated plots.

As part of research supported by the Maryland Grain Producers (, we planted corn under irrigated and rainfed conditions under a range of populations (20-40,000 seeds acre-1) and nitrogen (N) rates (75-300 lbs acre-1). This research was performed at the University of Delaware Warrington Irrigation research farm (Harbeson, DE)  in the summer of 2023 (Figure 1), where variable rate irrigation was used to create rainfed conditions across the field.

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Grid Sampling Soils to Improve Understanding of Soil Variability

Jarrod Miller and James Adkins, University of Delaware

Variability in soil land landscape characteristics reduces yield response to management techniques, particularly regarding seeding rates and fertilizer additions. Yield maps provide a spatial map of yield, which can be associated with drainage issues, soil nutrient holding, or nutrient concentrations. One method to uncover soil variability and crop response is to use precision soil sampling, including either grid or zone methods. Both increase the cost of taking soil samples, and each have their value depending on the desired outcomes.

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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.

Considering Fall Salt Remediation

Salts are a natural component of soils, including our soluble plant nutrients (Ca, NO3, ect). Excessive levels of any salt can be detrimental to both plant health and soil quality. On the Delmarva Peninsula, excessive salts can come through several sources, which include fertilizers, irrigation water, and salt water intrusion.

Fertilizer burn due to sidedress N applications.

Issues with fertilizers are related to seed germination and growth, where in-furrow recommendations of starter N+K fertilizers are limited to 10 lb/acre total due to salt effects. During sidedress applications, fertilizer burn (Figure 1) can damage leaf tissue, particularly UAN greater than 50lb/acre. As long as corn plants are younger, minimal tissue damage doesn’t affect yield. Considering the above recommendations, salt damage due to fertilizers should be easy to manage. Continue reading

2017-2018 Irrigated Soybean Seeding Rates

Cory Whaley, Phillip Sylvester, James Adkins, Jarrod Miller

Over the last ten years, several Universities have reported that planting soybeans at lower populations doesn’t necessarily reduce yield, but can increase margins. Studies at Ohio State between 2014-2016 indicated better returns with reduced seeding rates and an agronomic optimum planting rate of 150,000 seeds/acre. Both Iowa State and Nebraska have observed that a final stand of 100,000 plants (based on planting 120,000 seeds per acre) does not always reduce yield, but does improve margins due to lower seed costs. Regionally, Virginia Tech recommends higher planting rates for poor ground (120-140,000 seeds/acre), while fields with yield of 55-70 bu/acre could go as low as 90,000 seeds/acre.

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