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.
As part of research supported by the Maryland Grain Producers (https://marylandgrain.org/), 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.
Based on both N-rates and population, all irrigated plots averaged 30 bushels greater yield (Figure 2). This may not have had the same result across the region, as rainfall in Harbeson was only 15.8 inches for the growing season. Timing of rainfall is also important, where rainfed plots were not prominent until the dry period in August.
For N under rainfed conditions (Figure 2a), yields were highest when 250 lbs of N was applied (230 bushel yield), which was similar to 75 lbs of N under irrigated conditions (237 bushel yield). Under irrigation, yields were highest at 200 lbs of N acre-1 (247 bushels), and did not significantly increase with higher rates.
For population rates in rainfed fields (Figure 2b), planting at 28,000 seeds acre-1 produced the highest yield, dropping 15 bushels at 24,000 and 30 bushels at 20,000 seeds acre-1. For irrigated fields, the highest yields were observed at 36,000 seeds acre-1.
Tissue Nutrient Concentrations and Irrigation
Based on selected macronutrients, irrigation only effected the concentrations of K and Mg (Figure 3). Based on increasing N rates, K was higher under irrigated conditions at all fertilizer rates (Figure 3a). As K is known to need adequate soil moisture to move to plant roots, this is expected. The lower yielding rainfed plots also have the lowest K concentrations (Figure 3c). This could indicate that both lack of rainfall and access to K decreased yields.
For Mg, lower N rates (Figure 3b) and lower yielding plots overall (Figure 3d) had higher concentrations, some exceeding 0.20%. Over the years, most of our tissue concentrations in our research plots have ranged around 0.15 to 0.17%, so this result is interesting. It could possibly indicate stress in corn, but may also be related to reduced K uptake. Both K and Mg can have opposing effects on uptake, as higher K rates in pastures are known to suppress Mg uptake by the plant. In 2022, it was Mn and B that both were higher under rainfed, stressed conditions, so these types of results may vary by soil type and nutrient concentrations.
