Determination of Optimal Fall and Spring Nitrogen Rates for Maximizing Soft Red Winter Wheat Yield Sown at Variable Planting Dates.

Nitrogen (N) is a macronutrient and major constituent of several of the most important plant metabolites. It is also one of the most frequently deficient nutrients in plants despite being essential for growth and development. For winter wheat, nitrogen is one of the most expensive inputs and thus, management strategies must be efficient in supplying sufficient amounts of N at the proper growth stages to maximize yield and profitability. For example, supplying N shortly before the period of exponential growth is known to increase nitrogen use efficiency (NUE) since N demand is high. Alternatively, applying N prior to sowing can improve productivity in low-N-containing soils. Overall, higher NUE can be achieved by utilizing a timing and rate of N application that balances yield and profitability while minimizing N loss and over-application. Application of fall N has been hypothesized to increase yield, particularly when wheat is planted later than optimum. However, current data supporting this hypothesis is lacking which makes recommendations for producers difficult. Therefore, the objectives of this study are; 1) Evaluate wheat yield response to variable fall and spring N rates over three planting dates, 2) Determine interaction of fall and spring N rates with planting date and, 3) Determine the interaction of N rate by planting date by cultivar using both an early maturing and a full season variety. In fall 2014, field trials were sown in a split-plot design with four replications in three locations (Newport, Pinetree, and Rohwer Arkansas) at early, optimal and late planting date. A total of 10 treatments of fall and spring N rate combinations were applied, with total urea (46-0-0 + Agrotain) ranging from 60 to 180 lbs acre-1. The first application (0, 30, 60 lbs acre-1) was applied just after seedling emergence during the fall with the second application during the winter (60, 90, 120 and 150 lbs acre-1) applied in a two-way split. Replicated trials were conducted for the varieties Armor Havoc (full season) and Armor Vandal (early maturing) to determine cultivar interaction. In addition to yield, biomass and yield components were estimated by harvesting 50 spikes at maturity which allowed for determining the influence of nitrogen rate and timing on these traits. Normalized difference vegetative index (NDVI) readings with the GreenSeeker™ handheld sensor were also taken to verify the relationship between canopy reflectance and yield.