Effects of Disease, Nitrogen Source, and Risk on Optimal Nitrogen Fertilization Timing in Winter Wheat Production

Nitrogen source and timing can interact with glume blotch ( Stagonospora nodorum ) and take‐all root rot ( Gaeumannomyces graminis var. trittici ) to affect risk in wheat ( Triticum aestivum L.) production. The objectives of this research were to evaluate the effects of N source, N timing, and disease severity on expected yield and risk and to evaluate the risk–return trade‐offs between N sources for farmers with different risk preferences. A Just–Pope model was used to estimate separate mean yield and yield variance (risk) effects in evaluating the N timing decision. Wheat yields for 1998 through 2000 were obtained from an experiment on Collins silt loam (coarse‐silty, mixed, active, acid, thermic Aquic Udifluvents). The experimental design was a split plot with five replications. Main plots were fertilized on 15 February, 1 March, 15 March, 1 April, and 15 April. The N sources and fertilization rate were ammonium nitrate (AN) and urea ammonium nitrate (UAN), both applied at 101 kg N ha −1 . Glume blotch occurred in 1998, and take‐all occurred in 2000. Nitrogen timing, glume blotch severity, and take‐all severity significantly increased risk for AN but not for UAN. Nevertheless, at average disease ratings, fertilization with AN on 8 March was the utility‐maximizing N source and date regardless of risk preferences. The finding that AN was the optimal N source is worth $40.74 ha −1 to net‐return–maximizing wheat farmers who fertilize with AN instead of UAN. With take‐all severity at its higher 2000 level, risk increased for AN relative to UAN, but the net‐return advantage of AN was still positive at $26.41 ha −1