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ECONOMIC ANALYSIS

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

^a Dep. of Agric. Econ., The Univ. of Tennessee, 2621 Morgan Circle, Knoxville, TN 37996-4518
^b Dep. of Plant and Soil Sci., West Tennessee Exp. Stn., The Univ. of Tennessee, 605 Airways Blvd., Jackson, TN 38301

^* Corresponding author (rrobert3{at}utk.edu).

Received for publication August 19, 2003. 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.

Abbreviations: AN, ammonium nitrate • GS, Feekes' growth stage(s) • OLS, ordinary least squares • UAN, urea ammonium nitrate • WLS, weighted least squares

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