A site-specific N management approach has the potential to manage in-field variability and increase production and economic efficiencies by optimizing the nitrogen (N) inputs. Field studies were conducted to investigate the grain yield and protein responses of hard red winter wheat (Triticum aestivum L.) to several N management strategies across variable landscapes. Nine N treatments consisted of various combinations of N rates, sources and timings were applied at specific stages of crop development. Delta yield, delta protein and net returns were calculated to determine the spatial response to N across the field. Those parameters for each treatment varied spatially across the field. Normalized difference vegetation index and leaf area index could not explain the spatial response to N accurately. Overall, grain yield and protein responses to N strategies were highly dependent on the spatial position in each field; however, predicting the responses in time for deploying N management strategies were only weakly associated with canopy sensor data or soil characteristics.

Response from nitrogen and chloride fertilization was measured in field experiments on winter wheat (Triticum aestivum L. Em. Thell. var. 'Stephens' and 'Yamhill') grown in western Oregon in an environment with a range of susceptibility to take-all root rot (Gaumannomyces graminis var. tritici Walker). Cropping sequences and expected disease severity considered in the study were: first year wheat after clover (low risk of severe take-all root rot: Nixon I experiment), second year wheat with high disease infection in the previous crop (high risk of severe take-all root rot: Keyt II experiment), second year wheat with low disease infection in the previous crop (moderate risk of severe take-all root rot: Nixon II and Coon experiment), third year wheat (high risk of severe take-all root rot: Jones experiment), fifth year wheat (high risk of severe take-all root rot: Keyt II experiment), and eighteenth year wheat (take-all decline established: the pathogen is present in the soil but does not cause damage, Evers experiment). Nitrogen treatments were applied at 0, 67, 134, and 202 kg/ha in all experiments where wheat followed wheat and at 0, 45, 90, and 134 kg/ha in the experiment where wheat followed clover. Chloride treatments were applied at 0, 45, and 90 kg/ha in all experiments and a rate of 134 kg C1/ha was also used on the Jones experiment. Nitrogen was predominantly supplied from urea while ammonium chloride supplied chloride and ammonium sulfate supplied the crop requirement for sulfur (about 20 kg/ha). Fertilizers were top-dressed in split application with chloride and sulfur containing fertilizers applied first (February) and urea applied later (March) in all experiments but those conducted in the Nixon farm where a single fertilizer was applied in March. Crop response was measured through the effects of N and c1 treatments on dry matter production, plant nitrogen content, plant nitrogen uptake and plant percent nitrogen recovery, as well as grain yield, yield components, grain nitrogen content, grain protein content, grain nitrogen uptake, and grain percent nitrogen recovery. The results of the study strongly indicated that take-all root rot was only a problem in the Jones, Keyt I, and Keyt II experiments and was most severe in third year wheat (Jones experiment). This also was the only experiment with significant (p = 0.05) response from rates of 202 kg N/ha. Nitrogen fertilization was the main factor that greatly influenced the levels of the variables studied while chloride fertilization generally did not have a significant (p = 0.05) influence. 134 kg N/ha was generally the rate accounting for the best levels of each variable studied in all experiments except in the Jones experiment as precised earlier. Crop response was also affected by a relatively long 'dry' period (April 20th to June 20th), particularly in the experiment where take-all root rot was a problem. Levels of the variables studied accounted for by the optimum rates of N were consistently higher in the experiments where take-all root rot was not a problem than where it was a problem by the following amounts: dry matter yields-17%, plant nitrogen contents-18%, plant nitrogen uptake levels-30%, plant nitrogen recoveries-28%, grain yields-22%, grain nitrogen contents-only 4%, grain protein contents-only 2%, grain nitrogen uptake levels-26%, grain nitrogen recoveries-18%, and spikes/m2-24%.