The utilization of nitrapyrin in nitrogen (N) management may be an important tool for increasing soft red winter wheat yields in Missouri. The objectives of this study were to: (1) Determine the duration of ammonium retention in the soil profile during fall and winter growth periods from anhydrous ammonia fertilizer treated with nitrapyrin(2-chloro-6-(trichloromethyl) pyridine); (2) Evaluate fall and split applications of anhydrous ammonia with or without nitrapyrin for soft red winter wheat in Missouri; and (3) Relate crop yield to NH$\sb4\sp{+}$/NO$\sb3\sp{-}$ soil ratios. Anhydrous ammonia was applied with and without nitrapyrin on a Mexico silt loam (fine montmorillonitic, udolic ochraqualf). Three days after application of anhydrous ammonia, soil samples were collected from the injection zone (IZ) and the surrounding profile up to 8 cm from the IZ. Higher levels of ammonium (NH$\sb4\sp{+}$) were retained in the soil profile in plots treated with nitrapyrin during both 1991-92 and 1992-93 sampling periods. Overall nitrate (NO$\sb3\sp{-}$) levels in the soil profile increased during the first three weeks of the study, but then decreased following wheat establishment and remained relatively low during both 1991-92 and 1992-93 sampling periods. There was a negative relationship between declining NH$\sb4\sp{+}$ levels and cumulative heat units in treatments with and without nitrapyrin during both growing periods. Nitrapyrin improved NH$\sb4\sp{+}$ retention in the soil profile during fall and winter sampling periods. There was no yield response to improved NH$\sb4\sp{+}$ retention by nitrapyrin across sites during the three year study. Yield components and harvest index (HI) were not influenced by split N treatments with or without nitrapyrin. Nitrapyrin did not increase nitrogen use efficiency (NUE), however, NUE increased with increasing spring N application. Lack of yield response to nitrapyrin clearly indicates that nitrapyrin is not an important tool to be incorporated into the N management for soft red winter wheat production in Missouri.

Keywords: protein variability, delayed harvest, grain quality, winter wheat, remote sensing.