| The efficient use of nitrogen fertilizer is important to the economic sustainability of soft red winter wheat (Triticum aestivum L.) production and to meet environmental regulations to limit non-point source pollution from agriculture. The first objective of this research was to determine if remote sensing could be used to make early nitrogen application decisions and to predict optimum nitrogen application rates based on in-season nitrogen status in winter wheat. The second objective was to determine if site-specific nitrogen management would increase grain yield, reduce nitrogen inputs, and increase spring nitrogen use efficiency compared to field-specific nitrogen management based on the same logic or typical growers' practices. Research was conducted in 1998, 1999, 2000, and 2001 at five on-farm sites and five research stations representing a range of geographical regions in North Carolina.; A remote sensing technique based on near infrared digital counts and within-field tiller density references was developed and validated to predict when early nitrogen applications were required. This technique accounted for 76% of the variation between predicted and measured tiller density and mad accurate early nitrogen application decisions 85.5% of the time across 10 site-years and a wide range of environmental conditions.; Consistent with previous studies, biomass was found to influence spectral measurements of in-season crop nitrogen status. A strong exponential relationship between the normalized difference vegetation index and whole-plant nitrogen content (R2 = 0.69) and nitrogen uptake (R2 = 0.61) was found. The former could be used to estimate optimum N rate using remote sensing but only at high biomass sites.; Site-specific nitrogen management did not improve grain yield compared to field-specific management. Grain yield benefits up to 2267 kg ha −1 were realized by using either site-specific or field-specific nitrogen management compared to typical growers' practices. Large nitrogen fertilizer reductions, up to 48.6%, were also realized by using site-specific or field-specific nitrogen management compared to typical growers, practices. Spring nitrogen use efficiency was maximized at all sites by site-specific nitrogen management. |
