There is a need to develop more efficient soil fertility and nutrient management strategies in order to maximize wheat grain yields and increase grain protein levels. The feasibility of various sensor-based systems must be evaluated before a recommendation can be made what system is more efficient and appropriate for Montana conditions. The major objectives of this study are: 1. to evaluate two sensors - GreenSeeker, and Pocket Sensor - for developing NDVI-based topdress fertilizer N recommendations for dryland and irrigated spring wheat production in Montana, and 2. to determine whether sensor-based recommendations have to be adjusted depending on what N fertilizer source (liquid urea ammonium nitrate (UAN), or granular urea) is used. Three experiments (two dryland studies at Western Triangle Agricultural Research Center, Conrad, MT and in a producer’s field at near Valier, Pondera County, MT, and one irrigated study at Western Agricultural Research Center, Corvallis, MT) will be established using spring wheat Choteau variety. There will be a total of 9 treatments, each replicated 4 times. The preplant N rates of 0, 20, 40, 60, and 80 lb N ac^-1 will be applied as broadcasted urea. There will be two reference treatments at each site – the unfertilized check plot (0 lb N ac^-1), and the non-limiting N-rich reference (220 lb N ac^-1). The NDVI readings from each treatment will be collected at Feekes 5 growth stage. Topdress N rates (applied as urea, broadcasted or as UAN, foliar sprayed) will be prescribed using NDVI values and several algorithms experimentally developed at Oklahoma State University for spring wheat. Also, a new generalized algorithm will be tested. Grain yield, protein content, and NUE data will be analyzed to determine whether there were statistically significant differences depending what sensor was used to make fertilizer N recommendations.