Optimum nitrogen fertilization of cool-season grasses in the northeast USA

Many studies have looked at the economically optimum N application rate (cost of N vs. increased yield) for cool-season forage grasses, but few of these studies have also addressed the effect of N fertilization on livestock or environmental health. The objective of this study was to determine the N application rate to cool-season grass species that optimizes economic return without elevating herbage or soil NO₃-N levels. Four N rates were applied in split applications to established stands of orchardgrass (Dactylis glomerata L.), smooth bromegrass (Bromus inermis Leyss.), tall fescue (Festuca arundinacea Schreb.), and timothy (Phleum pratense L.) near State College, PA, on a Murrill silt loam (fine-loamy, mixed, mesic Typic Hapludults) soil. Harvest treatments of three and four times per year were imposed on each Grass X N treatment. Economically optimum N rates (EONR) were 284, 368, and 299 kg N ha^-1 or 26, 32, and 29 kg N Mg^-1 of forage harvested for orchardgrass, tall fescue, and timothy, respectively. Smooth bromegrass did not reach an economic plateau at the highest (402 kg ha^-1) N rate. At EONR, NO₃-N concentration exceeded 1000 mg kg^-1 in forage from early season harvests when four harvests per year were taken but not when three harvests per year were taken. Soil NO₃-N concentration generally was not elevated above background levels when EONR was applied to orchardgrass or tall fescue. Our results, in conjunction with results from New York, indicate that the EONR for the cool-season grasses and harvest systems evaluated is 5 to 10 kg N Mg^-1 of dry forage greater than current recommendations in New York and Pennsylvania. However, continued research is needed in a wider range of climates and soil types using various N sources.