Enhancing Nitrogen Fertilizer Efficiency and Seed Yield in Perennial Ryegrass Crops with Urease Inhibitors.

Nitrogen (N) increases seed yield in grass seed crops by increasing seed number and weight. Nitrogen use efficiency is reduced by ammonia volatilization losses; thus, seed growers may not receive maximum benefit from all of the N they apply.  A urease inhibitor [N-(n-butyl) thiophosphoric triamide] (NBPT) has shown considerable efficacy in reducing N losses due to volatilization and increasing yield in crops such as corn, but little is known its use in grass seed crops. Trials were conducted in first-year perennial ryegrass seed fields at three on-farm sites in 2013-14 in western Oregon. Ammonia volatilization losses of 5 to 25 percent have been measured in this high rainfall region. Fertilizer treatments included two N rates applied as 40-0-0-6, representing the range of recommended rates for perennial ryegrass seed crops in Oregon (120 and 160 lbs N/acre), with and without NBPT. A split application of 160 lbs N/a, with 50 percent applied as 40-0-0-6 with NBPT and 50 percent applied as liquid UAN was also included. No rainfall was measured for three days following N fertilizer application. Above-ground biomass, total tissue C and N, seed yield, and seed weight were measured. Data were analyzed using analysis of variance and means were separated by using Fisher’s Protected LSD values (P = 0.05). A 15 percent seed yield increase resulted at two sites when NBPT was used with 120 lbs N/a and seed yield increased by 10 percent at one site when NBPT was used with 160 lbs N/a.   There were no differences in seed weight, above-ground biomass, or total tissue C and N among N rate or urease inhibitor treatments. These results indicate that NBPT urease inhibitor products have potential for improving N fertilizer use efficiency and yield in grass seed crops when rainfall is absent immediately following fertilizer application.