Enhanced Efficiency Nitrogen Fertilizer Formulation Effect on Pasture Productivity: Forage Yield, Quality, and Clover Persistence.

The use of nitrogen (N) fertilizer is generally restricted on mixed species forage systems due to its stimulatory effect on grasses that increases competition with legume species. Reduced legume growth from this competition can compromise the forage nutritive value and prospective yields. The controlled-release nature of several enhanced efficiency (EE) fertilizer N products holds the potential to improve legume persistence in mixed species pastures while providing the supplemental N required by the grass component. This study evaluated the effect of different EE N formulations (ESN, methylene urea, SuperU, and a 75% ESN: 25% urea blend) and untreated urea on yield, nutritive value, and legume persistence in a ‘Wrangler’ bermudagrass (Cynodon dactylon) and ‘Durana’ white clover (Trifolium repens) mixture. Nitrogen was applied at four rates (0, 112, 224, and 448 kg N ha^-1) in two equal applications. The study was conducted at the University of Kentucky Spindletop Research Farm in Lexington, KY using a randomized complete block design. During the establishment year, the encroachment of volunteer white clover plants resulted in only small declines in the clover population but became larger in the higher N rate treatments in the second year. All EE N sources maintained white clover populations similar to the unfertilized grass/clover control, while urea significantly reduced white clover populations. Total forage yields for urea, methylene urea, SuperU, and the ESN blend treatments increased linearly with increasing N rates, while ESN use gave no increase except at the highest N rate. ESN increased IVDMD to 747 g kg^-1, as compared to other N sources (731 g kg^-1). Crude protein did increase throughout the growing season of the establishment year (from 198 to 232 g kg^-1). Continued research will allow evaluation of EE N effect on white clover persistence over multiple growing seasons.