NITROGEN-FERTILIZER USE EFFICIENCY IN STEER GAIN ON OLD-WORLD BLUESTEM

Old World bluestem (Bothriochloa ischaemum L.) is the major grass being planted for improved pastures on marginal farmland in western Oklahoma and adjacent areas in Texas, The farmland is often deficient in plant available N as a result of up to 100 years of cultivation and erosion. This study determined N fertilizer use efficiency on steer gain when grazing Old World bluestem in northwestern Oklahoma where average annual precipitation is 575 mm yr(-1). The study was conducted over 4 summer grazing seasons on Pratt soils (sandy, mixed thermic Psammentic Haplusalfs). Nitrogen rates of 0, 34, 68, and 102 kg N ha(-1) y(-1) were applied to paddocks in a randomized complete block design with 4 blocks, Steer gain averaged 220 kg ha(-1)yr(-1) and 3.3 kg per kg N applied at the 34 kg N ha(-1) y(-1) rate. Steer gain ranged from 2.3 to 4.3 kg yr(-1) per kg of N applied at the rate of 34 kg N ha(-1) yr(1). About an additional kg of steer gain per kg N applied was realized for the second 34 kg N ha(-1) yr(-1) increment from 34 to 68 kg N ha(-1) y(-1). Gain was negligible from the third 34 kg N ha(-1) y(-1) increment from 68 to 102 kg N ha(-1) yr(-1). Early summer grazing of N-fertilized Old World bluestem has high stocker production potential.