NITROGEN-FERTILIZATION OF A NATIVE GRASS PLANTING IN WESTERN OKLAHOMA

Native warm-season grass mixtures have been established on the Souther n Plains under the USDA Conservation Reserve Program. We studied respo nses to N fertilizer on such pastures in western Oklahoma over a 1-yea r period, Experimental pastures were previously cultivated fields with loamy soils seeded to a mixture of native warm-season grasses. Fertil izer treatments were 0 and 35 kg N ha(-1) year(-1) as ammonium nitrate . Pastures were intensively grazed from early June to early August ove r 4 years. Stocking rates averaged 52 and 104 AUD ha(-1) for the 0 and 35 kg N ha(-1) treatments, respectively. These stocking rates are hea vy for seasonal grazing in this region. Responses measured included fo rage mass and nutritive value before and after grazing, plant basal ar ea, and livestock performance. Precipitation was variable but generall y favorable over the study period. Peak forage mass was increased by N fertilization (2,480 versus 4,030 kg ha(-1) P < 0.01), producing 45 k g forage per kg N applied. Nitrogen fertilization increased crude prot ein concentration in June (8.2 versus 10.3 %; P < 0.05) and August (4. 1 versus 4.6%; P < 0,05), but had inconsistent effects on in vitro dry matter digestibility. Total vegetative cover and basal cover of blue grams (Bouteloua gracilis (H.B.K) Lag. ex Griffiths) increased in the fertilized pastures, Average daily steer gain was not different betwee n treatments (0.96 versus 1.02 kg hd(-1) day(-1)) even though stocking rates were substantially higher on fertilized pastures. Steer gain ha (-1) was increased by fertilization (83 versus 176 kg ha(-1): P < 0.01 ). This resulted in a fertilizer N use efficiency of 2.7 kg steer gain per kg N applied. Nitrogen fertilization combined with intensive summ er grazing provided a net return of $0.65 to $0.94 per kg N applied.