Nitrogen fertilization of a native grass planting in western Oklahoma

Native warm-season grass mixtures have been established on the Southern Plains under the USDA Conservation Reserve Program. We studied responses to N fertilizer on such pastures in western Oklahoma over a 4-year period. Experimental pastures were previously cultivated fields with loamy soils seeded to a mixture of native warm-season grasses. Fertilizer treatments were 0 and 35 kg N ha-1 year-1 as ammonium nitrate. Pastures were intensively grazed from early June to early August over 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 heavy for seasonal grazing in this region. Responses measured included forage mass and nutritive value before and after grazing, plant basal area, and livestock performance. Precipitation was variable but generally 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 kg forage per kg N applied. Nitrogen fertilization increased crude protein 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 grama (Bouteloua gracilis (H.B.K.) Lag. ex Griffiths) increased in the fertilized pastures. Average daily steer gain was not different between 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 summer grazing provided a net return of $0.65 to $0.94 per kg N applied.