|Berg, William - RETIRED USDA, ARS|
Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 23, 1997
Publication Date: N/A
Interpretive Summary: The USDA Conservation Reserve Program (CRP) established permanent vegetation on over 36 million acres of crop land in the U.S. In the Great Plains a large portion of the CRP fields were seeded to native grasses. These same grasses are found on adjacent rangeland that has never been farmed. The application of nitrogen (N) fertilizer is seldom used on rangelands because the increase in forage and livestock performance is not large enough to cover the cost. The use of N fertilizer may be more feasible on CRP lands because of the removal of soil nutrients while the land was used for crop production. We studied forage and livestock responses to N fertilizer on such pastures in western Oklahoma over a 4-year period. Yearly precipitation was variable but generally favorable over the study period. Forage production increased about 60% with N fertilization. Each pound of N applied produced an additional 45 pounds of forage. Fertilization also increased the amount of protein in the forage but did not improve the digestibility of the forage. The gain per head of yearling cattle was not changed by N fertilization. Total beef gain per acre doubled with N fertilization because the fertilized pastures supported twice as many animals. Each pound of N produced an additional 2.7 pounds of beef gain. Nitrogen fertilization was economically feasible under current economic conditions. We found native grasses on retired crop land respond differently to N fertilizer than the same grasses on rangeland and N fertilization will be a useful management tool when CRP contracts expire.
Technical Abstract: Native warm-season grass mixtures have been established on the Southern Plains under the USDA Conservation Reserve Program. We studied responses to nitrogen (N) fertilizer on previously cultivated loamy soils seeded to a mixture of native warm-season grasses. Treatments were 0 and 35 kg N/ha/year as ammonium nitrate. Pastures were intensively grazed from early June to early August over 4 years. Stocking rates were high and averaged 56 and 111 AUD/ha for the 0 and 35 kg N/ha treatments. Responses included forage standing crop and quality, plant basal area, and livestock performance. Precipitation was variable but generally favorable during the study. Peak standing forage was increased by N fertilization (2480 versus 4030 kg/ha, P<0.01), producing 45 kg forage/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). Forage digestibility was little affected by fertilization. Total vegetative cover and basal cover of blue grama (Bouteloua gracilis (H.B.K.) Lag. ex Griffiths) increased with N fertilization. Average daily steer gain was not different between treatments (0.96 versus 1.02 kg/hd/d. Steer gain/ha was increased by fertilization (83 versus 176 kg/ha, P<0.01). Fertilizer N use efficiency was 2.7 kg steer gain/kg N applied. Nitrogen fertilization with intensive summer grazing was economically feasible under current conditions.