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Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 9/18/2010 Publication Date: 1/24/2011 Citation: Waterman, R.C., Vermeire, L.T. 2011. Grazing Deferment Effects On Forage Diet Quality And Ewe Performance Following Summer Rangeland Fire. Rangeland Ecology and Management. 64:18-27. Interpretive Summary: Historically, rangeland fires in the Northern Great Plains occurred every 0-35 years with some magnification of fire intensity occurring due to fire suppression that has interrupted these historic fire regimes. Still, the majority of wildfires in the western U.S. occur during July and August. Research on summer fire effects is limited, but early studies in the Northern Great Plains indicated reduced standing crop for three or more year and at least a short-term reduction in soil moisture. Research has suggested that plant recovery was delayed by herbivore grazing. Consequently, land management agencies traditionally have favored multiple year deferments (minimum 2 yr) prior to allowing domestic livestock to reenter rangelands for grazing. Recently, a more methodical approach using site monitoring has been implemented by land management agencies aiding in the decision of when grazing can recommence. Research documenting the association between livestock production and diet quality of rangeland vegetation the year following summer fire is practically absent. Deferment of grazing until the later part of the growing season is believed to reduce plant stress and may favor post-fire recovery. However, a tradeoff in animal performance may be expected as weight gain has been shown to decrease on non-burned rangeland as the summer advances. Objectives of this research were to: 1) characterize forage nutritional quality for pastures deferred until spring, early summer or late summer (deferments ending in May, June and August) and 2) evaluate BW change in ewes grazing these pastures in spring, early summer and late summer. In the present study, forage characteristics followed similar trends to observations with unburned rangelands in the Northern Great Plains. Compared to other studies on unburned rangelands in the Northern Great Plains, our data might suggest that forage nutritional value was improved from the active growing season through periods when forages typically senesce and decline in nutritional value. However, ewe performance did not directly correspond with the observed forage characteristics, especially for the late summer grazing period. Although, extrusa samples collected from ruminally-cannulated ewes would suggest ewe performance should have been adequate to support BW gain, ewes maintained or lost body weight during late summer and consistently gained weight in spring and early summer. Thus, forage quantity not quality in the late summer grazing period did not allow ewes to gain body weight. Others have observed similar temporal patterns in livestock performance on non-burned rangelands in the Northern Great Plains. Decisions about grazing management following summer fire should reflect a number of factors including forage availability, soil stability, and potential reductions in plant stress with deferred grazing. The reduced gains we observed indicate effects on animal performance should be accounted for in deferment decisions as well. These results along with ongoing research at Fort Keogh, LARRL on plant community responses will aid in post-fire grazing management decisions. Technical Abstract: Complete rest or grazing deferment is a general recommendation following fire in the western U.S. to encourage vegetative recovery. However, effects of grazing deferments on animal performance have not been determined. Forage quality and ewe performance were evaluated for grazing trials with deferments until spring (May), early summer (June), or late summer (August) in two consecutive yr following summer fire in the Northern Great Plains. Forage quality was assessed each yr by complete rumen evacuations, subsequent grazing and collection of rumen extrusa on d 14 ± 0.87, 31 ± 1.38, 51 ± 1.34, and 68 ± 0.21 of each grazing period. Within each grazing period, three 1.5 ha pastures were each grazed by 12 ewes (yr 1) or 10 ewes (yr 2) including two ruminally-cannulated ewes. Ewe BW was measured on d 0 and 70 (yr 1) or 41 (yr 2) to evaluate BW changes at the same pasture utilization in both years. Extrusa CP resulted in a yr × grazing period × sampling period interaction (P < 0.01). Crude protein declined as grazing period and sampling period (d within grazing period) advanced in yr 1 whereas, CP concentrations were more intermittent and even greater as season and sampling period advanced in yr 2. An 8.2% reduction (P < 0.01) in extrusa NDF was measured in 2008 when compared to 2007. Furthermore, % NDF in extrusa samples peaked (P < 0.01) for early summer compared to the late summer grazing period. In vitro OM disappearance followed similar trends as measured for extrusa NDF. Average daily gain resulted in a yr × grazing period interaction (P = 0.03); with similar gains observed each yr for the spring and early summer grazing period (higher gains in 2008) and negligible gains in the late summer grazing period. Total gain (kg/ 1.5 ha) was 10.9 kg greater (P = 0.01) in 2008 and a quadratic (P < 0.01) response was measured for grazing period. Measurements of forage quality indicate improved or similar responses to those typical observed in the region. Late summer forage characteristics would suggest that ewes should have performed better than what was measured. The lack of response by ewes grazing in the late summer period may be contributed to insufficient quantity not quality of rangeland forage in the Northern Great Plains following summer fire the previous yr. |
