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Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings Publication Acceptance Date: 4/2/2008 Publication Date: 6/23/2008 Citation: Waterman, R.C., Vermeire, L.T. 2008. Influence of grazing deferments following summer fire on ewe performance and forage quality. Western Section of Animal Science Proceedings 59:110-113. Interpretive Summary: Historically rangeland fires in the northern Great Plains occur every 0-35 years with some magnification of fire intensity occurring due to fire suppression that has interrupted historic fire regimes. 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 aiding the decision of whether grazing can recommence. Minimal research is available documenting the association between livestock production and forage quality of rangeland vegetation following summer fire. Therefore, our objectives were: 1) to evaluate grazing performance of ewes via weight change during one of three 70-d grazing periods and 2) characterize forage nutritional quality from rumen extrusa collected throughout each grazing period using ruminally-cannulated ewes the year following summer fire. Decisions to graze rangelands in the northern Great Plains the year following a summer fire will greatly depend on forage availability and stability of soils. However, decisions based on reseeding (rehabilitation; excluding heavy forest sites) should be less of a priority since perennial cool- and warm-season graminoids comprise of the majority of biomass and 90% of that biomass is produced by July 1. Animals will obtain greater gains in spring and those gains will diminish as summer progresses. Livestock gains in northern Great Plains rangelands commonly decrease with the advance of summer, regardless of fire. The reduced gains we observed indicate effects on animal performance should be accounted for in deferment 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. Ewe performance and forage quality were evaluated for 70-d grazing trials with deferments until spring (May 17), early summer (June 21), or late summer (August 2) in the yr following summer fire in northern Great Plains. Within each deferment, three 1.5 ha plots were each grazed by 12 ewes (including two rumen-cannulated ewes). Ewes were weighed on d 0, 35 and 70 to evaluate BW changes to determine if length of time in a plot influenced performance. Forage quality was assessed by complete rumen evacuations, subsequent grazing and collection of rumen extrusa on days 15, 31, 51, and 68 of each grazing period. There was an interaction between deferment ' weigh d (P < 0.01) indicating ewe BW differed across deferments. However, BW gains for the initial 35 d in each 70-d grazing trial were similar. Body weight gains the last 35 d (P < 0.01 for deferment ' period (1st and 2nd 35 d)) were similar to initial 35 d for spring but remarkably lower for early summer and ewes actually lost BW the last 35 d in late summer deferment. Forage quality characteristics declined with later deferments and as time progressed within each grazing period. Forage CP (% OM) was highest (P < 0.01) during spring grazing (9.7%) and decreased to similar concentrations of 6.7 and 6.3% for early and late summer grazing, respectively. No differences (P > 0.05) were observed for extrusa NDF. However, in vitro NDF disappearance decreased (P < 0.01) from spring to late summer. Results show that ewe BW decreases as post-fire grazing deferment extends through summer and potential tradeoffs between livestock performance and plant recovery should be evaluated. |
