Submitted to: Livestock Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 22, 2008
Publication Date: March 15, 2009
Citation: Taylor, J.B., Moffet, C.A., Leeds, T.D. 2009. Body weight changes and subsequent lambing rates of western whiteface ewes grazing winter range. Livestock Science. 121:339-342. Interpretive Summary: These results were based on data collected from ewes that grazed shrub-dominated range for 3 consecutive winters. Regardless of breed, 2-yr-old ewes were lightest going on to winter range and had least number of lambs born. Overall, ewes lost weight while grazing winter range, but mature body weight, once achieved, was restored annually, with the exception of the 7-yr-old Targhee and Columbia ewes. Regardless of breed or age, ewes were able to achieve lambing rates >1.5 lambs following early-pregnancy weight loss. Lambing rates were greater in older ewes, which generally lost the greatest amount of body weight during winter grazing, except for 7-yr-old Targhee ewes. Despite evident BW loss demonstrated from this data set, early-pregnant western whiteface ewes, placed on shrub-dominated range for winter grazing, maintained productivity without supplemental feeds. As relevant in the Targhee breed and reported elsewhere (Chittenden et al., 1936), this seems to be somewhat contingent upon the ewe’s ability to replenish their previous BW annually.
Technical Abstract: What is the lambing performance of range ewes that experience early-pregnancy weight loss? To address this question, body weight change (BWC) and subsequent lambing performance data of Columbia, Polypay, Rambouillet, and Targhee ewes that grazed on shrub-dominated range during the winters of 1989, 1990, and 1991 were analyzed. In December (winter), after breeding each year (yr; October to December), ewes were moved to range. Depending upon severity of climatic conditions and vegetation accessibility, ewes grazed winter range for 30 to 60 days (d). Body weights were measured 2 d before grazing commenced and 2 to 7 d after ewes were transported off range. Daily BWC was calculated as a proportion of initial BW where: BWC, g kg initial BW-1 d-1 = ([exit BW – initial BW] / initial BW) / d of grazing. Data were analyzed within each breed, as repeated measures using mixed models with age and year included in the model. For all breeds, 2-yr old ewes were lightest going on to winter range and had the lowest lambing rate. Ewes lost weight while grazing winter range, but mature BW, once achieved, was restored annually with the exception of the 7-yr old Columbia and Targhee ewes. Regardless of breed or age, ewes were able to achieve lambing rates > 1.5 lambs following early-pregnancy weight loss. Lambing rates were greater in older ewes, which generally experienced substantial negative BWC during winter grazing.