Effects of ewe age and season of lambing on prolificacy in US Targhee, Suffolk, and Polypay sheep.

Factors affecting ewe prolificacy (number of lambs born per ewe lambing) were evaluated using records on 9705 Targhee, 12,721 Suffolk, and 7231 Polypay lambings from flocks participating in the US National Sheep Improvement Program. Factors considered were ewe age and season of lambing. Prolificacy was affected by age of the ewe (P<0.001) and was highest for ewes lambing between 4 and either 7 (Polypay) or 8 (Targhee and Suffolk) years of age. Effects of lambing at younger ages were similar for Targhee and Polypay. Relative to mature ewes, prolificacy was reduced by 0.6-0.7 lambs in 1-year-old ewes, about 0.3 lambs in 2-year-old ewes and about 0.1 lambs in 3-year-old ewes. Effects of ewe age were smaller in Suffolk, with reductions relative to mature ewes of 0.47, 0.13, and 0.01 lambs in 1-, 2-, and 3-year-old ewes. Prolificacy of older (>8-year-old) ewes was consistently reduced by 0.17-0.20 lambs relative to adult ewes. Within the 1-year-old age class, age in months further affected prolificacy of Suffolk and Polypay but not Targhee ewes. For these two breeds, prolificacy of 15-month-old ewes averaged 0.21 lambs more than that observed for 12-month-old ewes. Seasonal differences in prolificacy were large for Targhee and Polypay (P<0.001) but not for Suffolk (P=0.23) and the distribution of lambings among seasons differed (P<0.001) for all breeds. 22% of adult Polypay ewes, but only 1% of Targhee and Suffolk ewes lambed in summer and autumn. Prolificacy was highest in winter and spring lambings (December through May) and did not differ importantly among months within this period. Average prolificacy of 3- to 6-year-old ewes lambing in winter and spring was 1.75 for Targhee, 1.91 for Suffolk, and 2.13 for Polypay. For age groups with at least 10 observations, prolificacy in autumn (September through November) was reduced by an average of 0.14 lambs in Suffolk and 0.24 lambs in Polypay (very few Targhee lambed in autumn). Prolificacy in summer (June through August) was reduced by an average of 0.18 lambs in Targhee and 0.31 lambs in Polypay (very few Suffolks lambed in summer). Adjustment of prolificacy records for effects of ewe age is therefore necessary in genetic evaluation programs. Seasonal effects on prolificacy are probably best handled by placing ewes that lamb in different seasons in different contemporary groups.