Selection for lambing rate and clean fleece weight in sheep.

This study was designed to compare the effectiveness of selection for lambing rate (LB) and clean fleece weight (CFW) separately or by a combination of both traits at once. Two flocks of approximately 200 western white faced ewes per flock were maintained at two locations. Each flock was divided into four equal selection groups of similar initial birth type and age. The basis of selection in each flock was as follows: Line I, single trait selection for LB; Line II, single trait selection for CFW; Line III, multiple trait selection for LB and CFW; and Line IV; visual selection based on conformation and general fleece quality. Repeatability and heritability estimates were .18 and .16 +/- .04, respectively, for LB and .66 and .60 +/- .09, respectively, for CFW. The genetic correlation between LB and CFW was .26 +/- .16. Linear regression coefficients (number born/year) of annual response for LB in Lines I to IV were .018 +/- .008, .009 +/- .006, .011 +/- .005, and .009 +/- .005 (P > .05). Linear regression coefficients (kg/year) of annual observed response for CFW in lines I to IV were -.001 +/- .006, .027 +/- .009, .019 +/- .006, and .008 +/- .006. Response in Lines II and III was significant for CFW (P < .05) compared with Lines I and IV. In this study, CFW was increased by selection in white-faced sheep with little effect on lambing rate.