Efficiency of different selection criteria for increased lamb and wool production

Weaning and yearling data from 604 Rambouillet, Targhee and Columbia ewes of the Montana Agricultural Experiment Station, Bozeman, Montana, were used to estimate genetic parameters of production traits of economic importance. Data were available on all ewe lambs born in 1955 and 1956. Subsequent production records were available for all lambs which were kept in the flock for breeding purposes. The least-squares method of fitting constants were used to estimate the significance of environmental or nongenetic effects and to provide estimates of parameter constants. The Rambouillet data were the only data sufficiently large in volume to yield consistent and feasible results. These data were used with estimates of relative economic value to calculate selection indexes for use at weaning or yearling age. The following indexes at weaning and yearling ages had the highest correlations with aggregate genetic value when economic values were assigned to all traits except neck fold score. I[subscript wn] = 0.59(body weight) -- 1.50(face score) -- 2.04(neck fold score) + 0.40(type score) + 1.68(condition score) + 4.16(staple length) I[subscript yr] = 1.28(body weight) -- 2.24(face score) -- 3.39(neck fold score) -- 1.77(type score)+ 3.64(condition score) + 2.05(staple length) + 3.54(fleece weight)