Genetic analysis of growth and its relationship with lifetime production in Rambouillet, Targhee and Columbia ewes under range conditions of Montana
Chavez, Juan Francisco
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Growth characteristics of sheep from birth to maturity and the evaluation of the genetic and phenotypic relationships between growth traits with lifetime lamb and wool production were studied using data from 302 Rambouillet, 338 Targhee and 175 Columbia ewes born between I960 and 1976. The Brody (1945) growth model was used for the derivation of growth parameters mature weight (A) and maturing rate (k). The Fitzhugh and Taylor (1971) equation-free model was used for the estimation of growth statistics; Absolute Growth Rate (AGR), Relative Growth Rate (RGR) and Absolute Maturing rate (AMR) for five intervals from birth to 18 mo. Genetic and phenotypic parameters were estimated by half-sib intraclass correlation using Harvey (1977) least-squares method for each breed and for the pooled data. Targhees were superior to Rambouillet and Columbias for weight of lamb at weaning (ATWW) and efficiency index (El). ATWW was the lifetime yearly average of kg of lamb weaned and El was ATWW per unit of ewes mature weight (A). Columbias were superior for yearly average grease wool produced (ATFP). Age at maturity was estimated on 39 mo, 38 mo and 41 mo for Rambouillets, Targhees and Columbias, respectively. Columbias had the highest A and the smallest k, Targhees matured the fastest. Ewes born twins had the highest El. From the pooled data, heritability estimates of average total of lambs born (ATLB), average total of lambs weaned (ATLW), average total weight of lambs weaned (ATWW), average total grease fleece produced (ATFP) and El were .43±.15, .33±..15, .11±>15, 68±..16 and .15±,15, respectively. Genetic correlation, between ATWW and A, indicated that ewes with high additive genetic potential for ATWW will have high genetic potential for larger A. The genetic correlation between ATWW and k was zero. For the three Fitzhugh and Taylor (1971) growth statistics, highest heritabilities were obtained for the weaning-12 mo interval: AGR2 (.80+.16), RGR2 (.76+.16) and AMR2 (.81+.16). RGR2 had the highest positive genetic correlations with ATWW (.95+.81), El (.77+.S2) and ATFP (.39+.19) among growth statistics. Inclusion of AGR2 and RGR2 in the construction of selection indexes would improve accuracy of selection for ATWW, ATFP and El. However, the contributions of A and k were inferior to AGR2 and RGR2. The use of any of the growth traits studied in selection indexes for improvement of ATFP and El simultaneously gave no advantage in improving efficiency of selection.