|Van Vleck, Lloyd|
Submitted to: Livestock Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2005
Publication Date: 6/1/2006
Citation: Hanford, K.J., Van Vleck, L.D., Snowder, G.D. 2006. Estimates of genetic parameters and genetic trend for reproduction, weight, and wool characteristics of polypay sheep. Livestock Production Science 102(1-2):72-82. Interpretive Summary: Results from this study of several traits of Polypay sheep from records at the U.S. Sheep Experiment Station agree with those of previous, similar studies of the Columbia, Targhee, and Rambouillet breeds that multiple-trait analyses should be used rather than single-trait analyses when estimating genetic changes in traits because of the impact including correlated traits has on estimates of breeding values of other traits. The results also agreed with the previous studies that selection based on weaning performance over a long period could result in a moderate positive response in both litter size at weaning and weaning weight in flocks of dual-purpose breeds. However, a management decision concerning the number of lambs a ewe is allowed to raise, can have a greater impact on the response to selection for a prolific breed such as the Polypay, than for less prolific breeds, such as the Columbia, Targhee and Rambouillet. Although most of the correlations between fleece traits and weaning performance were in an undesirable direction, selection for increased weaning performance would offset decreases in wool traits with current market prices.
Technical Abstract: The objective was to describe genetic parameters and genetic changes in litter sizes at birth and weaning (LB and LW, n=9,081), birth weight (BW, n=11,896), weaning weight (WW, n=11,104), fleece weight and grade (FW and FG, n=8,872), and staple length (SL, n=1,805) of Polypay sheep. Direct heritability estimates from single-trait analyses were 0.11 for LB, 0.02 for LW, 0.17 for BW, 0.18 for WW, 0.68 for FW, 0.36 for FG, and 0.76 for SL. Estimates of direct genetic correlation were 0.40 between LB and LW, 0.57 between BW and WW, 0.65 between FW and SL, -0.37 between FW and FG and -0.70 between SL and FG. Breeding values (BV) from both single-trait and seven-trait analyses calculated using the parameters estimated from single-trait and two-trait analyses were compared across years of birth with respect to genetic trends. Estimated BV from both analyses for LB, LW, BW, WW and FW increased over time, while those for FG and SL were unchanged. Estimated changes in BV over time did not differ substantially for single-trait and seven-trait analyses, except for traits highly correlated with another trait that was responding to selection (i.e., LB, which was highly correlated to both LW and WW).