RESPONSES TO VARIOUS SELECTION PROTOCOLS FOR LAMB PRODUCTION IN RAMBOUILLET, TARGHEE, COLUMBIA, AND POLYPAY SHEEP

To investigate the effectiveness of four selection protocols for impro ving reproduction in sheep, we evaluated nine selection lines and two random-bred control lines for lamb and wool production. Results were b ased on 25,026 dam and 30,628 lamb records from Rambouillet (R), Targh ee (T), Columbia (C), and Polypay (P) sheep collected from 1976 throug h 1988. Phenotypic trends over years were positive (P < .01) for most reproductive traits, body weight, wool grade (coarser grades), and lam b weaning weight in nearly all selected lines. Small positive trends f or both random-bred control lines indicated there were improvements in management and(or) environment during the period. Small but significa nt phenotypic declines in fleece weights occurred in most lines, inclu ding controls. Substantial genetic gains (P < .01) in litter weight we aned (120 d), net reproductive rate (lambs weaned divided by ewes mate d), prolificacy, body weight, and weaning weight were made in nearly a ll selected lines. There were also small but significant improvements in milk score in most lines. There were significant genetic declines a nd improvements in fleece weights; however, the average genetic change in fleece weight for lines selected for litter weight weaned was negl igible. Genetic improvement in litter weight weaned was attributed app roximately 37% to prolificacy, 27% to percentage of lambs weaned, 17% to lamb weaning weight, 12% to fertility, and 7% to ewe viability from breeding to lambing. On average, selection based on independent culli ng levels (litter weight weaned plus yearling body weight), yearling b ody weight, or early puberty was only 85, 67, or 59%, respectively, as effective as that based solely on litter weight weaned for improving litter weight weaned. The net value of the average lover all breeds) a nnual increase in production per ewe resulting from selection for litt er weight weaned accumulated over 12 yr to an estimated $11.40 and $21 .51 annually for genetic and phenotypic increases, respectively.