Reproductive rate and genetic variation in composite and parental populations: experimental results in sheep

Mean and genetic variability for ewe performance of two composite populations were compared with those of their contemporary parent lines to assess relative potential for continued response from selection. Data were from the U.S. Meat Animal Research Center, 1973 to 1983. The 4,415 ewes by 508 sires of Finnsheep (F), Rambouillet (R), Dorset (D), and Composite 1 (C1 = F/2 + R/4 + D/4) lines were mated in April, August, and December of each year. The 1,763 ewes by 320 sires of Finnsheep (F), Suffolk (S), Targhee (T), and Composite 2 (C2 = F/2 + S/4 + T/4) were mated in October only. Data were adjusted for effects of year, season, and age of ewe at breeding, plus interval between lambings, for the accelerated lambing lines and for year and age at breeding for the annual lambing lines. Superiority of composite over parent lines in lambs weaned per ewe exposed was 36% for accelerated and 44% for annual lambing lines; that superiority arose largely from dominant heterotic effects for fertility and viability. Weighted mean paternal half-sib estimates of heritability for mean lifetime ewe performance of composite vs parental lines were 32 vs 23% for litter birth weight, 25 vs 28% for lamb birth weight, 26 vs 11% for number born, 11 vs 23% for conception rate, 5 vs 10% for number weaned, -1 vs 19% for lamb weight at 42-d weaning, but 6 vs 23% for number and 8 vs 28% for litter weight weaned per ewe exposed. Heritability was higher in composites than in parental lines for numbers born, but it was lower for the more heterotic traits (fertility, lamb viability) and, hence, for total weaned lamb output, without change in phenotypic variation. Thus, even an equal response in composites from continuing selection for weaned lamb output may depend mainly on more intense selection permitted by a higher reproductive rate.