Reproductive and productive performance of a composite sheep population, containing 50% Dorset, 25% Rambouillet and 25% Finn sheep breeding and under selection for ability to lamb in the fall, was evaluated. Traits considered were spring fertility, faIl litter size, live weights at birth, weaning, and at about 90 and 120 d of age, and scrotal circumference at weaning and at about 90 and 120 d of age. Fertility was defined as 1 or 0 depending upon whether a ewe lambed or failed to lamb, respectively, in fall. Litter size was defined as the number of lambs born per ewe lambing. A total of 1,102 exposures were used. Fertility averaged .59 for adult ewes (3 years and older), .45 for second-lambing ewes and .11 for yearling ewes (12 months old). Mean litter size averaged 1.89 across seasons and ages. Scrotal circumferences (SC) and body weights (WT) were measured at means of65, 96 and 128 d. Data were collected on 1878 lambs over 5 years; 63 sires and 420 dams were represented.

Genetic parameters were estimated by restricted maximum likelihood (REML) using various animal models. Heritability estimates for spring fertility ranged from .07 to .11. Litter size heritability was .05 for fall lambing and .10 for all lambings. Heritability estimates for weights from birth to 120 d ranged from .04 to . 19. Heritabilities for SC ranged from .02 to .25. Estimated genetic correlations among weights were large, and ranged from. 77 to unity. The genetic correlations of litter size with birth weight (BWT), 90-d weight (90WT) and 90-d scrotal circumference (90SC) were .17, .33 and .36, respectively. Genetic correlations of spring fertility with BWT, 90WT and 90SC were .22, -.31 and .29, respectively.

Mixed model methodology was applied to estimate genetic trends for fertility, litter size, BWT, weaning weight (WWT), 90WT and 90SC resulting from selection for fall lambing. Positive genetic gains in both spring fertility and fall litter size were observed. Rate of increase was higher for fertility than litter size, which was due to direct selection for fertility. Selection for spring fertility did not cause significant correlated changes in BWT, WWT, 90WT or 90SC. Thus, no genetic antagonisms resulted from the selection for fall lambing.