RESPONSE TO SELECTION FOR FERTILITY IN A FALL-LAMBING SHEEP FLOCK

Selection for fertility in a fall lambing system was applied for 5 yr (1.45 generations) in a crossbred sheep population. Three flocks were involved: a fall-lambing selected (S) flock of 125 ewes and 10 rams, a fall-lambing environmental control (EC) flock of 55 ewes and five ram s, and a spring-lambing genetic control flock of 45 ewes and five rams used to produce replacements for EC. Estimated breeding values for fe rtility (ewes lambing per ewe exposed) were calculated for animals mat ed in each year of the study using a single-trait animal model and inc reased more rapidly in S (.0138/yr) than in EC (.0067/yr). After accou nting for genetic drift, EBV of ewes mated in S in the final year of t he study averaged .070 +/- .032 (P = .08). A correlated response of .0 087 lambs/yr was observed in S for fall litter size. Genetic trends in all body weights were positive in EC, likely because of supplemental selection on body size in ram lambs used in EC. Ninety-day weight, mat ernal effects on weaning weight, and 90-d scrotal circumference increa sed in S, but direct effects on birth and weaning weights declined. In order to focus on realized selection response in fertility, EBV for f ertility of 330 replacement ewe lambs from S and EC were calculated us ing only data collected after their birth. These realized EBV were the n regressed on pedigree EBV calculated at the time the ewe lambs were born. The regression was significant when all relationships were used to calculate both EBV (.20 +/- .08) but only approached significance ( .13 +/- .07) when only records of the ewe lambs themselves, their desc endants, and their paternal half-sibs were used to calculate realized EBV. Also, EBV for al ewes were calculated at the start of each breedi ng season and were higher (P < .01) for ewes that subsequently lambed than for open ewes. These results suggest that selection can be used t o improve fertility in fall-lambing sheep flocks.