RECIPROCAL RECURRENT SELECTION FOR 21-DAY LITTER WEIGHT OF CROSSBRED GILTS. II. REPRODUCTIVE PERFORMANCE OF PUREBRED FEMALES PRODUCING PUREBRED AND TWO-WAY CROSS LITTERS AND PERFORMANCE OF PUREBRED AND CROSSBRED PIGS
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This paper reports estimates of correlated genetic change in reproductive performance of purebred gilts producing two-way cross litters and purebred sows producing purebred litters as well as postweaning performance of two-way cross and purebred pigs produced during reciprocal recurrent selection (RRS) between Line 8 (Duroc) and Line 9 (Beltsville No. 1) for 21-d litter weight production of crossbred gilts. A randomly mated control line developed from a crossbred foundation was maintained to monitor environmental change. Data were adjusted for age of dam, inbreeding of dam, inbreeding of litter (or pig) and linear and quadratic effects of day born within season. The traits evaluated were: litter size, litter weight and average pig weight/litter at birth and weaning (42 d); postweaning average daily gain; age at 90.7 kg and backfat thickness at 90.7 kg. Two data sets were analyzed; the first set included seven seasons of data with purebred gilts producing two-way cross litters, and the second set included seven seasons of data with purebred sows producing purebred litters. No estimates of environmental trend were significant in either set of data. In the first data set, only the estimate of genetic change in backfat thickness of two-way cross pigs was significant and it was in the desired direction. All other estimates were small and did not approach significance. In the second data set, estimates of genetic trend were greater in Line 9 than in Line 8. Estimates of genetic trend in Line 9 were significant for average pig weight at birth, age at 90.7 kg and backfat probe at 90.7 kg, and approached significance for litter size at weaning and average daily gain. The estimates were undesirable for preweaning traits and desirable for postweaning traits. The estimates of genetic trend in Line 8 were of the same sign as those in Line 9, but only the estimate for backfat thickness was significant. The decrease in size of purebred litters in Line 9, and to some degree Line 8, suggests an accelerated accumulation of homozygosity beyond that accounted for by adjustment for pedigree inbreeding. The fact that Line 9 showed a greater decrease than Line 8 suggests that most of the increase in level of reproduction of crossbred gilts may have resulted from genetic change in Line 9 rather than Line 8, or that favorable alleles were being fixed in Line 8 and unfavorable alleles in Line 9.