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Mass Selection in a Pig Population: Experimental Design and Responses to Direct Selection for Rapid Growth and Minimum Fat

University of Alberta, Edmonton, Alberta, Canada T6H 2H1

Abstract

Time trends in performance were examined over nine generations of mass selection in three replicated lines defined as G (maximum postweaning growth rate), B (minimum backfat) and I (a phenotypic index combining these two traits). Performance was measured under a continuous self-feeding regimen with males reared in confinement and females reared on pasture. Backfat, measured by scalpel probe, and age were adjusted to the average weight of contemporaries at test completion, approximately 91 kg for males and 92 to 131 kg for females, depending on generation of test. Replicates accounted for less than 4% of the total variance observed in each generation for both sexes but the variance due to lines, near zero in generation 1, increased to 36% (growth) and 52% (backfat) for the males and 9% (growth) and 17% (backfat) for the females tested in the final generation. This sex difference, coupled with low genetic correlations between the sexes for growth rate (.34) and back fat (.71) was interpreted as evidence that use of different test environments for males and females resulted in the phenotypes measured being genetically different traits in the two sexes. The practical and technical relevance of this interpretation is discussed. Cumulative selection differentials paralleled the performance time-trends observed for males. Index selection resulted in a progressive increase in growth rate equivalent to that observed for the G line, and a progressive decrease in fat equivalent to the B line. However, fat of the G line increased, and growth of the B line decreased, as generations advanced, and the net merit of both lines for the two traits was inferior to the I line in the final generation. Statistical estimates of genetic parameters are presented and discussed.

¹ Research conducted by senior author at the Canada Agr. Res. Sta., Lacombe.

² Mailing address: Box 1810, Lacombe, Alberta, Canada T0C 1S0.

³ Natl. Res. Council Post-Doctorate Fellow at Lacombe, 1975 to 1977. Present address: Natl. Inst. of Anim. Ind., Tsukuba Norindanchi, P. O. Box 5, Ibaraki 305, Japan.