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Mass Selection in a Pig Population: Correlated Responses in Reproductive Performance¹

University of Alberta^,4, Edmonton, Alberta, Canada T6H 2H1

Abstract

Time trends in female reproductive performance were evaluated for a population of 1,203 dams contributing to a nine-generation selection experiment involving replicated lines selected for maximum growth (G), minimum fat (B) and an index (I) combining these two traits. Although inbreeding advanced to substantial levels during the course of this experiment, the regression trends for litter size at birth and weaning were positive and significant (P.01) for the I line and near zero for the B and G lines. The line rankings for weaned litter size in the final generation was I>B>G. In comparisons with the control population in this generation, with weaned litter size adjusted for line differences in inbreeding of the dams, the ranking was I>B>G>C. Indirect selection for traits of the sow (e.g., F_x, weights at farrowing and weaning, litter size and weight) was negligible, averaging less than .l/generation for most traits. Contrary to expectations, there was no evidence that mass selection favored the retention of breeding stock from litters of above average size. Repeatabilities of litter size and litter weight ranged from .12 ± .04 to .16 ± .04 for data pooled across lines. However, repeatabilities were not uniform for each line, being approximately 50% of the pooled estimates for the G line and ranging around 150% of the pooled estimates for the B line.

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.

⁴ Dept. of Anim. Sci.

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