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An Explanation for the Low Heritability of Litter Size in Swine^{1, 2,}

North Carolina State University, Raleigh 27607

Abstract

Data from 1,078 two-generation and 710 three-generation pedigrees were used to assess the causes of low heritability of litter size in swine. The data were divided into high (18%), middle (64%) and low (18%) groups, based on the first generation litter size. In three of six breed-parity groups, the daughter's litter size of the low group exceeded the high and/or the middle group. The three-generation pedigree also exhibited an oscillation. These results suggest a negative environmental correlation between the litter size of dam and daughter. The linear regression of daughter on dam (750 pairs) resulted in a heritability estimate of 0.13±.06. However, a significant curvilinear relation was evident: =0.0624+0.0578X–0.0125X². Linear regressions for the high, middle and low groups were –.065, 0.005 and 0.098, respectively. The heritability estimate from granddaughter-granddam regression (539 pairs) was 0.28±.26. These results provide further evidence of negative maternal effects on litter size. Such effects would result in low heritabilities and ineffective selection for increased litter size. It is suggested that the physiological maturation of gilts in larger litters is delayed due to increased stress and/or competition. Thus females from large litters are unable to express their genetic superiority. Further, these results suggest a moderate heritability for litter size if the stress of being reared in large litters is removed. Additional evidence was provided by age at puberty on 180 gilts. Significant cubic effects of litter size on age at puberty were present. This regression suggests that gilts from litters of six to 12 would reach puberty at approximately the same age, whereas gilts from larger litters would be increasingly older at puberty.

¹ Paper No. 3959 of the Journal Series of the North Carolina State University Agricultural Experiment Station, Raleigh.

² The authors express their appreciation to the personnel of the Tidewater Research Station and the Swine Breeding Unit for their assistance in collecting the data and to Carroll Perkins for his assistance in computer programming.

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