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Selection of Pigs under Farm Conditions: Kind and Amount Practiced and a Recommended Selection Index¹

University of Wisconsin³

Abstract

The records analysed in this study were collected on Wisconsin farms during the period 1947–1952. Data were available for at least two consecutive years on each herd studied. A total of 161 herds with 969 litters by 218 sires are involved. The characters studied are the number of pigs per litter at birth and at 154 days of age, the litter weight and the individual pig weight at 154 days.

The average total selection differentials in actual units are .48 and .78 of a pig for litter size at birth and at 154 days respectively, 135 pounds for litter weight and 9.8 pounds for individual pig weight at 154 days. One-half to three-quarters of the selection practiced for litter characters was automatic.

The estimates of heritability vary from .07 to .11 and are not significantly different from 0. Since they are the best estimates available on the population sampled, they were used in calculating the genetic variance for each trait.

The phenotypic correlations between litter characters vary from .39 to .91, the largest standard error being .03. Those between litter characteristics and individual pig weight are close to 0. The estimates of genetic correlations between litter characteristics are highly positive. However due to the size of their sampling variances, there is considerable doubt as to their true magnitude. The estimates of genetic correlations involving individual pig weight indicate very little if any genetic association between those traits. The relative economic importance of litter size and individual pig weight at 154 days of age, in terms of net income, probably lies within the range of 15:1 to 30:1, with an average ratio of 20:1. Number farrowed and litter weight at 5 months were given no direct economic importance, but were included in the index as an aid in predicting the genetic differences between animals with respect to litter size at 154 days.

The index which best estimates the transmitting ability of 5-month-old pigs is:

where the subscripts 1 to 4 refer to the number farrowed in the litter in which the pig in question is born, and the number in the litter, the litter weight and the individual pig weight at 154 days of age, respectively. It is correlated .399 with the aggregate genotypic value of the pigs.

A somewhat simpler index and with approximately the same precision in estimating the transmitting ability of a pig for its economically important traits is:

¹ Paper from the Department of Genetics (No. 535) and the Department of Animal Husbandry. Published with the approval of the Director of the Agricultural Experiment Station.

² Present address: Agricultural Experiment Station, Lennoxville, Quebec, Canada.

³ The authors wish to acknowledge aid from: The Numerical Analysis Laboratory, University of Wisconsin for the use of the IBM equipment and F. E. Gruenberger of that laboratory for help in parts of the analysis; the farmers who provided the data; G. E. Bradford, R. M. Durham, L. L. Felts, T. E. Hamilton and D. H. Williams for help in collecting the data; and Oscar Mayer and Company, Madison, for a grant in partial support of this work.