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Inheritance of Scrotal Hernia in Swine¹

Iowa State College

Abstract

Over the last ten years approximately 5 percent of the male pigs weaned in this herd have had scrotal hernia.

The differences between lines are greater than could be explained by chance alone. These differences have fluctuated from year to year, but little if any more than would be expected by chance.

The yearly differences in the incidence of scrotal hernia were not statistically significant.

The estimates of heritability for scrotal hernia were about 0.15 when based on differences among sires within lines. A less dependable estimate, based on differences among lines, was about 0.07. These figures estimate the amount to which the additive effects of his genes determine whether a male pig which reaches weaning age had scrotal hernia or not.

Whether or not a male pig had scrotal hernia was also 15 percent determined by a maternal effect. It was not determined to what extent this unexpectedly large maternal effect was genetic or environmental.

The large maternal effect indicates that a breeder should practice strong selection against sows that have produced herniated pigs. If the maternal effect can be identified as some environmental effect and if the environment which causes hernia can be easily corrected, that would offer a way to lower the incidence of hernia.

Since scrotal hernia is a threshold characteristic with low heritability, hernia could be reduced by forming many inbred lines, selecting those which have the least hernia and crossing them together to form new lines. Scrotal hernia is not important enough by itself to warrant forming inbred lines but, since many lines are being formed for other reasons, the incidence of hernia should be considered when selecting among the lines.

¹ Journal Paper No. J 1936 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No. 1127.

² The assistance of Dr. J. L. Lush and Dr. L. N. Hazel is gratefully acknowledged.