HERITABILITY OF NONRETURN RATE OF CANADIAN HOLSTEIN-FRIESIAN BULLS

Abstract

Conception rates of Canadian Holstein-Friesian bulls in artificial insemination were recorded as percent 60- to 90-day nonreturns to first service. A total of 5, 923, 974 first services to 784 bulls by 207 sires were analyzed using a mixed model which included age of bull (1–14 yr), years (1956–1976), months and AI units as fixed effects. Bulls and sires of bulls were random. Significant differences were found in all classifications involving fixed effects. Nonreturn rates at one year of age were highest and significantly different from nonreturn rates at 9 yr of age and older. Lowest estimates of nonreturn rates were in January and December, and highest were in September and October. Differences were found among some AI units. Best linear unbiased predictions of differences among all service sires were obtained. Variance components were estimated iteratively by maximum likelihood. Heritability was 0.25 by regression of sire on son with 30 sires having four or more sons. Repeatability was 0.41. Many factors affecting nonreturn rates could not be identified, especially at the cow or herd level. Sire selection programs in AI units for high nonreturn rates in sires of young bulls should be successful. Key words: Fertility, dairy bulls, heritability