Expectations of Heritability Estimates for Non-Return Rate of Bulls and Conception Rate of Cows

Abstract

Differences between heritability estimates based on sires' non-return rates and those of conception rate based on single records of matings were studied in terms of individual, paternal and maternal genetic and environmental effects. This was accomplished by expressing the outcome of a mating as a trait of the potential calf (0 if conception did not occur or if the fetus did not survive to a specified date, 1 otherwise). Because the heritability estimates of non-return rate depend on number of inseminations, the effect of the structure of the data on the heritability estimates also was studied. When non-return rate was based on a large number of services, the heritability estimate was insensitive to how number of services varied among sons or sires. In the absence of environmental paternal effects (Ep), non-return rate heritabilities will approach 1 as the number of services increases, whereas in the presence of Ep the limit is less than 1. The fact that 23 literature estimates ranged from .10 to .55 indicates the importance of Ep. Heritability estimates from the literature based on single records (conception rate of cows that are paternal half-sibs) were small (46 of 51 were less than .03), but heritability of mean conception rate of daughters also would approach 1 as the number of daughters increases. Literature estimates of the heritability of conception rate (based on potential calves being paternal half-sibs) were slightly larger than those calculated from paternal half-sib cows.