Heritability Estimates of Fertility in Replacement Heifers

“Heritability” is that portion of the difference in the performance of cattle that is due to genetics. The remainder of the differences are presumed to be due to differences in the environment (i.e. management, pastures, weather, etc). Previous estimates of the heritability of pregnancy rates in heifers ranged from 0 to 0.28. Iowa State University scientists studied records of 3144 heifers from 6 herds in 5 states. In the Iowa State study, the heritability of pregnancy rate was 0.13. Pregnancy rate is the percentage of the heifers exposed to artificial or natural breeding that were diagnosed pregnant after their first entire breeding season. First service conception rate is the likelihood that the heifer became pregnant on the first artificial insemination attempt to breed her. The heritability of first service conception rate was even lower at 0.03. This implies that 97% of the differences in the first service conception rate are due to the management environment in which the heifers were raised. (Source: Minick and co-workers. 2004 Iowa State University Beef Research Report.)

These low heritability estimates suggest that painfully slow progress could be made by selecting sires that produced heifers with greater pregnancy rates. Keeping heifers from cows that calve early in the calving season should also select for genetically improved reproduction. This data also reminds us that in any one year, management is still the key to successful pregnancy rates in replacement heifers. Remember, 87% of the differences in pregnancy rates were due to the "environment."

Although reproductive performance is a lowly heritable trait, some heifers are born with problems and they should be identified as soon as possible and removed from the herd. Spring born heifers are in their first breeding season now and should be checked for pregnancy about 60 days after the end of their first breeding season. Identifying and culling open heifers early will remove sub-fertile females from the herd. Lifetime cow studies from Montana indicated that properly developed heifers that were exposed to fertile bulls, but DID NOT become pregnant were often sub-fertile compared to the heifers that did conceive. In fact, when the heifers that failed to breed in the first breeding season were followed throughout their lifetimes, they averaged a 55% yearly calf crop. Therefore keeping them or rolling them over to a fall-calving herd is a bad bet. Selecting against poor reproduction may be painfully slow due to the low heritability. However, "painfully slow" progress is still better than no progress!