With the on-going debate about heifer development strategies, it would seem to be valuable to review some of the basics concerning population genetics and how these principles could affect our management decisions. As we evaluate growing programs for replacement heifers and their impact on long term productivity of the herd, let us remember that both the genetic makeup of the heifer and environment in which she is raised will affect her ability to reproduce and raise progeny.
Reproduction in cattle has historically been reported to be a lowly heritable trait. “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 (management). The environmental component could include nutrition, health, weather stresses, AI technique, bull fertility, or a neighbor’s dog that gets in the pasture and chases the cattle. Previous estimates of the heritability of pregnancy rates in heifers ranged from 0 to .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 .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 attempt to breed her. The heritability of “first service conception rate” was even lower at .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. These low heritability estimates suggest that only slow progress could be made by selecting sires and dams that produced heifers with greater pregnancy rates. This data also reminds us that management is still the key to successful pregnancy rates in replacement heifers. Source: Minick and co-workers. 2004 Iowa State University Beef Research Report.