Genetic trends in gestation length
Gestation length has become of increasing importance in dairy genetic evaluations worldwide. This has occurred for several reasons, with the initial focus being on the improvement of calving ease. Calves which are born earlier are typically of smaller size. This was generally thought to reduce the incidence of dystocia (although this has been disputed in some countries). However, the effects of gestation length on reproductive performance have become of greater interest. A reduction in gestation length has positive effects on calving interval, cows which have a greater number of days from calving to mating are more likely to be cycling and have demonstrably higher conception rates than their late-calving counterparts. However, this benefit for herd reproductive performance is not without its drawbacks. A reduction in gestation length does not directly improve a cow’s ability to resume estrus cyclicity post-calving, or to achieve fertilization after insemination. Gestation length is, rather, a trait that improves reproductive performance indirectly. Moreover, gestation length, if it is reduced too significantly, may have adverse effects on the health and survival of dairy calves, whose welfare is an increasing target of ethical scrutiny from consumers and society in general. Genetic evaluation for gestation length is being performed in a number of countries, for example in the United States since 2017. In Australia it has been introduced in 2020. This paper will look into genetic trends for gestation length in countries that perform genetic evaluation of this trait or conduct research in this space. We will discuss 1) the reasons for those genetic trends, for example trying to answer the question of whether selection for which fertility traits could be placing indirect selection pressure on gestation length; 2) if there are differences between the countries that can be explained by seasonal or all year round calving patterns; 3) how gestation length is being used as a tool to manage dairy cows’ calving patterns, including breeding and marketing of sires with extremely short gestation length breeding values; 4) evidence in the literature on genetic and phenotypic associations with calving traits, fertility and other traits; 5) potential long term consequences of selection for change in gestation length; 6) economic value of gestation length and its inclusion in (economic) selection indexes.
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