ABSTRACT
Genetic selection has resulted in a great increase in milk production per cow and it is generally accepted that unfavourable genetic correlations exist between milk yield and reproductive performance in dairy cows (Berry et al., 2016). This is often given as the explanation for the decrease in fertility seen in dairy cows over the last 40 years. To reverse this trend there is great interest in selecting for reproductive traits in selection programmes; however, this is challenging as the mean heritability of traditional reproductive traits is low and varies between 0.02 and 0.05 (Berry et al., 2014). As a consequence there has been great emphasis on solving the low fertility problem by addressing the physiological issues (addressed above) that contribute to fertility. While this approach is necessary, it is not the complete solution because poorly managed cows will not reproduce and also because more emphasis is now being placed on the potential for the genetic improvement of genetic traits. It is now known that the genetic correlation between reproduction and milk production on not one (Berry et al., 2014) and as such simultaneous selection for both milk production and reproduction traits are possible, although in this situation genetic gain for both traits will be reduced compared with selecting for either trait on its own (Berry et al., 2016).
