Milk production, fertility and the modern dairy cow

Over the last 50 years, there has been a clear decline in fertility in the dairy herd (Royal et al, 2000; Butler, 2003; López-Gatius, 2003). Conception rates to first service have reduced from 55% to less than 40% (Royal et al, 2000). During this time, the dairy industry has also achieved a remarkable increase in the volume of milk produced per cow, likely achieved through a substantial increase in the proportion of Holstein-Friesian genetics in the herd (Royal et al, 2000). The push for more milk per cow has been widely discussed as an important contributor to the decline in reproductive performance (Figure 1). Within the cow, the effects on reproduction that result from lactation are complex.

There are an array of hormonal cascades that must occur in order to achieve successful reproduction (Lucy, 2001). The hypothalamo-pituitary-ovarian axis and the growth hormone-insulin-like growth factor axis are examples of two of these. A key driver of the success of the hormonal cascades, which has been identified in many studies, is energy balance within the cow. From the pre-calving period through until after mating has been completed, energy balance has a significant influence on reproductive success (Walsh et al, 2011).