Within a dairy herd, total milk production is determined by the proportion of cows producing milk at any given time and the level of milk production of the individual cows within the herd (Vandehaar, 1998). Reproductive efficiency dramatically affectsboth of these factors. Thus, improving reproductive efficiency helps to maintain the maximal number of cows in a herd producing milk at optimal levels. The body of scientific literature on dairy cattle reproduction supports the idea that fertility of lactating dairy cows has decreased over the past 50 years (Lucy, 2001). The rate at which cows become pregnant in a herd is termed the pregnancy rate, and pregnancy rate is determined by an interaction between the service rate and the conception rate. Reproductive efficiency in dairy cattle currently is suboptimal due to poor artificial insemination service rates and poor conception rates and pregnancy rate can be improved by increasing either the service rate or the conception rate. This paper will overview recent research that has begun to address physiologic and management factors that affect both service and conception rates in lactaing dairy cows.
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