The economics of fertility in the dairy herd

A method to establish the economic optimum (minimum) cost of fertility in t he dairy herd is described and demonstrated. A Markov chain model is used i teratively to establish the gross margin of the herd in the long term at va rious levels of oestrous detection rate and under two different rebreeding strategies. These gross margins are required by the optimization methodolog y. Under the initial assumptions reflecting current commercial practice in the United Kingdom, gross margin was pound 806 per cow. This figure varied by proportionately 0.15 over the range of oestrous detection rates assumed (0.4 to 0.7) while delaying rebreeding by 20 days caused guess margin to dr op by approximately 0.04. It was concluded that if is important to optimize fertility control as well as rebreeding strategy in order to establish the economic impact of fertility in the dairy herd. The economic value of fertility was also expressed per unit of calving inte rval and adjusted calving interval (ACI). ACI was calculated by dividing ca lving interval by the proportion of cows not culled for reproductive failur e. Under the assumptions made, the marginal value of calving interval at th e optimum oestrous detection rate was pound 6.22 per day, rising to pound 7 .44 per day if rebreeding was delayed. The corresponding figures for ACI we re pound 1.57 per day and pound 1.24 per day. The range in marginal values at sub-optimal oestrous detection rates were pound 4.38 for calving interva l and pound 0.61 for ACI. It was concluded that the lower variation in ACI at different levels of fertility may make it a more representative trait fo r inclusion in a selection index provided the necessary genetic parameters call be reliably estimated.