Contribution of variables to the optimum policy for insemination and replacement of dairy cows

The revenues from dairy cows originate from the sale of milk, calves and cows while the costs are associated mainly with feeding, housing and rearing of young stock. These factors were accounted for in determining the optimum replacement policy. However, the contribution of the variation occuring in these variables to the optimum replacement policy has not been studied so far. In developing management guides for individual cows, it is important to know which variables influence the optimum decisions for insemination and replacement. The optimum replacement policy is not changed when a constant amount is subtracted from the monthly net revenues from present and replacement cows. For example the monthly costs of housing, which are equal for all cows in the herd, will have no effect on optimum decisions for insemination or replacement. It explains also the relatively small influence of changes in the average milk production of the herd on the optimum replacement policy. The influence of a trait on the optimum replacement policy will therefore depend on the variation in monthly revenues or costs associated with it. The objective of a commercial dairy farmer in deciding whether or not to replace a cow immediately, or at a later stage in lactation, is maximization of income over time. Assuming a constant herd size this implies that the cash flow expected from a cow presently in the herd has to be weighted against that of replacement heifers in determining the optimum replacement policy. The calculation of the expected cash flows could be simplified by excluding the variation in a number of traits. Techniques which have been used to determine the optimum replacement policy based on the expected cash flows were described and evaluated by the author in 1984. In this study the consequences of excluding variation in a number of traits for the optimum replacement policy are quantified. In addition the average performance is determined for cows when applying the optimum policies resulting from the different alternatives.