Simulation of the lifetime productivity of a dairy herd in Japan according to the number of days to first service of cows.

To determine the optimal voluntary waiting period to first service after calving for maximizing the lifetime productivity of cows in a dairy herd, we simulated the changes in lifetime productivity due to increasing the number of days from calving to first service. We also assessed the effects of changes in the maximum number of services within a lactation, maximum number of calvings, and calf sale prices on lifetime productivity. We evaluated lifetime productivity as lifetime average productivity, that is, the sum of daily revenues from milk and calf sales from first calving to culling. The lifetime productivity at ≤65 days to first service was low because of the low conception rate during early lactation. In addition, lifetime productivity at ≥185 days to first service decreased as the number of days to first service increased. The number of days to first service for maximum lifetime productivity decreased as the maximum number of calvings increased. Furthermore, the decrease in lifetime productivity associated with decreasing the maximum number of services within a lactation from five to four was greater than that when the maximum number of calvings was decreased from six to five.