Modelling performance consequences on the probability of reproducing, and thereby on productive lifespan in dairy cows

Reproductive success is a key component of lifetime efficiency (ratio of total energy in milk to total energy intake over the lifespan) as failure to get in calf results in culling and thus has a negative effect on productive lifespan. At the animal level, breeding and feeding management can substantially affect milk yield, body condition, and energy balance of cows, which are all major contributors to reproductive failure in dairy cattle. This study developed a reproductive module that was incorporated into an existing lifetime performance model to enable prediction of the performance consequences of different breeding and feeding strategies on probability of reproducing, and thereby on productive lifespan. This then allows more realistic prediction of cow lifetime efficiency. The model is dynamic and stochastic with an individual cow being the unit of modeling and one day being the unit of time. To evaluate the reproductive module, data from a French study including Holstein and Normande cows fed with high concentrate diet and data from a Scottish study including Holstein cows selected for high and average genetic merit for fat plus protein, fed with high versus low concentrate diets were used. On average, the model consistently simulated reproductive performance of various genotypes of cow across feeding systems. Relative to the French data, the model significantly under-predicted first service conception rate for Normande cows (48% vs. 58% for predicted vs. observed). On the Scottish data, simulated conception to first service was not significantly different from observed but interval traits (days to first service, days open) were under predicted, which was mainly due to the discrepancy between simulated and observed voluntary waiting periods. Simulation showed that genetic selection for greater milk production impaired reproductive performance and thus reproductive lifespan, but not lifetime efficiency. However, the definition of lifetime efficiency used did not include associated costs or consider herd-level effects, which should be included to allow more accurate simulation of lifetime profitability in different scenarios