Modelling performance consequences on the probability of reproducing, and thereby on productive lifespan in dairy cows
Résumé
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.
Domaines
Sciences du Vivant [q-bio]
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