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Association between somatic cell count early in the first lactation and the lifetime milk yield of cows in Irish dairy herds
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2013-05-18
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Archer, S. C., Mc Coy, F., Wapenaar, W., Green, M. J. Association between somatic cell count early in the first lactation and the lifetime milk yield of cows in Irish dairy herds. Journal of Dairy Science, 2013, 96(5), 2951 - 2959. doi: https://doi.org/10.3168/jds.2012-6294
Abstract
Change in lifetime milk yield is an important component of the cost of diseases in dairy cows. Knowledge of the likelihood and scale of potential savings
through disease prevention measures is important to
evaluate how much expenditure on control measures
is rational. The aim of this study was to assess the
association between somatic cell count (SCC) at 5 to
30 d in milk during parity 1 (SCC1), and lifetime milk
yield for cows in Irish dairy herds. The data set studied
included records from 53,652 cows in 5,922 Irish herds.
This was split into 2 samples of 2,500 and 3,422 herds
at random. Linear models with lifetime milk yield and
first-lactation milk yield as the outcomes and random
effects to account for variation between herds were
fitted to the data for the first sample of herds; data
for the second sample were used for cross-validation.
The models were developed in a Bayesian framework
to include all uncertainty in posterior predictions and
parameters were estimated from 10,000 Markov chain
Monte Carlo simulations. The final model was a good
fit to the data and appeared generalizable to other Irish
herds. A unit increase in the natural logarithm of SCC1
was associated with a median decrease in lifetime milk
yield of 864 kg, and a median decrease in first-lactation
milk yield of 105 kg. To clarify the meaning of the
results in context, microsimulation was used to model
the trajectory of individual cows, and evaluate the
expected outcomes for particular changes in the herdlevel prevalence of cows with SCC1 ≥400,000 cells/mL.
Differences in mean lifetime milk yield associated with
these changes were multiplied by an estimated gross
margin for each cow to give the potential difference in
milk revenue. Results were presented as probabilities
of savings; for example, a 75% probability of savings of
at least €97 or €115/heifer calved into the herd existed
if the prevalence of cows with SCC1 ≥400,000 cells/
mL was reduced from ≥20 to <10 or <5%, respectively, and at least €71/heifer calved into the herd if the
prevalence of cows with SCC1 ≥400,000 cells/mL was
reduced from ≥10 to <5%. The results indicate large
differences in lifetime milk yield, depending on SCC
early in the first lactation and the findings can be used
to assess where specific interventions to control heifer
mastitis prepartum are likely to be cost effective.