Milk production was compared between multiparous Holstein cows that were induced to calve and herdmates that calved spontaneously at approximately the same time in 88 dairy herds from Victoria and Tasmania. Yield averages for each herd were calculated for induced and spontaneously calving cows for milk volume, milkfat and protein during the first 120 days of lactation and for 305 days. Associations between induced and spontaneously calving groups for 305-day yields were described using linear equations. Slopes ranged from 0.82 (for milk yield) to 0.87 (for protein yield) and R2 values ranged from 0.84 to 0.88 (P<0.001). The average reduction in milk yield over the 88 herds was 366 litres (7%). The estimated milk yield reduction for induced groups in herds where spontaneously calving contemporaries averaged only 3500 litres per 305 days was 40 litres (1.1%) compared to an average estimated reduction of 915 litres (11%) in herds averaging 8500 litres. Similar proportional reductions were calculated for 305-day yields of milkfat and protein. Collectively these results show that yield reductions following induced premature parturition are substantially higher in absolute as well as proportional terms in herds with higher milk yields.
Proceedings of the New Zealand Society of Animal Production, Volume 66, Napier, 344-349, 2006
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