Many experiments have measured the responses of grazing dairy cows to supplementary feeds, but few have studied the reasons for the large differences in responses between experiments. Data derived from two trials, in which supplementary feeds of either maize grain or a nutritionally balancing ration were offered to cows in early, mid, and late lactation in each of spring, summer, autumn and winter, were used to calculate the marginal milksolids responses to supplements. The data set created was subject to multiple regression analysis in an attempt to quantify the effect of the different variables on the magnitude of the response to supplementary feeds. The factor that was of greatest importance was the relative feed deficit (RFD) of the cows measured by the reduction in milksolids yield (kg MS/cow/day) of the respective control groups that had occurred when the restricted feeding treatments had been imposed. Total marginal milksolids responses were greatest when severe feed restrictions, relative to current feed demand, resulted in large reductions in milksolids yield of the control groups. Total marginal milksolids response increased (P<0.01) by 0.9g MS/MJME offered as supplement per 0.1 kg MS/cow/day RFD. Total marginal milksolids responses also declined (P<0.01) by 0.2 g MS/MJME offered as supplement as pasture allowance increased by 10 MJME/cow/day.
Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 229-233, 2001
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