Studies on the properties of New Zealand butterfats: V. The effect of stage of lactation

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Abstract
Milk and butterfat yields, and the properties of butterfats from the milks, were compared in three successive years for early- and late-calving cows of monozygotic twin pairs grazed on the same pastures, over the period when both cows of the twin pairs were in lactation.The seasonal fluctuations in the properties of the butterfat were closely similar for the two groups of cows in all three trials. This was taken to indicate that the known regular pattern of seasonal change in the properties of New Zealand butterfat over all North Island districts is not due to change in stage of lactation of the cows but is probably the result of a regular change in the feeding conditions.The average milk and butterfat yields were lower for the early-calving cows during the comparison period, the iodine values were higher, and the Reichert and saponification values were lower. The early-calving cows were uniformly heavier than the late-calving cows. It is suggested that the consistent differences in the butterfat characteristics over the whole comparison period could be due to differences in the extent to which body fat reserves were drawn upon by the cows for maintenance of lactation.The average carotene and vitamin A contents of the butterfats were not appreciably different for the two groups of cows, indicating that these values are not directly related to stage of lactation of the cows or to yield of milk or butterfat.