Summary A study of 34,176 lactation records not exceeding 305 days in length shows a definite decrease in the butterfat percentage of the milk of Ayrshire cows from the first lactation to old age that is statistically significant. Because of this decline in butterfat percentage with age it is suggested that it would be more nearly accurate to use age conversion factors for both milk and butterfat production, or to use a fat percentage differential as a correction factor when using only one set of age conversion factors for both milk production and butterfat production.
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Putnam et al. (1944) studied this question.
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