Part Lactations. I. Age-Correction Factors for Monthly Milk Fat Yields

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Abstract
Age correction factors are developed for monthly butterfat yields of dairy cows in New Zealand. The 1st test month of the lactation is taken into account because in any calendar month cows are at different stages of lactation as well as at different ages. The factors obtained decrease as lactation progresses, indicating that in the later part of lactation the production of young cows is very similar to that of mature ones. This is probably due to the poor environment at this time of year masking the age differences. The factors for 2-year-olds differ greatly according to the 1st month but this effect is not apparent in the 3- and 4-year-olds, which vary little and are close to the factors for lactation production. Factors are also presented for correcting for age and 1st test month simultaneously, so that monthly records of cows calving in different months can be grouped together. The means of records corrected by these factors show that longer-milking cows give greater yields not only because of being milked longer but also because they produce more each month. Cows starting lactation at a high level had, on average, longer lactations than those starting at a low level of production.