Sources of Variation Affecting the Difference Between Morning and Evening Daily Milk Production

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Abstract
Starting and finishing times of daily milkings were recorded on 184,521 records on 19,905 cows of five dairy breeds located in 555 New York Dairy herds. Analysis of variance of Am minus P~ milk produc- tion for 6 milking intervals, 12 months in lactation, 12 levels of herd production, and 5 ages at calving indicated additive correc- tion factors would be necessary to correct A~ or P~ production in a single-milking testing scheme for each month of lactation- milking interval subclass. There was a significant effect of herd production on ~ minus P~ production, but for lack of a pattern it was disregarded. Significant age effects showed no pattern except in the Holstein breed. Correction factors are nec- essary for single-milking testing schemes since a cow producing 20.4 kg of milk per day would be expected to produce 37, 26, 18, 9, 2, and --5% more milk in the A~ for milking intervals of > 839, 810 to 839, 780 to 809, 750 to 779, 720 to 749, and < 720 minutes.