Relations between Test-Day Milk Production of Holstein Cows

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Abstract
Milk production on test day from 6,715 Holstein-Friesian ttIR 2 × and 3 × records during 1952-54 was studied, to determine the relations between production for single months and cunmlative production, and to develop factors for extending' part production to a 305-day basis. Correlations between milk produced on test days were largest for adjacent months and ranged from .38 to .89 for 2 ×-cows under 3 yr.; from .23 to .88 for 2 ×-3 yr. and over; from .39 to .89 for 3 ×-under 3 yr., and from .21 to .88 for 3 ×-3yr. and over. Correlations between nlilk production on test day, and the sum of nlilk produced on ten test days, ranged from .71 to .93, were higher for the young cows, were similar for different milking frequencies in the same age group, and were highest during the 4th to 7th too. The regressions of the sum of production for ten test days on single test-day production had relationships similar to the corresponding correlations. Factors developed from the ratio of total to cumulative part production, and from the regression of total on part production, are presented. The ratio method may underestimate total production of low-producing cows and overestimate total produc-tio~ of high-producing cows, since the ratio method corrects only for the incompletehess of the lactation and does not take into account the incomplete repeatability of the parts of the lactation, which is a part of the regression method. This difference is largest during the early months.

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