Environmental Influences on Monthly Variation in Milk Constituents

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Abstract
Least-squares techniques were used to estimate the effects of age of cow, stage of lactation, month of year, and the 2 factor interactions of age with stage and month with stage on milk and its constituents. The analyses were done within brecds, and herd effects were absorbed. There were 303,193 monthly observations from 26,414 cows of 6 brecds. All cows were enrolled in Dairy Herd Improvement Associations in their respective states. The''traits considered were the test day production of milk, milk fat, solids-not-fat (SNF), total solids (TS), and the percentage composition of the 3 constituents. SNF percentage was determined by the Golding plastic bead procedure. The reduction due to fitting the set of constants for each main effect and each of the interactions was large and gave a highly significant (P <.001) F-ratio for all traits. The interaction of month and stage accounted for a very small proportion of the total variance. Although this effect is real, it is of small magnitude. The interaction of age with stage indicated that incomplete lactations of cows of different ages require different factors for extrapolating to complete lactations, for all constituents.