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Potential of Milk Yield in Short Segments of Early Lactation as Covariates in Feeding Experiments¹

Biometrics and Information Systems Center, University of Nebraska, Lincoln 68583
Department of Animal Science, University of Nebraska, Lincoln 68583

ABSTRACT

Milk yields in early segments of lactation were investigated as covariates for increasing sensitivity in measurement of response of lactation yield to experimental treatments imposed in early lactation. These segments were 3 through 6, 3 through 10, 7 through 10, 3 through 14, 7 through 14, and 11 through 14 days postpartum. Correlations between milk yields of early short segments and 305-day milk yields were computed for first, second, and third or later lactations. Partial correlations pooled over lactation-season subclasses also were computed. Covariances of 3 05-day milk yields (variance for season of freshening and lactation number removed) with milk yields of various short segments were analyzed.

Correlations were relatively low for cows in first lactation but increased for each succeeding lactation. The 11 through 14 day yields were correlated more highly with total lactation yield than other early segments. The coefficient of variation of 305-day milk yield was reduced from 19.3% with no covariate adjustment to 16.6% with adjustment for 11 through 14 day yields, and cows needed per treatment to detect 20% differences in milk yield were reduced from 16 to 12. The 3 through 14 and 7 through 14 day segments gave similar improvements but were slightly less effective in reducing variance than the 11 through 14 day segment.

¹ Published with approval of the Director as Paper No. 6024, Journal Series, Nebraska Agricultural Experiment Station.