Environmental Influences on Regression Factors for Estimating 305-Day Production from Part Lactations

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Abstract
Cumulative milk and fat production on test days from 11,420 Michigan DHIA-IBM records from 1953 to 1957 were used to derive regression factors for estimating 305-day production from part lactations. The data included 8993 Holstein, 1457 Guernsey, 651 Jersey, and 319 Brown Swiss lactations. Linear regression coefficients of total on part lactation within herd-season-lactation-age groups were used to measure the relationship between cumulative production on test day and total production for the first 10 test days. Analyses of regression coefficients for the 1st, 3d, and 7th cumulative months of the Holstein data indicated season was not an important influence on the part-to-whole relationship. Visual inspection of mean coefficients for age within lactation numbers failed to reveal where ages and parities should be separated or combined. Intra-herd factors for extending incomplete records were developed from the average regression coefficients of total on part lactations within herds, seasons, lactations, and ages for each of the 9 cumulative test days. Each regression coefficient included in the average was weighted by the number of records from which it was computed. For comparison, extension factors disregarding herd, season, lactation number, and age effects also were derived. Although the differences between inter- and intra-herd factors were not tested for significance, the marked similarity between Holstein factors suggests that herd had little or no influence on the part-to-whole relationship. Correlations between cumulative part and 305-day production ignoring herd differences were not less than 0.7 for the first month, increased steadily as lactation progressed, and were 0.9 by the fifth test day for all breeds. These findings support data in the literature which suggest that production records of only 1 or 2 mo. are valuable guides to what a cow will produce in that lactation. Estimating 305-day production from such records would be a useful tool in early culling and progeny testing.