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Biases in the Estimation of the Regression of Milk Production on Age

Dairy Cattle Research Branch, Animal Husbandry Research Division, U.S.D.A., Beltsville, Maryland

ABSTRACT

The regression of production on age was computed by the paired and gross comparison methods within four geographical regions of the United States. A total of 405,330 lactations of registered DHIA Holsteins calving between 1951 and 1962 were studied. Bias due to selection was assumed to be the primary cause of variation between the paired and gross comparison regressions. These differences were found to be extreme only for the Western Midwest and Plains area. The standard DHIA age factors appear acceptable for most purposes in other areas, except for cows calving beyond 11 years of age. Gross regression curves for five regions were used as a criterion of regional differences in rate of maturity. Differences at immature ages were not large and do not indicate that cattle in the Deep South mature more slowly than those of more temperate areas.