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

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Abstract
The regression of lactation milk yield on age at freshening was computed by the paired and gross comparison methods within 4 geographical regions of the United States. A total of 405,330 lactations of registered DHIA Holstein cows calving between 1951 and 1962 were studied. Bias due to selection was assumed to be the primary cause of variation between the results of the paired and gross comparison regressions. These differences were extreme only for the Western Midwest and Plains area. The standard DHIA factors appear acceptable for most purposes, except at ages of 11 years or more. Gross regression curves for 5 regions were used as a criterion of regional differences in rate of maturity. Differences at immature ages were small and do not indicate that cattle in the South mature more slowly than those of more temperate areas.

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