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The Effect of Age and Dry Period on Production at Different Levels of Producing Ability^*

G. E. Dickerson and A. B. Chapman

University of Wisconsin

Abstract

This study is an attempt to learn whether the effect of developmental and management factors on production is proportional to the level of production, as is assumed in using percentage correction factors. Records of 274 cows tested during at least their first five lactations were selected from 41 Holstein herds for this study. The average 305-day lactation production of 1574 records of these cows was 362 pounds.

The herds were divided into three groups according to the average production of the cows in each herd (305-day lactation yield for the first five lactations of each cow, corrected for age and short preceding dry periods). Within each of these three groups of herds, the individual cow's average corrected production was adjusted for the difference between the mean of the group and the mean of that particular cow's herd. This adjustment was made by multiplying each cow's average production by the ratio of the group mean to the individual herd mean.

^* Paper from the Department of Genetics, Agricultural Experiment Station, University of Wisconsin, No. 255.

The writers wish to acknowledge the assistance given by W.P.A. in analyzing the data.