The effect of selection for heifer milk yield on the production level of mature cows

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Abstract
The effect of selection of dairy bulls on their daughters' 1st lactation yields has been investigated, using contemporary comparison information availreable for some 1273 bulls. It was shown that incomplete repeatability of individual records would introduce a negative bias into the regression of the increase in production of a bull's daughters with age on his contemporary comparison and this bias was calculated. The observed regressions on 1st lactation contemporary comparison were close to zero after correction for bias both for increase from 1st lactation yield to 2nd (— 0·08) and for the increase from 2nd lactation to 3rd (+ 0·09). Selection on 1st lactation yield should not, therefore, change the increase of yield with age, in agreement with results of other workers. The results are related to the problem of the heritability of the separate lactations.

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