RELATION OF PREPUBERTAL SERUM STEROID PROFILES TO FIRST LACTATION MILK YIELD

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Abstract
Correlation between prepubertal serum sex steroids and subsequent lactation performance was investigated with 98 Holstein, Ayrshire and crossbred heifers in the hope of finding a precise criterion for early selection. Blood samples were collected from the heifers at 100, 130, 160 and 190 d of age. Initially samples from the first 25 animals to provide a complete set of samples and completing their first lactation, were analyzed for progesterone, androstenedione, testosterone, estrone and estradiol. The relation between each steroid level at each age and first lactation milk yield was assessed by linear regression analysis. Day 130 androstenedione was found to be the variable most highly correlated with milk production. The day 130 steroid levels of all 98 heifers were then analyzed and relation of steroid level to first lactation milk yield was assessed by multiple linear regression procedures which treated breed as a dummy variable. Although the best model using the complete set of data accounted for 23% (R2) of the variation in milk production, 18% was due to breed alone. The reported usefulness of prepubertal steroid levels for predicting lactation performance was not confirmed. Key words: Prediction, lactation yield, steroids, heifers