Effect of Body Weight and Age at Calving on Milk Production in Holstein Cattle

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Abstract
The 180-day production records of 1344 lactations of Holstein cows including records of lactations one through eight, collected from six herds, were used to determine the independent influences of age and weight on production. A multiple regression analysis of first lactation data indicated that for a constant age milk production increased 134 lb and fat 7.8 lb for each 100-lb increase in body weight, and for a constant weight each increase of one month of age was accompanied by an increase of 46 lb of milk and 1.2 lb of fat. When all lactations were combined and the number of the lactation ignored, milk production increased by 400 lb and fat production by 14.4 lb for each 100 lb increase in weight. For a constant weight each month increase in age was accompanied by an increase of 28.4 lb of milk and 0.9 lb of fat. Heritability estimates based on 385 daughter-dam pairs for body weight, milk production, and fat production were .29, .44, and .40, respectively, when obtained from first lactation data, and were .19, .43, and .41 for all lactations (1 through 4) combined. Genetic correlations between total milk and fat production were of the order of .90. Those between body weight and production ranged from near zero in the first lactation to -.53 in the second lactation. The genetic correlation between weight and milk was -.12 and that between weight and fat -.23, when the first four lactations were combined. Formulas were developed for adjusting milk and fat production for variations in weight and age independently of the breeding value for production.