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

SUMMA!~YThe 180-day production records of 1,344 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 100-1b 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 100-1b 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 .0, 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 fore" lactations were combined.Because of the negative correlations, less progress would be expected in selecting for both increased body weight and production than if they were independent or positively correlated.Analysis of the dsta indicated that as much and probably more of the variation in production was associated with body weight as with age.Breeders of dairy cattle have talked much about size, and size has received considerable attention in most type-score cards; however, no cattle breeders and few researchers have investigated the genetic relationships between size and production.Some have considered