Weight and age at calving and weight change related to first lactation milk yield.

Daily milk yields from 400 first lactations collected from one herd over 16 yr were utilized to ascertain relations of weight and age at calving and change of body weight during first lactation on milk yield and calving interval. These relationships were evaluated for the complete lactation and for each of five 60-day segments of the lactation. The influence of sire on the interrelationship between body weight, age at calving, and milk yield also were measured in data from sires with 10 or more daughters. Average milk yield (300 day) and gain of body weight during first lactation for all records were 5544 and 56.2 kg. Both year and season of calving influenced weight at calving, milk yield, and the relationship between the two. Milk yield was the greatest and body weight gain the least for heifers calving in the fall. Analysis of all records revealed that calving weight but not calving age accounted for a significant portion of variation of milk yield during the first four 60-day periods. Both calving weight and age accounted for a significant amount of the variation of total milk yield. There was a significant effect of sire on calving weight and milk yield but not on total weight gain, age at calving, number of services, or calving interval. There was an increase of number of services and a trend toward a longer calving interval with increasing milk yield. Although age and weight at calving were nearly equal for explaining variation of total yield of milk of first lactation, age at calving was of little value in explaining variation of milk yield of the 60-day intervals. The relationship of these observations to the use of age correction factors for extended first lactation records is discussed.