Sources of Variation in the Birth Weight of Holstein-Friesian Calves

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Abstract
Birth wts. of 395 [male] and 399 [female] Holstein-Friesian calves were obtained from 3 Wisconsin State Dept. of Public Welfare herds. The data were analyzed to determine some of the causes of variation in birth wt. and to compare the birth wt. of outbred and inbred (Fx > 0.06) offspring by the same sire. The average birth wts. were 92.9 and 85.5 lb. for the outbreds and inbreds, respectively. Male calves averaged 5.2 lb. heavier than [female] [female] , and calves from "first calf" heifers were about 10 lb. lighter than calves from older dams. Herd differences accounted for about 22%, sex for 7% and calving sequence for 14% of the total variation in birth wt. Season and year of calving had little or no effect on birth wt. Approx. 60% of the remaining variation in birth wt. was ascribed to the heredity of the calf. The average intra-sire partial regression of birth wt. on inbreeding percentage, after adjustment for influence of size of dam, was [long dash]0.28 lb. The partial regression coeffs. varied significantly from sire to sire, apparently due to differences in the average birth wt. transmitted by different sires. The inbred calves of sires transmitting heavy birth wts. tended to be heavier than the out-bred calves because of having more of their sires'' genes, which cancels, at least in part, the reduction in birth wt. from increased homozygosity.

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