Genetic parameters of body size in purebred and crossbred dairy cattle.

Breed additive, breed maternal, and heterozygotic effects on body weight, height at withers, depth of fore-chest, girth of fore-chest, length from withers to pins, length from withers to hips, and length from hips to pins at 6, 12, 18, and 30 mo of age were estimated from linear regression analyses. Data were available on 87 purebred Ayrshire, Brown Swiss, Holstein, and 213 two-way, three-way, and five-eights crosses. Birth year, birth month, and age of dam were significant environmental effects. Ayrshire additive effects were smaller than Holstein for all traits at all ages except length from hips to pins at 6 and 12 mo of age. Additive effects for Brown Swiss were smaller than for Holstein for body weight at 12 mo, height at withers at all ages, depth of fore-chest and girth of fore-chest at 6, 12, and 18 mo, length from withers to pins and length from withers to hips at 12 and 30 mo, and length from hips to pins at 18 mo. Breed maternal effects were not significant for any trait at any age. Means of crossbreds generally exceeded means of parental breeds, especially body weight. Average heterosis for body weight was 3.0% at 6 mo, 4.6% at 12 mo, 5.0% at 18 mo, and 3.5% at 30 mo of age. Heterosis for weight increased with age through 18 mo and declined by 30 mo of age. Heterosis for other traits was generally 1 to 2% with no trend with age.