Grading and rotational crossbreeding of beef cattle. II. Calf performance to weaning.

Purebred Angus (A), Polled Hereford (PH) and Santa Gertrudis (SG) bulls were mated to grades, two-breed and three-breed rotational crosses of these breeds (seven herds) to produce three generations (G1, G2, G3) of calves. Birth weight and weaning traits were evaluated for herd differences, heterosis and effects of various genetic components. For each generation, the mean birth weight of SG exceeded that of PH which exceeded A (P less than .05). Averaged over generations, A and PH performed similarly for daily gain, 205-d weight and 205-d weight/cow exposed while SG exceeded A and PH for these traits. For feeder score, the breeds ranked A, PH, SG. For birth weight, daily gain, 205-d weight and 205-d weight/cow exposed, breeds ranked SG, PH, A for average additive genetic effects on the traits. Heterozygotic effects due to breed crosses, averaged over the three generations, were positive for all traits and generally significant, with PH X SG displaying more heterosis than the other two crosses. The maternal effects for A and SG were greater than PH for all traits except birth weight. Of the traits studied, from 12 (205-d wt/cow exposed) to 52% (205-d wt) of the variation averaged over generations was accounted for by a partial regression model that determined breed additive, breed maternal, heterosis and average maternal heterosis effects. Also averaged over generations, from .3 (205-d wt/cow exposed) to 1.7% (avg daily gain) additional variation was due to factors that may have included specific maternal heterosis, epistasis and linkage.