Abstract
Heterosis and reciprocal differences for immature weights, mature weight, degree of maturity of body weight (u), absolute growth rate (AGR), absolute maturing rate (AMR), relative growth rate (RGR) and age at puberty were examined for 208 straightbred and crossbred Hereford, Angus and Shorthorn females. The cows were born from 1960 to 1963 at the Fort Robinson Beef Cattle Research Station, by 16 Hereford, 17 Angus and 16 Shorthorn sires. Mature weight was defined as weight, adjusted by regression for condition score, taken in January, 1969, when the cows were 6 to 9 years of age. Average heterosis for mature weight was 2.5% (P<.05), but neither heterosis for specific two-breed crosses nor reciprocal differences were significant for mature weight. The increased AMR's of crossbred calves through 550 days of age supported the hypothesis that a primary effect of heterosis is to accelerate the maturing process. Heterosis for u was significant at all constant ages except at birth. The largest heterotic effect was for crosses involving Hereford. Reciprocal differences were unimportant for u, AMR and RGR. Heterosis for RGR was significant (P<.05) only during the first wintering period—a time of increased environmental stress. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.