FIXED EFFECTS IN THE FORMATION OF A COMPOSITE LINE OF BEEF-CATTLE .1.EXPERIMENTAL-DESIGN AND REPRODUCTIVE-PERFORMANCE

Red Angus (RA) dams were mated to Charolais (C) or Tarentaise (T) sire s to produce crossbred (F1) progeny. Members of the F1 generation, dif fering in breed direct effects, were mated to produce an F2 generation with an expected breed composition of 1/2 RA, 1/4 C, and 1/4 T. Two b reed groups within the F2 generation differing in breed maternal effec t ([C x RA] x [T x RA] and [T x RA] x [C x RA]) were identified separa tely. These breed groups were crossed to produce an F3 generation and, likewise, the two resulting F3 generation breed groups were crossed t o produce an F4 generation. No distinction was made among breed groups subsequent to the F3 generation. Pregnancy rates averaged 90.9% over 11 yr, with 82.0% of cows exposed weaning a calf. Among formative gene rations of this composite population, F2 had greatest pregnancy, calvi ng, and weaning rates. Age of dam significantly affected pregnancy rat e, calving difficulty, and gestation length. Older cows tended to expr ess higher pregnancy rates and longer gestation lengths than did young er cows (P < .01). Males calves had a 1.7% greater weaning rate than f emale calves (P < .05), but matings producing male calves had longer g estation lengths (P < .05) and were 8.5% more likely to experience cal ving difficulty (P < .01). Individual breed additive effects (calves f rom C or T sires mated to RA dams) were important for calving difficul ty only (P < .05), where C-sired matings experienced greater calving d ifficulty. Differences between C x RA and T x RA dams, indicative of m aternal breed additive effects, were not detected. Individual heterosi s was large and positive for pregnancy rate (P < .01) and calving rate (P < .01) but negative for weaning rate (P < .01). Maternal heterosis was significantly positive for pregnancy rate and calving rate.