COMPARISON OF CROSSBRED COWS CONTAINING VARIOUS PROPORTIONS OF BRAHMAN IN SPRING-CALVING OR FALL-CALVING SYSTEMS .4. EFFECTS OF GENOTYPE X ENVIRONMENT INTERACTION ON LIFETIME PRODUCTIVITY OF YOUNG COWS

Lifetime productivity of young (2 to 6-yr-old) crossbred cows containing various proportions (0, 1/4, or 1/2) of Brahman breeding was evaluated using 188 spring-calving and 154 fall-calving cows. Cows were mated to Limousin sires to produce 1983 through 1986 calf crops. The 1987 calf crops were produced using Limousin and Salers sires. Significant (P < .10) genotype (crossbred cow group) x environment (season of calving) interactions were found for age at first calf and lifetime calving percentage and mature cow weight. Within the spring calving group, no differences were found between crossbred groups for age at first calf or lifetime calving percentage. However, within the fall-calving group, 0 and 1/2 Brahman cows from Angus dams calved earlier than did those from Hereford dams. Age at first calf also tended to increase as proportion of Brahman increased in the fall-calving group. Within the fall-calving group, lifetime calving percentage for 0 and 1/2 Brahman cows from Hereford dams was lower than for those from Angus dams. The opposite was true for the 1/4 Brahman crosses. In general, as the proportion of Brahman breeding increased, productivity, measured as weight per calf or weight weaned per year, increased; however, age at first calving increased also as proportion of Brahman increased. All spring-calving groups calved earlier in life (P < .05) than their respective fall-calving counterparts and with the exception of the 0 Brahman groups, which were similar, spring-calving groups produced a higher (P < .05) percentage of calves than did the fall groups. Differences between base cow breed (Angus vs Hereford) were found for age at first calf, lifetime calving percentage, and weight weaned per year with cows from Angus dams having an advantage over cows from Hereford dams. Cow weight differences were found with 1/4 Brahman cows being lighter at breeding than either 0 or 1/2 Brahman cows.