176 Relationships Among Early Life Weight and Growth Among Halfblood Bos IndicusMale Calves and Associations with Carcass Weight

Nellore-Angus steers (n = 572) born in 2009-2013, 2015, and 2019 and Brahman-Angus reciprocal steers (n = 73) born in 2019 and 2020 were evaluated for calf and carcass weights (HCW). Calves were born and reared at the McGregor Research Center and included F2, F3, F4, and F5Nellore-Angus and F1Brahman-Angus. Calves were weighed at 4 approximate times: within 48 h of birth (BWT), 50 d age (spring-working, SWWT, not available for 2009-2010), 180 d age (pre-weaning, PRWWT), and approximately 210 d age (weaning, WWT). Animals born in the same year were managed similarly, and cow herds were managed together in most cases except during annual natural service, multi-sire breeding seasons. Animals born in the same year were managed as contemporaries post-weaning and for feedlot finishing. Mixed model analyses of variance for calf traits included genetic type (nested within calf birth year), calf birth year, age of dam category, and covariates of Julian birth date (BWT, SWWT) or calf age (PRWWT and WWT) as fixed effects. In each year at least 2 genetic types were present; however, because of imbalance across years, only genetic types produced in the same years were compared following significant F-tests. For these analyses cow age category did not affect calf BWT or SWWT, but showed influence for PRWWT (P= 0.006) and WWT (P <0.001); cow age category did not affect calf ADG. Differences in calf weights (P <0.001) and calf ADG (P <0.001) due to the genetic type were driven by reciprocal differences between Brahman-Angus crosses and potential maternal hybrid vigor advantages for F2Nellore-Angus calves from F1dams vs. other genetic types. Brahman-sired F1(B × A) males were 12.7 to 14.1 kg heavier for BWT (P <0.001) and 14.9 to 19.2 kg heavier for SWWT (P <0.001) than Angus-sired F1males (A × B), but persistence to weaning varied across years. It appears in these data that some genetic types may be more sensitive to individual year effects, but it is uncertain why. Correlations of calf weight traits with HCW were inconsistent across genetic types. No calf weight was associated with HCW in A × B crosses, and only PRWWT in B × A related to HCW (r = 0.44, P= 0.007). Among Nellore-Angus crosses, BWT showed small to moderate correlation to HCW (0.23 < r < 0.46); however, among the F4and F5calves, later weights (SSWT, PRWWT and WWT) had large correlations with HCW (0.50 < r < 0.79) whereas these correlations ranged from 0.24 to 0.37 in F2and F3steers. It appears animals of similar Bos indicus percentage may have quite different calf growth patterns related to HCW.