57 Relationship between Proportion ofBos indicus Genetics of Dams and Calf Performance from a 31-Year Multibreed Herd Dataset

Abstract This is a 31-year retrospective cohort study aiming to evaluate the relationship between the proportion of B. indicus genetics of crossbred dams and the weaning weight of their calves. Our hypothesis was that crossbred dams with intermediate degree of B. indicus genetics wean heavier calves than those with low-to-high degree of B. indicus genetics at the subtropical condition. The cow-calf pairs (n = 4,901) were in six evenly distributed breed groups according to the proportion of Brahman: G0–19%, G21–34%, 38%, G41–59%; G63–78% and G81–100%. Dams reared contemporaneously were undergone to controlled breeding seasons where the six breed groups of dams and sires were reciprocally mated. Non-adjusted and 205d-adjusted weaning weight of calves were considered as dependent variables and analyzed using MIXED procedure of SAS 9.4. The following independent variables were included in the model as fixed effects: parity category order (primiparous, secundiparous and pluriparous), calving period (0-30d, 31-60d, 61-90d and 91-120d), gender of calf (male or female) and breed group. Year was included as a random effect. All possible interactions were tested and only those with P < 0.20 remained in the final model. Most of all, for the non-adjusted weaning weight, the breed group effect (P < 0.001) varied according to the calving period. Dams calving after 30d of calving season weaned the lightest calves. However, the magnitude of such losses was variable within breed groups. Calves from G0-19% and G81-100% of dams were the most and least impacted by calving period, respectively. The difference on weaning weight between calves born at 0-30d and 91-120d was 60kg for G0-19% of dams and 30kg for G81-100% of dams. For every other breed group, this difference ranged from 45kg to 55kg. In spite of such interaction effect, calves from G0-19% and G81-100% of dams were lighter (230.4 and 225.0 kg, respectively; SEM: 3.4) than any other breed group (238.3 to 242.5 kg). For the 205d-adjusted weaning weight, there was a breed group effect (P < 0.001), regardless of calving period. Lighter calves were still weaned from G0-19% and G81-100% of dams (214.9 and 206.5 kg, respectively, SEM: 2.8) compared with any other breed group (223.4 to 227.6 kg). Thus, crossbred dams with intermediate degree of B. indicus genetics weaned the heaviest calves, probably as result of outbreeding enhancement provided by the adaptability of B. indicus to the tropics and productivity of B. taurus.