PSX-17 Effects of hair shedding and Mashona service sires on the performance of spring-calving Dexter cows.

Abstract Hyperthermia adversely affects cattle performance in the southeastern US. One approach to mitigating heat stress is selecting individual animals better able to handle summer temperature and humidity within a heat-sensitive breed. Another option is crossbreeding with a heat-tolerant breed. This study investigated how service sire breed and hair shedding level affected the performance of Dexter (Bos taurus) cows bred in summer for spring calves. Hair-shedding scores, ranging from 0% (no winter hair loss) to 100% (full winter hair loss), were recorded for 108 cows at 2-week intervals from April to July over 5 years. Across the study period, 381 matings to Dexter and Mashona (Sanga) bulls occurred for 8 weeks starting in early July. Mean daily temperature in early July was 28.6°C. Calving rate (CR) and adjusted calf weaning weight were evaluated. Calves (n = 196) were weaned in December at 7 months of age. Cows were classified as high (HiS) or low (LoS) hair shedders using shedding threshold values that varied by recording date. Data were analyzed using SAS models that included shedding status group, service sire breed, and their interaction along with other sources of variation. The mean hair shedding in the herd increased from 12% in late April to 93% in late July across the five years. Hair shedding group as determined in early July and service sire breed interacted (P < 0.05) to affect CR. Within the Mashona sire group, HiS cows had a higher CR rate than LoS cows (81 vs 53 ± 9%). Hair shedding did not affect (P > 0.1) CR within the Dexter sire group (HiS: 58 ± 9%; LoS: 48 ± 9%). Within the HiS cow group, Mashona sires generated higher (P < 0.05) CR than Dexter sires (81 vs. 58 ± 9%). Mashona and Dexter sires did not differ (P > 0.1) within the LoS cow group for CR (53 vs. 48 ± 9%). As a main effect, the hair shedding level affected CR as HiS cows had a higher (P < 0.05) CR than LoS cows (73 ± 9 vs. 53 ± 12%). Hair shedding group as determined in mid-May and sire breed did not interact (P > 0.1) for calf weaning weights. Weaning weights were heavier (P < 0.05) for calves of HiS cows compared to LoS cows (127.0 ± 9.5 vs. 120.6 ± 9.6 kg). Mashona sires produced heavier (P < 0.05) calves at weaning than Dexter sires (134.1 ± 9.3 vs. 114.2 ± 9.2 kg). Preliminary results indicate that elevated hair shedding level and crossbreeding with Mashona bulls enhanced cow-calf performance in the spring-calving Dexter herd. Both test factors represented viable options to use for reducing the impact of summer heat on cow herd performance.