Effect of sustained-release antioxidant bolus on body condition score, blood parameters, uterine health, and some reproductive parameters in transition dairy cows under heat stress condition

The transition period for dairy cows is stressful, and if this occurs during heat stress conditions, it will become more challenging for them. This study aimed to evaluate the effect of sustained-release bolus (Each bolus consisted of a mixture of mineral salts including copper sulfate (8 g), sodium selenite (0.17 g), manganese sulfate (3.9 g), zinc sulfate (2.4 g), and vitamin A (0.47 g) on body condition score (BCS) change, serum metabolites, uterine health, and some reproductive parameters in transition cows with moderate or high pre-calving BCS. Four experimental treatments were (1) moderate BCS without bolus consumption (MB-Bo, n = 35), (2) moderate BCS with bolus consumption (MB + Bo, n = 35), (3) high BCS without bolus consumption (HB-Bo, n = 35), and (4) high BCS with bolus consumption (HB + Bo, n = 35). Results showed that after calving, negative energy balance occurred in all experimental groups. However, cows with high BCS (HB-Bo and HB + Bo) had greater (P = 0.02) BCS change during the postpartum period (0-40 days). Bolus administration decreased white blood cells count 14 days after calving (P = 0.02). Cows with moderate BCS (MB-BO and MB + Bo) had higher (P < 0.01) red blood cell count than cows with high BCS (HB-Bo and HB + Bo) on 14 days after calving. The cows in MB + Bo group had higher (P < 0.05) serum glucose and albumin and lower (P < 0.01) non-esterified fatty acids and beta-hydroxybutyrate. Moreover, this group of cows had higher (P < 0.05) serum total antioxidant capacity, glutathione peroxidase and superoxide dismutase, and lower malondialdehyde (P = 0.03) than other groups. In this regard, the increase in antioxidant capacity with the consumption of blues caused the HB-Bo group to have more incidence of metritis (P = 0.08) and endometritis (P = 0.08). The HB-Bo group had about 12 days longer (P < 0.01) days open than MB + Bo group. It was concluded that consumption of slow-release bolus containing antioxidant elements had positive effect on the metabolic and reproductive status of high-producing dairy cows under heat stress condition.