The effects of different vitamin mixtures on growth performance, rumen fermentation parameters, serum metabolites, and heat stress biomarkers in Hanwoo heifers under high temperature-humidity condition.

OBJECTIVE: Heat stress impairs feed intake, metabolic balance, and productivity in cattle, and nutritional strategies are needed to maintain productivity under hot conditions. This study evaluated the effects of two complete commercial vitamin premixes on growth performance, rumen fermentation, blood profiles, and stress-related biomarkers in Hanwoo heifers under high temperature-humidity conditions. METHODS: Fifteen Hanwoo heifers (initial body weight 647 ± 35 kg; age 27.7 ± 0.40 months) were assigned to a replicated 3 × 3 Latin square design with three 21-day periods. The treatments were a control, Mix A, and Mix B, with both premixes top-dressed onto the morning ration at 0.4% of daily dry matter intake. Growth performance, feed intake, rumen fermentation, hematological and serum biochemical profiles, and serum cortisol and heat shock proteins 70 and 90 were measured. RESULTS: The mean temperature-humidity index was 78.6, with a mean of 74.6 at 05:00, indicating sustained heat load. Mix A increased forage dry matter intake compared with the control (1.27 vs. 1.18 kg/d; p = 0.0187), whereas Mix B was intermediate. Mix B reduced the molar proportion of ruminal iso-butyrate (p = 0.0086) and serum total protein (p = 0.0023) and showed a tendency toward greater average daily gain (p = 0.0820). Serum albumin was lower in Mix B than in Mix A, with the control being intermediate (p = 0.0460). Mix B also showed a lower serum cortisol than the control, whereas Mix A was intermediate (p = 0.0372). CONCLUSION: The premixes produced distinct mixture-specific responses under high temperature-humidity conditions. Mix A increased forage intake, whereas Mix B altered ruminal iso-butyrate, serum protein variables, and the cortisol concentration.