Influence of a feed additive containing vitamin B12 and yeast extract on milk production and body temperature of grazing dairy cows under high temperature-humidity index environment

A commercial feed additive, OmniGen-AF (R) (OMN; Phibro Animal Health, Teaneck, NJ), has shown positive impacts on thermoregulation and milk production in confined dairy cows. This study was carried out to evaluate whether OMN has similar effects on grazing dairy cows. For the latter purpose, feed intake, milk production, blood metabolites, and body temperature (including surface, rectal, and thermal imaging) were assessed. This study was performed in a commercial dairy farm at Mato Grosso do Sul State in Brazil and lasted for 84 d. Thirty-six multiparous Holstein cows (136 +/- 52 days in milk, 23.8 +/- 4.96 kg/d milk yield, and 518 +/- 65 body weight, at the start of experiment) were enrolled to a completely randomized experiment and assigned either to control (0 g/d of OMN) or OMN (50 g/d). Cows were maintained under a rotational grazing system in pens uniformly covered with Panicum maximum (cv. Mombasa). After milking, cows were fed fixed amounts of corn silage with concentrate level adjusted according to average milk yield from the previous wk. OMN was top dressed and hand mixed in the upper portion of corn silage and concentrate mixture. The average temperature humidity index (THI) throughout experiment was 77.8 (29.8 degrees C air temperature and 52 +/- 15.6% relative humidity), suggesting that heat stress (THI > 68) likely impacted cows. OMN increased pasture intake, milk and milk fat yields, and milk fat content. OMN decreased urea N concentration and somatic cell count in milk. Treatments neither influenced body weight nor body condition score. OMN increased blood glucose and ionized calcium concentrations, and decreased blood aspartate aminotransferase and alkaline phosphatase concentration. OMN reduced rectal temperature, body surface temperature, and respiration rate of cows. This study showed evidence that OMN improves performance and reduces signs of heat stress of grazing cows under an environment with high THI.