| The objective of this study was to assess the effects of a commercial carbohydrate-based feed additive on dry matter intake (DMI), milk yield, milk composition, and plasma metabolites during an acute period of heat stress (HS). Forty-eight mid-lactation Holstein dairy cows were blocked according to milk yield, days in milk, and parity and were then randomly assigned to one of two dietary treatments within block. Treatments were calculated to provide 100g (as fed) daily of either sucrose (control; CTL) or a commercial feed additive (RallyRTM, Purina Animal Nutrition, Shoreview, MN; RAL) administered twice daily as part of the total mixed ration (TMR). The experiment was divided into two periods consisting of 6.25 +/- 0.3 d under thermoneutral (P1) conditions, followed by 11.75 +/- 0.3 d of heat stress (P2) conditions (daily cyclical temperatures ranging from 23.8 to 30.2°C, temperature-humidity index of 69.2 to 75.5) in temperature-controlled environmental chambers. Daily DMI was determined using feed issue and refusal records. Milk yield was recorded daily and milk components were assessed for one 24-hour period on a twice weekly basis. Blood was sampled twice weekly and analyzed for concentrations of plasma glucose, beta-hydroxybutyrate, insulin, and nonesterified fatty acids (NEFA). All variables were analyzed using the Mixed procedure of SAS with repeated measures. There was no significant treatment difference during P1 for any of the variables measured. During P2, RAL cows displayed increased DMI (treatment by time, P = 0.05) and milk yield (treatment by time, P = 0.05) relative to CTL cows. Milk fat percentage tended to decrease to a greater extent in cows fed RAL (treatment by time, P < 0.07), but milk fat yield was not different between treatments (P > 0.20). Plasma NEFA concentrations of RAL cows tended to be lower (P < 0.1) than those of CTL cows during P2. Feeding RAL prior to and during a period of acute cyclical HS increased DMI and milk yield, and appeared to favor improved energy balance in heat-stressed mid-lactation dairy cows. |
