Effects of heat stress conditions and dietary organic acid and pure botanical supplementation on milk production in Holstein cows.

To evaluate the effects of heat stress (HS) environmental conditions and dietary organic acid and pure botanical (OA/PB) supplementation on lactation performance, we enrolled 48 Holstein cows (208 ± 4.65 d in milk [mean ± SD], 3.0 ± 0.42 lactations, 122 ± 4.92 d pregnant) in a study with a completely randomized design. Following a 7-d acclimation in thermoneutral conditions (temperature-humidity index [THI] 68), cows were assigned to 1 of 4 groups (n = 12/group): thermoneutral conditions (TN-Con), HS conditions (HS-Con; diurnal THI 74 to 82), thermoneutral conditions pair-fed to match HS-Con (TN-PF), or HS fed OA/PB (HS-OAPB; 75 mg/kg of body weight; 25% citric acid, 16.7% sorbic acid, 1.7% thymol, 1.0% vanillin, and 55.6% triglyceride; AviPlusR, Vetagro, Italy) for 14 d. Cows were fed a corn silage based total mixed ration top-dressed without (triglyceride only) or with OA/PB. Cows were milked twice daily. Blood was collected on d −1, 1, 2, 3, 7, and 14 for plasma urea N (PUN) quantification. Data were analyzed using a mixed model including fixed effects of treatment, time, and their interaction. Planned contrasts included HS-Con versus (vs.) TN-Con, HS-Con vs. TN-PF, and HS-Con vs. HS-OAPB. Rectal and skin temperatures, and respiration rates were greater in HS-Con and HS-OAPB, relative to TN-Con and TN-PF (P < 0.01). HS-Con had greater water intake, and lower yields of milk and milk lactose and protein, relative to TN-PF (P < 0.01). Yields of energy-corrected milk (ECM) were lower in HS-Con, relative to TN-PF (P < 0.01). HS-OAPB had greater water intakes (P = 0.05) and tendency for greater dry matter intakes, relative to HS-Con (P = 0.14). HS-OAPB had greater ECM yields, relative to HS-Con (P = 0.05). This was due to greater milk protein yields (P = 0.05) and tendency for greater milk yields (P = 0.12) in HS-OAPB, relative to HS-Con. Milk urea N and PUN concentrations were lower in HS-OAPB cows, relative to HS-Con (P < 0.01), which suggests improved N efficiency. We conclude that dietary OA/PB supplementation partially restores lactation performance in heat-stressed dairy cows.