Organic acids and botanicals supplementation improves protein metabolism in heat-stressed dairy cows

To evaluate the effects of dietary organic acids and pure botanicals (OA/PB) on protein metabolism in heat-stressed dairy cattle, we enrolled 48 Holstein cows (208±4.65 days in milk [mean ± SD], 3.0±0.42 lactations, 122±4.92 d pregnant) in a completely randomized design trial. Following a 7-d acclimation in thermoneutral conditions (temperature-humidity index [THI] 68±0.32), cows were assigned to 1 of 4 groups (n=12/group): thermoneutral conditions (TN-Con), HS conditions (HS-Con; diurnal THI 74 to 82), TN 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; AviPlus R,Vetagro S.p.A, Italy) for 14 d. Cows were fed a corn-silage based total mixed ration top-dressed with OA/PB. All cows received top-dress equivalent for triglyceride used for microencapsulation. Blood was collected for plasma insulin quantification using a radioimmunoassay. Cows were milked twice daily. Data were analysed using a mixed model including fixed effects of treatment, time, and their interaction. Planned contrasts included HS-Con vs TN-Con, HS-Con vs TN-PF, and HS-Con vs HS-OAPB. Higher plasma insulin concentrations were observed in HS-Con, relative to TN-PF (P=0.03). Notably, HS-OAPB displayed similar plasma insulin concentrations compared to HS-Con (P=0.38; 0.97 and 0.86 ng/ml, respectively). HS-OAPB had greater milk protein yields and less milk urea-N (MUN) concentrations, and greater energy-corrected milk (ECM) yields, relative to HS-Con (P<0.05). The tendency for higher dry matter intakes (P=0.15; +1.4 kg/d) and increased water intakes (P<0.01; + 25 l/d) was observed for HS-OAPB, relative to HS-Con, may explain in part the increases in ECM. Such observations, combined with the elevated plasma insulin concentrations strongly suggest that OA/PB supplementation may be a means to partially restore milk production and improve N use efficiency in heat-stressed dairy cows.