Effects of heat stress conditions and dietary organic acid and pure botanical supplementation on gastrointestinal permeability and plasma trimethylamine N-oxide concentrations in lactating cows.

In dairy cows, heat stress may develop with a modified gut microbiome, thus altering plasma concentrations of microbial-derived trimethylamine N-oxide (TMAO) with a concomitant change in gastrointestinal permeability (GP). Dietary organic acid and pure botanical (OA/PB) feeding may prevent these outcomes. Forty-eight Holstein cows (208 ± 4.65 d in milk [mean ± SD], 3.0 ± 0.42 lactations, 122 ± 4.92 d pregnant) were enrolled 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), heat stress (HS) conditions (HSCon; 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. An oral Cr-EDTA challenge was performed to measure GP on d 3 and 13. Blood was collected on d −1, 3, and 14. Plasma Cr and TMAO were quantified. Data were analyzed using a mixed model including fixed effects of treatment, time, and their interaction. Contrasts included HS-Con vs. TN-Con, HS-Con vs. TN-PF, and HS-Con vs. HS-OAPB. HS-Con had greater plasma Cr area under the curve (AUC; P = 0.05) and tendency for greater Cr AUC (P = 0.12) on d 3, relative to TN-Con and TN-PF, respectively. HS-Con had similar plasma Cr AUC on d 13, relative to TN-PF and TN-Con. TN-PF tended to have greater plasma Cr concentrations from h 12 to 24 post bolus on d 13, relative to TN-Con (Treatment × Time, P = 0.13). HS-Con had lower plasma TMAO concentrations on d 3 and 14, relative to TN-Con or TN-PF (P < 0.01). HS-OAPB plasma Cr AUC or TMAO concentrations were not different from HS-Con on d 3 or 14. We conclude that heat stress increases GP in cows independent of changes in intake or OA/PB feeding, and decreases in plasma TMAO are suggestive of a modified gut microbiome during HS.