Effects of heat stress conditions and dietary organic acid and pure botanical supplementation on milk fatty acid composition in relation to gut permeability.

To evaluate the effects of heat stress (HS) and dietary organic acid and pure botanical (OA/PB) supplementation on milk fatty acids (MFA), we enrolled 48 Holstein cows (208 ± 4.65 d in milk; 3.0 ± 0.42 lactations) in 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: 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 total mixed ration top-dressed without (triglyceride only) or with OA/PB. Cows were milked twice daily. Milk samples were collected on d −2, 3 and 13. Cr-EDTA (gut permeability [GP] marker) was drenched on d 3 and 13. 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. Pearson correlation was used to define relationship between MFA and GP (area-under-the-curve) on d 3 and 13. Milk concentrations and yields of de novo and mixed MFA were greater while preformed, total monounsaturated, and C18:0 concentrations were lower in TN-Con, relative to HS-Con (P < 0.01). Milk concentrations and yields of cis-9,trans-11-C18:2 and de novo MFA were increased while C18:0 concentrations were decreased in TN-PF, relative to HS-Con (P < 0.05). MFA concentrations and yields were not modified by HS-OAPB, relative to HS-Con. Concentrations of mixed MFA on d 3 (r = −0.43; P< 0.01) and yields of cis-9,trans-11-C18:2, total polyunsaturated, trans-11-C18:1 and cis-9,cis-12-C18:2 on d 13 (r = −0.30 to −0.33; P ≤ 0.05) were negatively correlated with GP. We conclude that MFA production is modified by HS but OAPB feeding does not restore these responses. Moreover, the concentrations of select MFA are weakly correlated withgut permeability in dairy cattle.