Immune and Metabolic Effects of Rumen-Protected Methionine During a Heat Stress Challenge in Lactating Holstein Cows.

Multiparous, lactating Holstein cows (n = 32) were randomly assigned to 1 of 2 dietary treatments [TMR with rumen-protected Met (RPM) or TMR without RPM (CON)], and within each dietary treatment group cows were randomly assigned to 1 of 2 environmental treatment groups in a split-plot crossover design. In phase 1 (9 d) all cows were fed ad libitum and in thermoneutral conditions (TN). In phase 2 (9 d), group 1 (n = 16) was exposed to a heat stress (HS) challenge (HSC). Group 2 cows (n = 16) were pair-fed (PFTN) to HSC counterparts and remained in TN. After a 21-d washout period, the study was repeated (period 2) and the environmental treatments were inverted relative to treatments from phase 2 of period 1, while dietary treatments remained the same for each cow. During phase 1, cows in RPM had greater plasma Met concentration compared to cows in CON (59 µ M and 30 µ M, respectively; P < 0.001). Cows in PFTN had a greater decrease (P < 0.05) in plasma insulin than cows in HSC at 4 h (-2.7 µIU/mL vs. -0.7 µIU/mL) and 8 h (-7.7 µIU/mL vs. -0.4 µIU/mL) during phase 2. Compared to cows in PFTN, cows in HSC had an increase (P < 0.05) in plasma serum amyloid A (-59 µg/mL vs. +58 µg/mL), serum haptoglobin (-3 µg/mL vs. +33 µg/mL), plasma lipopolysaccharide binding protein (-0.27 µg/mL and +0.11 µg/mL), and plasma interleukin-1β (-1.9 pg/mL and +3.9 pg/mL) during phase 2. In conclusion, HSC elicited immunometabolic alterations; however, there were limited effects of RPM on cows in HSC. [ABSTRACT FROM AUTHOR]