Response of mid-lactation primiparous Holstein cows to the supplementation of rumen-protected methionine during the summer.

This study aimed to evaluate the effects of rumen-protected methionine (RPM) supplementation on productive and physiological responses of primiparous Holstein cows during summer. We hypothesized that RPM supplementation would maintain or improve milk yield and composition due to beneficial physiological, redox, and inflammatory responses in cows exposed to summer heat. The trial was conducted in a randomized block design during nine weeks in Brazil using 80 primiparous cows (182 ± 64 DIM; 42.9 ± 4.7 kg/d milk). Cows were blocked by milk yield and DIM and assigned to a control diet (CON; no added RPM) or the same diet supplemented with RPM (Mepron®, Evonik) at 0.75 g/kg diet dry matter, targeting 20 g/cow/day (product contains 62% metabolizable methionine) to the average cow. Milk yield and composition, vaginal temperature, respiratory rate, and plasma samples were collected in weeks 3, 6, and 9. Data were analyzed using mixed models including treatment, week, and their interaction as fixed effects, and block and cow as random effects. Cows were maintained under naturally occurring summer conditions. Environmental monitoring during weeks 3, 6, and 9 indicated elevated temperature-humidity index (THI) values, with values remaining above the heat-stress threshold (THI > 68) for 68.3% of the monitored hours (mean THI = 70.6; range 61.0-84.4). Overall (least squares mean across weeks 3, 6, and 9), RPM increased milk yield by 2.0 kg/d (44.9 vs. 42.9 kg/d), protein yield by 50 g/d (1,464 vs. 1,414 g/d), lactose yield by 108 g/d (2,109 vs. 2,001 g/d), and total solids yield by 176 g/d (5,331 vs. 5,155 g/d). Lactose concentration was lower in RPM (4.71 vs. 4.76%). Fat yield was unaffected, but a treatment × week interaction was observed for fat content. Milk fatty acid (FA) profile was unchanged, although treatment × week interactions were observed for individual fatty acids (C16:0, C18:0, C18:1, and preformed FA). Plasma glucose was lower, and insulin was higher in RPM than in CON cows (39.3 vs. 43.2 mg/dL and 0.52 vs. 0.35 ng/mL, respectively). Antioxidant capacity improved, with RPM cows having greater ferric reducing antioxidant power (32.9 vs. 28.5 µM) and lower malondialdehyde (2.48 vs. 2.78 nmol/mL). Other biochemical, inflammatory, and immune markers were unaffected. Respiratory rate was slightly higher in RPM than in CON cows (55 vs. 50 breaths/min). Mean vaginal temperature did not differ between treatments; however, a treatment × time × hour interaction was observed. Supplementation with RPM improved milk and solids yield, and enhanced antioxidant capacity and insulin levels, supporting its use to improve metabolic resilience under warm conditions.