Effects of Rumen-Protected Methionine on Milk Production Performance, Feed Efficiency, and Economic Parameters in Early Lactation Dairy Cows Under Heat Stress

Background and Objective: Heat stress is a fundamental challenge in dairy cattle production in hot regions, particularly during early lactation, causing reduced feed intake, metabolic disorders, and decreased milk production. This stress also disrupts the metabolism of proteins and essential amino acids. Methionine, the limiting essential amino acid in dairy cow diets, plays a key role in protein synthesis, energy metabolism, and metabolic health, but its bioavailability is limited due to ruminal degradation. Under heat stress conditions, the requirement for specific amino acids such as methionine may increase due to metabolic changes and increased need for antioxidants (such as glutathione, in which methionine plays a role in synthesis). However, limited information exists regarding the effects of rumen-protected methionine on the performance of early lactation dairy cows under heat stress. Rumen-protected methionine, which is resistant to microbial degradation in the rumen, can improve milk production, protein percentage, and feed efficiency. Under heat stress conditions, methionine requirements likely increase due to metabolic changes and increased antioxidant needs. This research was conducted to investigate the effect of rumen-protected methionine on milk production performance and economic parameters in early lactation Holstein cows under heat stress.
Materials and Methods: This study used 24 lactating Holstein cows (12 cows per treatment) with average days in milk (91 ± 12), milk production (37 ± 3), and parity (2.4 ± 0.1) at the University of Tehran research farm. The present study was conducted using repeated measures designs with 2 experimental treatments including control treatment: basal diet (containing zero levels of rumen-protected methionine) and methionine treatment: basal diet plus methionine (containing 25 grams of rumen-protected methionine per day per lactating cow).
Results: Results showed that raw milk production (37 vs. 34.8 liters per day) (P < 0.05) and energy-corrected milk (38.8 vs. 35.8 liters per day) (P < 0.05) were significantly higher for cows receiving rumen-protected methionine compared to the control group. Feed efficiency calculated based on raw milk, fat-corrected milk, and energy-corrected milk was significantly higher in the methionine group. Regarding milk composition, fat percentage (3.42 vs. 3.18 percent) and total milk solids (12.9 vs. 11.1 percent) significantly increased in the methionine-containing treatment. Milk fat yield (1.32 vs. 1.17 kg per day) and total milk solids yield (4.97 vs. 4.10 kg per day) were also significantly higher in the rumen-protected methionine group. Economic parameters showed that milk sales revenue for the methionine treatment compared to the control treatment (13.44 vs. 12.99 US$) was significantly higher (P< 0.02), and income over feed cost for the methionine treatment compared to the control treatment (5.86 vs. 5.33 US$) was also significantly higher (P<0.03). In fact, daily consumption of 50 grams of methionine supplement equivalent to 0.53 US$ per cow generated profit for the farmer.
Conclusion: The use of rumen-protected methionine in early lactation dairy cow diets can be considered as an ideal additive in nutritional management of early lactation dairy cows under heat stress due to improved milk production and composition as well as improved economic parameters.