Short‐Term Impact of Betaine Supplementation on Ruminal Microbial Relative Abundance, Nutrient Digestibility, Serum Metabolites, and Milk Composition in Heat‐Stressed Dairy Cows

This short-term study evaluated the effects of dietary betaine supplementation over 4 weeks on feed intake, milk production, nutrient digestibility, blood metabolites, and ruminal microbial communities in heat-stressed Holstein cows. Thirty lactating cows were randomly divided into a control group and a treatment group supplemented with 80 g/day of natural betaine extract for 4 weeks during summer. Betaine supplementation significantly increased dry matter intake (21.0 vs. 18.7 kg/day; p ≤ 0.05) and milk yield (35.4 vs. 31.2 kg/day; p ≤ 0.05) compared to the control. Milk composition, including fat and protein content, also improved (p ≤ 0.05). Cows receiving betaine had lower rectal temperatures (p = 0.02) and respiration rates (p = 0.01), indicating reduced heat stress. Serum concentrations of inflammatory markers-ceruloplasmin, haptoglobin, and serum amyloid A-were significantly lower in the betaine group by Week 4 (p ≤ 0.05). Plasma and rumen amino acid profiles, particularly lysine and methionine, were better preserved in the betaine group (p ≤ 0.05). Ruminal concentrations of volatile fatty acids increased (p ≤ 0.05), and the abundance of beneficial microbial species, such as Fibrobacter succinogenes, Butyrivibrio proteoclasticus, and Megasphaera elsdenii, was enhanced (p ≤ 0.05). These findings suggest that betaine supplementation is an effective nutritional strategy to improve dairy cow performance, reduce inflammation, and stabilise ruminal microbiota during heat stress.