Monolaurin in the Diet of Feedlot Finishing Cattle: Effects on Performance, Metabolism, Ruminal Environment, and Meat Fatty Acid Profile

This study evaluated the effects of monolaurin intake per finishing feedlot cattle on growth performance, metabolic status, ruminal environment, and meat fatty acid profile. Twenty-four castrated Holstein males (379 ± 8.5 kg; 12 months old) were randomly assigned to two treatments: basal diet (control) or basal diet with α-monolaurin (treated: 0.762 g/kg dry matter intake; ≈6.63 g/animal/day) for 79 days. Feed intake, body weight, and feed efficiency were recorded, and blood and ruminal samples were collected during the trial. Ruminal fermentation parameters, protozoa counts, hematological and biochemical variables, oxidative status biomarkers, ruminal microbiota composition (16S rRNA sequencing), and Longissimus dorsi fatty acid profile were analyzed. Monolaurin feed did not affect dry matter intake or final body weight, but increased total weight gain, average daily gain, and feed efficiency (p ≤ 0.05), indicating improved nutrient utilization. Hematological and serum biochemical variables were largely unchanged, although total leukocyte counts were lower in treated cattle. Animals receiving monolaurin showed reduced reactive oxygen species and lower superoxide dismutase activity, suggesting improved oxidative balance without changes in lipid peroxidation. During the adaptation phase (day 14), treated cattle exhibited lower acetate, propionate, valerate, and total volatile fatty acid concentrations and higher protozoa counts, but these differences disappeared by day 79, indicating ruminal adaptation. Microbiota diversity was not altered overall, although specific genera differed in relative abundance between treatments. In meat, monolaurin increased lauric, linoleic, and arachidonic acids, reduced palmitic and heptadecanoic acids, decreased total saturated fatty acids, and increased polyunsaturated fatty acids (p ≤ 0.05). Overall, dietary monolaurin improved feed efficiency, modulated oxidative status, induced transient ruminal microbial adjustments, and enhanced the nutritional quality of beef lipids without compromising metabolic health.