Effects of dietary fatty acids on milk production and ruminal greenhouse gas emissions in lactating dairy cows
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With climate change becoming a growing concern, ruminal methane emissions from livestock have come under increased pressure, especially in the U.S. dairy industry. As a result, evaluating feeding strategies has become increasingly important, as they offer a practical and accessible approach for farmers to reduce environmental impacts. For this experiment we investigated the effects of alternative dietary fatty acid (FA) supplementation strategies, substituting either fiber in non-isoenergetic (NONISO) plot or starch in isoenergetic (ISO) plot, on milk production, digestibility, FA profile, and ruminal gas emissions in lactating dairy cows. Forty-eight multiparous Holstein cows (94.4 ± 20.7 days in milk; 46.3 ± 6.5 kg milk/day) were assigned to a split-plot 4 × 4 Latin square design with 21-day periods. Main plots were based on ISO and NONISO plots containing 42% forage dry matter, with non-forage neutral detergent fiber (aNDFom) primarily from almond hulls. Within each plot (n = 24), cows received one of four treatments: (1) no FA supplementation (CON), (2) palmitic acid (HPA; 85% C16:0), (3) calcium-salts of FA (CSFA; 59% C16:0, 25% oleic acid [C18:1], and 3% eicosapentaenoic acid [EPA] plus docosahexaenoic acid [DHA]), or (4) whole cottonseed plus canola oil (OIL), each providing 2% supplemental FA on a dry matter basis. A mixed-effects model was used, including treatment, plot, period, square, and treatment × plot interaction as fixed effects, and cow nested within plot as a random effect. Dry matter intake (DMI), milk yield (MY), and energy-corrected milk (ECM) yield were affected by treatment only. In the NONISO plot, cows fed OIL had greater milk fat concentration relative to CON. In the ISO plot, cows fed HPA had higher milk fat concentration relative to CON. Milk protein concentration in the ISO plot was lower across all FA treatments relative to CON. Milk fat yield in the NONISO plot was highest in cows fed OIL, compared to other treatments. In the ISO plot, HPA increased milk fat yield relative to CON. Milk protein yield was not affected by treatment in the NONISO plot but was reduced by HPA and OIL in the ISO plot compared to CON. Methane production was unaffected by treatment in the NONISO plot. In the ISO plot, all FA treatments increased methane yield relative to CON. In the NONISO plot, HPA increased hydrogen yield relative to all other treatments. In the ISO plot, hydrogen yield increased for all FA treatments relative to CON. These results indicate that dietary FA supplementation alters milk composition and gas emissions, and that these effects are modulated by dietary energy density and ingredient substitution.