Potential roles of nitrate and live yeast culture in suppressing methane emission and influencing ruminal fermentation,digestibility, and milk production in lactating Jersey cows

Concern over the carbon footprint of the dairy industry has led to various dietary approaches to mitigate enteric CH₄ production. One approach is feeding the electron acceptor NO₃^?, thus outcompeting methanogens for aqueous H₂. We hypothesized that a live yeast culture (LYC; Saccharomyces cerevisiae from Yea-Sacc 1026, Alltech Inc., Nicholasville, KY) would stimulate the complete reduction of NO₃^? to NH₃ by selenomonads, thus decreasing the quantity of CH₄ emissions per unit of energy-corrected milk production while decreasing blood methemoglobin concentration resulting from the absorbed intermediate, NO₂^?. Twelve lactating Jersey cows (8 multiparous and noncannulated; 4 primiparous and ruminally cannulated) were used in a replicated 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments. Cattle were fed diets containing 1.5% NO₃^? (from calcium ammonium nitrate) or an isonitrogenous control diet (containing additional urea) and given a top-dress of ground corn without or with LYC, with the fourth week used for data collection. Noncannulated cows were spot measured for CH₄ emission by mouth using GreenFeed (C-Lock Inc., Rapid City, SD). The main effect of NO₃^? decreased CH₄ by 17% but decreased dry matter intake by 10% (from 19.8 to 17.8 kg/d) such that CH₄:dry matter intake numerically decreased by 8% and CH₄:milk net energy for lactation production was unaffected by treatment. Milk and milk fat production were not affected, but NO₃^? decreased milk protein from 758 to 689 g/d. Ruminal pH decreased more sharply after feeding for cows fed diets without NO₃^?. Acetate:propionate was greater for cows fed NO₃^?, particularly when combined with LYC (interaction effect). Blood methemoglobin was higher for cattle fed NO₃^? than for those fed the control diet but was low for both treatments (1.5 vs. 0.5%, respectively; only one measurement exceeded 5%), indicating minimal risk for NO₂^? accumulation at our feeding level of NO₃^?. Although neither apparent organic matter nor neutral detergent fiber digestibilities were affected, apparent N digestibility had an interaction for NO₃^? × LYC such that apparent N digestibility was numerically lowest for diets containing both NO₃^? and LYC compared with the other 3 diets. Under the conditions of this study, NO₃^? mitigated ruminal methanogenesis but also depressed dry matter intake and milk protein yield. Based on the fact that few interactions were detected, LYC had a minimal role in attenuating negative cow responses to NO₃^? supplementation.