Enteric methane emissions and lactational performance of Holstein cows fed different concentrations of coconut oil

To determine if dietary medium-chain fatty acids (FA; C₈ to C₁₄) may mitigate enteric methane emissions, 24 cows were blocked by body size (n=2) and randomly assigned to 1 sequence of dietary treatments. Diets were fed for 35 d each in 2 consecutive periods. Diets differed in concentrations of coconut oil (CNO; ∼75% medium-chain FA): 0.0 (control) or 1.3, 2.7, or 3.3% CNO, dry matter basis. The control diet contained 50% forage (74% from corn silage), 16.5% crude protein (60% from rumen-degradable protein), 34% neutral detergent fiber (NDF; 71% from forage), and 28% starch, dry matter basis. Data and sample collections were from d 29 to 35 in environmentally controlled rooms to measure methane (CH₄) production. Methane emitted was computed from the difference in concentrations of inlet and outlet air and flux as measured 8 times per day. Control cows emitted 464g of CH₄/d, consumed 22.9kg of DM/d, and produced 34.8kg of solids-corrected milk/d and 1.3kg of milk fat/d. Treatment with 1.3, 2.7, or 3.3% dietary CNO reduced CH₄ (449, 291, and 253g/d, respectively), but concomitantly depressed dry matter intake (21.4, 17.9, and 16.2kg/d, respectively), solids-corrected milk yield (36.3, 28.4, and 26.8kg/d, respectively), and milk fat yield (1.4, 0.9, and 0.9kg/d, respectively). The amount of NDF digested in the total tract decreased with increased dietary CNO concentrations; thus, CH₄ emitted per unit of NDF digested rose from 118 to 128, 153, and 166g/kg across CNO treatments. Dietary CNO did not significantly affect apparent digestibility of CP but increased apparent starch digestibility from 92 to 95%. No FA C₁₀ or shorter were detected in feces, and apparent digestibility decreased with increasing FA chain length. Coconut oil concentrations of 2.7 or 3.3% decreased yields of milk FA <C₁₂ and >C₁₄. The highest milk fat concentration (3.69%; 1.3% CNO) was due to the greatest yields of C₁₂ to C₁₆ milk FA. Milk FA concentrations of C₁₈:₂ ₜᵣₐₙₛ₋₁₀,cᵢₛ₋₁₂ were related to increased dietary CNO concentrations and presumably to depressed ruminal NDF digestion. Moderate dietary CNO concentrations (e.g., 1.3%) may benefit lactational performance; however, CNO concentrations greater than or equal to 2.7% depressed dry matter intake, milk yield, milk fat yield, and NDF utilization. If mitigation of enteric CH₄ emissions is due to decreased digestion of dietary NDF, then this will lessen a major advantage of ruminants compared with nonruminants in food-production systems. Thus, CNO has limited use for enteric CH₄ mitigation in lactating dairy cows.