Mitigation of enteric methane emissions through improving efficiency of energy utilization and productivity in lactating dairy cows

The data set used in the present study was obtained from 20 energy metabolism studies involving 579 lactating dairy cows (511 Holstein-Friesian, 36 Norwegian Red, and 32 Jersey-Holstein crossbreds) varying in genetic merit, lactation number, stage of lactation, and live weight. These cows were offered diets based on grass silage (n = 550) or fresh grass (n = 29), and their energy intake and outputs, including methane energy (CH₄-E), were measured in indirect open-circuit respiration calorimeter chambers. The objective was to use these data to evaluate relationships between CH₄-E output and a range of factors in animal production and energetic efficiency in lactating dairy cows under normal feeding regimens. The CH₄-E as a proportion of milk energy output (E_l), E_l adjusted to zero energy balance (E_l(0)), or intakes of gross energy (GE), digestible energy (DE), or metabolizable energy (ME) was significantly related to a wide range of variables associated with milk production (E_l and E_l(0)) and energy parameters (energy intake, metabolizability, partitioning, and utilization efficiencies). Three sets of linear relationships were developed with experimental effects removed. The CH₄-E/GE intake (r² = 0.50-0.62) and CH₄-E/E_l (r² = 0.41-0.68) were reduced with increasing feeding level, E_l/metabolic body weight (MBW; kg^0.75), E_l(0)/MBW, GE intake/MBW, DE intake/MBW, and ME intake/MBW. Increasing dietary ME/DE decreased CH₄-E/E_l (r² = 0.46) and CH₄-E/GE intake (r² = 0.72). Dietary ME concentration and ME/GE were also negatively related to CH₄-E/GE intake (r² = 0.47). However, increasing heat production/ME intake increased CH₄-E as a proportion of E_l (r² = 0.41), E_l(0) (r² = 0.67) and energy intake (GE, DE, and ME; r² = 0.62 and 0.70). These proportional CH₄-E variables were reduced with increasing ratios of E_l/ME intake and E_l(0)/ME intake and efficiency of ME use for lactation (r² = 0.49-0.70). Fitting CH₄-E/E_l or CH₄-E/E_l(0) against these energetic efficiencies in quadratic rather than linear relationships significantly increased r² values (0.49-0.67 vs. 0.59-0.87). In conclusion, CH₄-E as a proportion of energy intake (GE, DE, and ME) and milk production (E_l and E_l(0)) can be reduced by increasing milk yield and energetic efficiency of milk production or by reducing energy expenditure for maintenance. The selection of dairy cows with high energy utilization efficiencies and milk productivity offers an effective approach to reducing enteric CH₄ emission rates.