Nutrient use and methane emissions in growing beef fed different protein sources and a pasture-based diet.

This study investigated the effects of different protein sources on feed intake, nutrient, and energy utilization, growth performance, and enteric methane (CH4) emissions in growing beef cattle, also evaluated against a pasture-based diet. Thirty-two Holstein × Angus growing beef were allocated to four dietary treatments: a total mixed ration (TMR) including solvent-extracted soybean meal as the main protein source (n = 8), TMR with local brewers' spent grains (n = 8), TMR with local field beans (n = 8), and a diet consisting solely of fresh-cut Italian ryegrass (GRA; n = 8). Every 4 wk, animals were moved to digestibility stalls within respiration chambers to measure nutrient intakes, energy and nitrogen (N) utilization, and enteric CH4 emissions. Feed intake (Calan gates), nutrient intakes, and CH4 emissions (GreenFeed) were also measured when animals were group-housed. In respiratory chambers, enteric CH4 yield per kg of dry matter intake (DMI), per kg of organic matter intake (OMI), and per kg body weight were lower (P < 0.05) for GRA. Feces and urine energy outputs were higher (P = 0.007 and P < 0.001, respectively) for GRA steers than concentrate-fed steers. Urinary nitrogen output (UNO, P = 0.026), manure (feces + urine) nitrogen output (MNO, P = 0.034), UNO/nitrogen intake (P = 0.002), and MNO/nitrogen intake (P = 0.006) were higher for GRA. During group-housing periods, CH4 emissions, measured by GreenFeed, were similar to those measured in chambers. Similar CH4 yield between treatments, expressed per kg digestible DMI and digestible OMI, may indicate that the lower diet digestibility was likely the reason for the reduced enteric CH4 emissions in pasture-based diets. The higher energy output and nitrogen losses, and the reduced nitrogen utilization for steers fed the fresh-cut ryegrass diet indicate less efficient energy and nitrogen utilization, which can be considered environmentally undesirable. The lower growth rates in the pasture-based system should also be accounted for when this is adopted for reducing production costs.