A study was conducted to determine whether enteric methane (CH4) emissions from growing feedlot cattle fed backgrounding diets based on barley silage could be reduced through grain supplementation. A second objective was to determine the effects of feed intake on CH4 emissions. Eight Angus beef heifers (initial and final body weight, 328 28 and 430 29 kg) were used. The experiment was designed as a split-plot crossover with two diets and two 8-week periods. The main plot was the diet [dry matter (DM) basis]: high forage (70% barley silage, 30% barley-based concentrate) or high grain (30% barley silage, 70% corn-based concentrate). The sub-plot was the feeding level: unrestricted (ad libitum feed intake, 5% orts) or restricted (65% of ad libitum intake) feed intake. Methane emissions were measured during each sub-plot over 3 d using whole animal chambers. Changing the forage to concentrate ratio and substituting barley for corn did not affect CH4 emissions (141.5 g d(-1); P = 0.26), and the average emission was about 10% higher than the emission calculated using the International Panel on Climate Change (IPCC) Tier I approach. Methane conversion rate was also similar for both diets [6.23% of gross energy intake (GEI), P = 0.29], and was similar to the value of 6.0 used in the IPCC Tier 2 approach to calculating CH4 emissions from cattle. Restricting intake reduced CH4 emissions (169 vs. 114 g d-1; P < 0.002), with the reduction in CH4 proportional to the decline in intake. Level of intake relative to maintenance energy requirements was moderately inversely related (r = -0.30; P = 0.04) to CH4 (% GEI). The proportion of GEI lost as CH4 declined by 0.77 percentage units per unit increase in level of intake above maintenance. This study shows that supplementing barley-silage-based diets with corn grain to increase diet quality has only small effects on reducing CH4 emissions. In contrast, maximizing feed intake above maintenance energy requirements increases daily CH4 emissions, but improves efficiency of CH4 conversion because CH4, as a percentage of GEI, declined. Thus, feeding cattle for maximum gain is an important CH4 mitigation strategy for the cattle industry as it reduces the proportion of feed energy lost as CH4 each day, as well as, reduces the number of days to market and associated CH4 production.
