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A study was conducted to determine whether enteric methane (CH 4 ) 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 CH 4  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 CH 4  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 1 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 CH 4  emissions from cattle. Restricting intake reduced CH 4  emissions (169 vs. 114 g d -1 ; P &lt; 0.002), with the reduction in CH 4  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 CH 4  (% GEI). The proportion of GEI lost as CH 4  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 CH 4  emissions. In contrast, maximizing feed intake above maintenance energy requirements increases daily CH 4  emissions, but improves efficiency of CH 4  conversion because CH 4 , as a percentage of GEI, declined. Thus, feeding cattle for maximum gain is an important CH 4  mitigation strategy for the cattle industry as it reduces the proportion of feed energy lost as CH 4  each day, as well as, reduces the number of days to market and associated CH 4  production. Key words: Cattle, methane, greenhouse gasses

Enteric methane emissions from growing beef cattle as affected by diet and level of intake