Methane emissions and rumen microbiome response during compensatory growth on either a forage or grain-based finishing diet in beef cattle.

The aim of this experiment was to evaluate the effect of the level of prior nutritional restriction during backgrounding in Angus steers on methane (CH₄) emissions, diet digestibility, rumen fermentation, and ruminal microbiome under either a forage or grain-based finishing diet. Eighty steers (body weight [BW]: 444 ± 39 kg, age: 18 ± 1 mo) were blocked and randomly assigned within the block to either an optimal (0.6 to 0.7 kg/d) or suboptimal (0.3 to 0.4 kg/d) growth rate to exploit compensatory growth (CG), during 97 d of backgrounding. Following, for 84 d, half of the steers in each group were finished on a forage diet while the other half were finished on a grain-based diet. During the backgrounding period, CH₄ emissions tended (P ≤ 0.07) to be higher; however, CH₄ intensity expressed by BW gain was 50% lower (P < 0.01) for optimal compared to suboptimal growth steers. BW gain, dry matter intake, diet digestibility, and ammonia nitrogen in the rumen were greater (P < 0.01) for optimal compared to suboptimal steers. During the finishing period, CH₄ emissions in either forage or grain finishing diets were similar (P > 0.05) for both backgrounding treatments. However, due to greater BW gain in suboptimal steers (1.20 vs. 0.97 kg/d), their CH₄ intensity-related coefficient decreased (P < 0.05) during the finishing period. Diet digestibility or any fermentation parameter was unaffected (P > 0.05) by previous backgrounding during the finishing period. In fact, rumen microbial abundance measured during finishing was not modified (P > 0.05) by previous backgrounding. Steers finished under grain conditions, had lower (P < 0.01) daily CH₄ emissions and CH₄ intensity. Additionally, grain-fed steers increased (P < 0.05) BW gain, diet digestibility, propionic, lactic, and valeric acids, Succinivibrionaceae family and Succiniclasticum, Erysipelotrichaceae UCG-002, Sharpea, and Megasphaera bacteria genera, compared to forage-fed steers. In conclusion, ruminal microbiome and fermentation, diet digestibility, and CH₄ emissions were unaffected during finishing between prior levels of backgrounding growth. However, given the higher BW gain in suboptimal steers in both finishing diets, CH₄ intensity was reduced in comparison to the optimal backgrounded steers. Nevertheless, lifetime emissions of the steers need to be assessed with the different dietary regimens, since suboptimal steers reduced CH₄ emissions during the backgrounding period but, additional days of finishing were required to achieve the same BW as their contemporaries.