# Can Biochar Improve the Sustainability of Animal Production?
## 3 Animal Feed
### Enteric Fermentation Methane Emissions  
Ruminants, including beef and dairy cattle, sheep, goats, and bison utilize their unique digestive system and fermentation process to break down forage in the gut. This process is called enteric fermentation and produces large volumes of CH\({}_{4}\) emissions into the atmosphere [9]. This CH\({}_{4}\) loss takes away 2-12% of the rumen's energy intake, posing significant production concerns among climate and animal scientists [84]. Since enteric emissions contribute the largest source of CH\({}_{4}\) in the US, there are efforts to utilize feed additives to mitigate enteric fermentation emissions, and biochar has been explored for this purpose [84].  
Biochar addition to biological systems increases access to carbon, leading to mixed results in terms of microbial activity and emissions. On the one hand, biochar inhibits methanogenesis (enteric fermentation pathway for CH\({}_{4}\) production) by increasing aeration and O\({}_{2}\) content. It has also been hypothesized that the high surface area, pore structure, and ion exchange capacity of biochar promotes the formation of biofilms, inhibiting ruminal CH\({}_{4}\) emissions [85]. However, some studies (e.g., [PERSON] et al. [86]) report increased CH\({}_{4}\) and CO\({}_{2}\) emissions since microorganisms have access to an additional liable carbon source.  
[PERSON] et al. [87] incorporated cocoa-pod husk biochar at 0.1, 0.2, 0.3, and 0.4 mg ml\({}^{-1}\) to a basal diet of grass and pollard (wheat milling byproduct) to dairy cows. In-vitro ruminal fermentation samples were incubated for 48 h. Compared to the control, biochar addition did not impact rumen pH, NH\({}_{3}\), or total gas production. There was a noticeable shift in volatile fatty acid (VFA) production, with a 21% increase in acetate production, and 7, 18, 29, 50, and 33% decrease in propionate, butyrate, isobutyrate, valerate, and isovalerate, respectively. While gas production was not affected, VFAs are the final product of ruminal fermentation and serve as the main source of energy and carbon for the formation of proteins.  
A higher ratio of acetate:propionate has a positive impact on animal efficiency. Additionally, propionate competes with CH\({}_{4}\) in rumen fermentation, while acetate and butyrate can be used by methanogens in the rumen to reduce CO\({}_{2}\) and CH\({}_{4}\). Increased levels of biochar decreased protozoa counts by 6-7%, which then contributed to a decrease in total bacteria and in vitro ruminal dry matter and organic matter degradability. This is contradictory to [PERSON] et al. [88], who suggested that decreases in the protozoa population are harmful to fiber digestion, which is crucial for the rumen diet. [PERSON] et al. [89], [PERSON] et al. [90], and [PERSON] et al. [91] all reported similar findings, with no changes in the CH\({}_{4}\) production regardless of the biochar chemical/physical properties or inclusion rates. [PERSON] et al. [72] suggested this is due to the indigestible, recalcitrant nature of biochar in the rumen.  
[PERSON] et al. [92] had more positive results in reducing CH\({}_{4}\) emissions via biochar feed addition. They evaluated rice-husk biochar at a 0.6% DM addition rate as a feed additive to young beef cattle basal diet. Over the 98 day experiment, there was a 22% reduction in CH\({}_{4}\) emissions and a 25% increase in live weight gain. The authors attributed the positive results to the porous characteristics of the biochar in helping facilitate redox reactions between bacteria and overall favored biofilm formation. Therefore, there may be a dependency on the stage of animal growth at which biochar is introduced in the feed.