Globally, livestock systems account for 12% of anthropogenic greenhouse gas emissions, with enteric fermentation from ruminants being the largest contributor. Reducing enteric methane (CH4) emissions is therefore essential for achieving national and international climate goals. While it has been suggested that moving towards a plant-based diet would progress towards these goals, livestock production remains vital for enhancing nutrition, sustainability, gender equity, and food security worldwide. Animal-sourced foods provide critical bioavailable nutrients that are often absent from the diets of the 768 million people who face undernourishment globally. Thus, it is imperative to identify strategies to reduce enteric CH4 emissions without diminishing the role of livestock in global food systems.

Given the global diversity of cattle production systems, these strategies must be adapted to suit specific regional contexts. In high-income countries (HICs), feed additives have been identified as an effective strategy for reducing enteric CH₄ emissions, largely due to the prevalence of confined feeding systems in these regions. Feed additives mitigate enteric CH4 production by directly disrupting the methanogenesis pathway (CH4-inhibitors) or by indirectly altering the rumen environment (rumen modifiers). However, the selection of both CH4-inhibitors and rumen modifiers with proven and consistent in vivo efficacy is limited, underscoring the need for further research. The first chapter of this dissertation outlines current CH4-inhibiting and rumen modifying feed additive research, with a focus on in vivo studies, highlighting the most promising and immediate solutions to mitigate enteric CH4. The following two chapters evaluate the effects of two novel feed additives on enteric gas emissions and animal performance parameters in beef cattle.

The second chapter of this dissertation evaluates the effect of two levels of biochar inclusion on enteric CH4 emissions and animal performance in feedlot cattle across three dietary phases. Biochar is a solid carbonaceous residue produced through biomass gasification under high-temperature, low-oxygen conditions. Twenty-four Angus crossbred steers were randomly assigned to one of three treatment groups: control (n=8, no biochar), low biochar (n=8, 1% of dietary dry matter, DM), and high biochar (n=8, 2% of dietary DM). Enteric CH4, carbon dioxide (CO2), and hydrogen (H2) emissions were measured every two weeks over a 14-week trial period using a GreenFeed system. Biochar inclusion had no significant effect on CH4 and H2 production (g/day), yield (g/kg DMI), and intensity (g/kg ADG). While CO2 production was 9% lower in the high biochar group compared to the control group during the final dietary phase, no significant differences were observed for CO2 yield and intensity. Additionally, biochar inclusion did not significantly affect dry matter intake (DMI), average daily gain (ADG), or feed conversion efficiency (FCE), suggesting no detrimental effect on animal production. Given the variability in biochar’s physical and chemical characteristics, which are determined by source material, production conditions, temperature, particle size, and pre- or post-production manipulations, further research is needed to optimize biochar for potential CH4 mitigation.

The third chapter of this dissertation investigates the effect of Rumin8 Investigational Veterinary Product (IVP), a bromoform based feed additive, on enteric CH4 emissions, animal production parameters, and the rumen environment in feedlot cattle. Although various approaches have been proposed to decrease enteric CH4 emissions, feed additives containing bromoform (CHBr3) have shown promise with minimal impact on animal production parameters. This study examined the effects of two Rumin8 IVP containing synthetic CHBr3 on enteric gas emissions, animal production parameters, and the rumen environment. Twenty-four Angus beef steers were randomly assigned to one of three treatment groups: Control, Oil (8 mL Rumin8 oil IVP/kg DMI), and Powder (1.2 g Rumin8 powder IVP/kg DMI). The Rumin8 oil IVP treatment resulted in a CHBr3 intake of 32.2 mg/kg DMI, while the Rumin8 powder IVP provided a CHBr3 intake of 2.0 mg/kg DMI during weeks 1-8. In week 9, a new batch of Rumin8 powder IVP increased the CHBr3 intake to 17.9 mg/kg DMI. The Oil group exhibited 95.0, 95.0, and 96.1% reductions in CH4 production, yield, and intensity, respectively, accompanied by 925, 934, and 858% increases in H2 production, yield, and intensity, respectively. Neither treatment significantly affected animal production parameters or rumen environment variables. These findings suggest that Rumin8 oil IVP containing synthetic CHBr3 has the potential to reduce enteric CH4 emissions.

The CH4 mitigation potential of intensifying animal production is greater in low-producing systems, such as those in low- and middle-income countries (LMICs), compared to high-producing systems common in HICs. Consequently, CH4 mitigation efforts in LMICs focus on sustainable intensification, which, for ruminant production, relies on accurate energy requirement estimates. However, such data are lacking for the Bos taurus × Bos indicus crossbreeds that dominate LMIC’s production systems. Refining these estimates could enhance sustainable intensification efforts and help close livestock productivity gaps and is the focus of the fourth and final chapter of this dissertation. To determine the net energy for lactation (NEL) requirement for maintenance and efficiency of utilization of metabolizable energy intake (MEI) for milk production (kL) of Bos taurus × Bos indicus crossbred dairy cows in the tropics a meta-analysis using 141 observations from 38 independent studies in tropical regions with crossbred dairy cows was conducted. The energy produced in milk corrected for zero energy balance (EL0) was regressed by MEI including other covariates. This meta-regression analysis was conducted by frequentist inference via Bayesian optimization in RStan. The best-fit model predicted a net energy for lactation value at maintenance of 0.323 MJ/kg BW0.75/day with variations for each specific study. The efficiency with which MEI is used for milk production was estimated to be 0.554, which was common for all studies. Nutritional requirement tables should be specifically estimated for Bos taurus × Bos indicus crossbred dairy cows as their requirements differ from Western breeds. Using appropriate nutritional requirements of crossbred cattle would lead to better nutrition and increased production, aligning with their genetic potential.