Downstream effects of brown seaweed supplementation in beef cattle diets on greenhouse gas emissions and coprophagous beetles

Abstract

The Canadian livestock sector is a significant source of greenhouse gas (GHG) emissions. To reduce enteric methane emissions, the sector has started using mitigation strategies, such as dietary interventions. While studies have shown that supplementing beef cattle diets with brown seaweed can reduce enteric methane emissions, the downstream effects of dietary supplementation remain unclear. This thesis explores the downstream effects of supplementing beef cattle diets with brown seaweed on dung and beetle-mediated GHG emissions, as well as on coprophagous beetles’ reproductive success and resource use and selection. To assess these effects, dung was collected from beef cattle assigned to four treatment groups based on the dietary level of brown seaweed inclusion (i.e. 0%, 0.5%, 1%, 2%). Laboratory-based experiments revealed that dietary supplementation, at a rate of 2% of dry matter intake (DMI), reduced dung CH₄ emissions by 49% and CO₂ emissions by 26%, resulting in a 30% reduction in dung CO₂e emissions. While dietary supplementation with brown seaweed reduced dung’s baseline CH₄ emissions, the relative beetle-mediated reductions remained identical. This suggests that supplementing beef cattle diets with brown seaweed had no effect on C. erraticus’ CH₄ mitigation potential, as the smaller absolute effects noted in supplemented dung samples were caused by lower initial CH₄ emissions. Additionally, dietary supplementation had no effect on beetle-mediated CO₂e emissions. Through a combination of field- and laboratory-based experiments, this project found that the inclusion of brown seaweed in beef cattle diets reduced both Onthophagus nuchiocornis’ resource use for brood ball production and the proportion of major males in beetle offspring. These results suggest that dietary supplementation reduced dung quality. However, when evaluated under field conditions, coprophagous beetles’ resource selection was not affected by dietary supplementation. Across studies, there is a lack of consensus on the impact of dung beetles on GHG emissions from livestock dung. Using a series of laboratory-based experiments, this work shows that methodological decisions strongly influence dung and beetle-mediated GHG emissions. For example, the homogenization of dung prior to GHG sampling, dung’s surface-area-to-volume ratio, and dung mass all influence beetle-mediated GHG emissions from dung. Overall, results from this research reveal the downstream effects of supplementing beef cattle diets with brown seaweed on dung beetle ecology and dung GHG emissions. These results highlight the need to assess the downstream effects of management interventions when trying to determine their ability to improve the environmental sustainability of livestock production.