Supplementing beef cattle diets with brown seaweed affects coprophagous beetles' dung use

Abstract Cattle dung deposited on Canadian pasture systems supports a diverse insect community, including dung beetles (Coleoptera: Scarabaeoidea). Although the inclusion of brown seaweed in beef cattle diets can reduce enteric methane emissions, its downstream effect on coprophagous beetles is poorly understood. This study evaluates the effects of supplementing beef cattle diets with brown seaweed on dung beetle resource use and reproductive success, as well as its effects on coprophilous beetles’ resource selection. Dung was collected from beef cattle assigned to four treatment groups based on the rate of dietary supplementation with brown seaweed (i.e. 0% DMI, 0.5% DMI, 1% DMI, 2% DMI). Results from this study found that dietary supplementation negatively impacted Onthophagus nuchicornis ’ resource selection for brooding purposes. When given a choice between dung from two treatment groups, beetles produced 53% more brood balls using dung from the lower supplemented treatment group included in the pairing. However, under field conditions, dietary supplementation had no effect on attractiveness to coprophilous beetles. Additionally, in a no‐choice experiment, dietary supplementation with brown seaweed did not impact Onthophagus nuchicornis ’ reproductive endpoints; however, results showed evidence of stress on male beetles, as dietary supplementation marginally reduced the proportion of major males in beetle offspring. The combined effects of dietary supplementation on brood ball production and on the proportion of major males suggests that dietary supplementation marginally reduced dung quality. Collectively, this work demonstrates that the wider environmental impacts of supplementing cattle diets should be considered alongside its potential to mitigate enteric methane emissions.