Grazing of white-tailed deer with shrubs and trees modifies ruminal biogas production in the presence of guanidinoacetic acid and live yeast cells as feed additives

The effects of shrubs and trees on rumen biogas production (GP) in the presence of feed additives—guanidinoacetic acid (GAA) and live yeast (Saccharomyces cerevisiae)—in White-tailed deer were assessed. Total GP, ruminal methane (CH4), carbon monoxide (CO), and hydrogen sulphide (H2S) production were determined. Opuntia leptocaulis + Yeast had higher GP (P < 0.0001) at 48 h than others, but with Acacia rigidula + Yeast producing the least. Opuntia engelmanni + Yeast and Hilaria mutica + Yeast and Prosopis glandulosa were observed to have the highest dry matter digestibility and pH, but with the least value obtained in Hilaria mutica + Yeast and Setaria leucophylla with GAA or Yeast addition, respectively, predicated largely on fibre contents of the forages. Addition of GAA to Leucophyllum frutescens and Mimosa zygophylla was observed to have the highest CH4 production at 4 h, 24 h, and 48 h, respectively, while H. mutica + Yeast consistently had the lowest. P. glandulosa + Yeast (24 h and 48 h) resulted in higher values (P < 0.0001) of CO gas kinetics and CO gas production. Lower value with Diospyros texano + Yeast (24 h) compared to others, except that at 4 h, 48 h was lower in S. leucophylla + Yeast, and O. engelmannii + GAA, respectively. However, H. mutica + GAA was consistently higher in b, c, Lag, and 24 h, except at 48 h, where P. glandulosa + Yeast was higher than other treatments. However, H2S production at 48 h had the least value in O. engelmannii + GAA. Forage–additive interactions significantly influenced rumen biogas profiles in White-tailed deer, with fibre and fat contents determining their extent. Yeast generally enhanced GP and digestibility while reducing CH4 kinetics, particularly in H. mutica. In contrast, GAA increased methane output in select forages, with species-specific effects observed for CO and H2S. These findings highlight the potential of targeted forage–additive combinations to modulate ruminal fermentation and gas emissions, offering insights for sustainable deer nutrition and ecosystem management.