Regulation of rumen fermentation during seasonal fluctuations in food intake of muskoxen

We studied rumen fermentation of castrated adult muskoxen (Ovibos moschatus; n = 4) during periods of low (May) and high (August) food intake. Turnover time (17 ± 1.8 h) and volume (26 ± 3.9 L) of rumen fluid were consistent between May and August and among days within each season. Rumen temperature did not vary significantly during the day (38.8 ± 0.29 °C) in either season. Rumen osmolality (271.9 ± 16.4 vs. 245.9 ±11.4 mOsm kg− 1) and pH (6.81 ± 0.31 vs. 6.39 ± 0.15) were higher in May than in August indicating a shift in the allostatic set point. Rumen fluid pH was more variable in May than in August both before and after a single meal of fermentable substrate even though fermentation acids were lower in May than in August (101.0 ± 11.0 vs. 126.0 ± 8.74 mM). Changing proportions of minor fermentation acids indicated a shift in metabolic pathways even though bacterial numbers were similar between seasons (6.4 ± 5.8 × 109 mL− 1). Allostatic set points probably alter the homeostatic range of conditions and the microbial diversity of fermentations in herbivores from highly seasonal environments.