Can methane suppression during digestion of woody and leafy browse compensate for energy costs of detoxification of plant secondary compounds? A test with muskoxen fed willows and birch.

Digestion and metabolism of woody and leafy browse requires detoxification of plant secondary compounds that can incur an energy cost. Browse, however, inhibits methane (CH(4)) production and therefore could offset some costs of detoxification. We measured an index of heat increment of feeding (HIFi) and CH(4) production in muskoxen (Ovibos moschatus) given a single test meal (at 10 g/kg BM(0.75)) composed of hay mixed with one of three browse species (Willow: Salix alaxensis, S. pulchra; Birch: Betula nana). Detoxification cost was estimated as HIFi of browse diet-HIFi of hay diet and CH(4) compensation as CH(4) production of hay diet-CH(4) production of browse diet. CH(4) compensation was noted in 47% of 15 trials in which a detoxification cost was evident; six trials were with woody browse and one with leafy browse. Separate controls were responsible for the difference in CH(4) compensation for leafy browse vs. woody browse. Detoxification costs for twigs and leaves of B. nana were underestimated because of their low digestibility. In only one of six treatments was CH(4) compensation documented for B. nana. We conclude that energy saved by CH(4) suppression was small (<6%) compared with detoxification costs.