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Effects of Woody Browse on Dietary Efficiency
and Methane Production in Musk Oxen

by James P. Lawler, Institute of Arctic Biology

Methane is important as a greenhouse gas and represents an important loss in the efficiency of conversion of dietary energy to metabolic energy in ruminants. Investigations of methane production in cattle have linked a variety of forage attributes to the amount of methane produced. However, little work has been done with wild ruminants and the consequences on methane production of their diet choices. Therefore, estimates of wild ruminant methane emissions could be in error when extrapolated from domestic ruminates fed agricultural diets. Musk oxen are an ideal choice as a model organism for this type of study because they feed on a variety of forages from sedges to woody browse. In addi- tion, they live in an environment that demands efficient use of diet. Since a change in global climate would affect plant distribution and change species abundance, it would undoubtedly affect ruminant diet choice. Woody plants may replace grasses and sedges during warming of arctic tundra which would affect herbivore diet choice, distribution, numbers, and productivity. Vegetation change and diet choice has consequences for efficiency of energy use, as well as total methane production by arctic ruminants. We hypothesized that wild ruminant diet choice has evolved to minimize methane production.

To test this hypothesis, 3 young musk oxen were fasted for 24 hours and then placed in an open-circuit calorimeter. We used oxygen consumption, carbon dioxide production, and methane production to evaluate energy expenditure (EE) and energy lost as methane (ECH4). Once a background level of EE and ECH4 was determined (2 hours), the musk ox was fed a single meal composed of brome hay (Bromus inermis), or hay mixed with chipped twigs of either felt leaf willow (Salix alaxensis) or diamond leaf willow (Salix pulchra) fed at 10 g/kg body weight.75. Energy expenditure and ECH4 were measured continuously for 8 hours. Diets offered were 100% brome hay and brome hay mixed with 20%, 40%, 60%, and 80% of one of the two different willow species. Energy expenditure and ECH4 above the previously determined background levels represent that derived from the diet.

Our results show that increasing the percentage of browse in the diet significantly reduced methane production (Fig. 1). We could not detect a significant difference in methane production between the two types of browse. Methane production was highest for the 0 and 20% browse diets and decreased progressively between 20% and 80% browse.

Net EE above fasting for both browse species increased showing a curvilinear response with a peak EE at 20-60% browse and a decrease at 80% (Fig. 1). However, we could not detect a difference in EE statistically between browse species or between percentages of browse in the diet.

Fig. 1 - graph
Figure 1. Mean net energy expenditures and mean net
methane energy losses of 3 musk oxen on 5 levels of dietary
browse intake. Each of the 3 musk oxen were fed each level
of browse on 2 seperate occasions.

Energy efficiency was measured as a decrease in energy lost as ECH4 relative to EE from the meal. Although this efficiency improved with increasing levels of dietary browse, the trend was not statistically significant.

This study suggests that consumption of woody browse has a potential lowering effect on methane production at high dietary proportions. However, consumption of small amounts of browse tended to raise methane output. The fact that browse in this study caused changes in methane production suggests that estimates of methane production by wild ruminants be evaluated more carefully with regard to natural diets. Since small changes in energy efficiency can have a multiplicative effect on animal productivity, changes in methane production and dietary efficiency have important implications for arctic ruminant productivity, distribution, and numbers.

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