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Sources of Rumen Carbon Dioxide and Methane in the Lactating Dairy Cow¹

Department of Animal Husbandry and Department of Physiological Sciences University of California, Davis 95616

ABSTRACT

The objective of this study was to investigate substrates as sources of ruminal CO₂ and CH₄. Acetate-1-¹⁴C, propionate-1-¹⁴C, propionate-2-¹⁴C, butyrate-2-¹⁴C, DL-lactate-U-¹⁴C, and barley straw ¹⁴C were introduced into the rumen of lactating dairy cows in separate experiments. Small quantities of the introduced isotope were recovered in rumen CO₂ within five hours; (1–2%) when acetate-1-¹⁴C, propionate-2-¹⁴C, or butyrate-2-¹⁴C was the substrate. However, when propionate-1-¹⁴C was introduced, 18% of the ¹⁴C was recovered in rumen CO₂ within five hours. When barley straw ¹⁴C served as a substrate, 4–8% of the ¹⁴C was recovered in rumen CO₂ within ten hours. The time distribution of rumen ¹⁴CO₂ suggested there was considerable microbial fermentation of barley straw. Methane-¹⁴C accounted for 0.2 to 3% of the ¹⁴C introduced in the short-chain fatty acids and 2– 4% from barley straw. Transfer quotient calculations indicated that 40– 60% of the rumen CH₄ arose from the carbonate pool. However, this was substrate-dependent. When DL-lactate-U-¹⁴C or barley straw ¹⁴C was used, the transfer quotient varied from 0.8 to 1.2, indicating more direct sources of rumen CH₄ than the carbonate pool.

¹ This work was supported in part by NTH Grant no. RO1AM09821.

² Present address: Metabolic Laboratory, Colorado State University, Port Collins.