SOURCES OF RUMEN CARBON DIOXIDE AND METHANE IN LACTATING DAIRY COW

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. © 1969, American Dairy Science Association. All rights reserved.