Methane's Impact on Animal Performance – VFAs
The primary energy sources for beef cattle are starch and cellulose. These energy sources are fermented by microorganisms in the rumen to volatile fatty acids (VFA) and gases. The majority (50-85%) of the energy cattle receive is in the form of VFAs.
The primary VFAs produced are acetate, propionate, and butyrate. Type of diet (forage vs concentrate), microbial species present in the rumen, and ruminal pH are the major factors that influence the percentage of each VFA produced. For example, a forage-fed animal may have VFA ratios of 70:20:10 for acetate, propionate, and butyrate (respectively). In concentrate-fed animals the ratio changes to a greater proportion of propionate at 50:40:10.
The conversion of structural carbohydrates such as starch and cellulose to VFA is inefficient due to the loss of energy in heat and methane. Propionate has 3-carbons, butyrate has 4-carbons, and acetate has 2-carbons. When acetate is produced in the rumen there is a loss of one carbon as CO2 which can be used by a group of archea known as methanogens to form methane. Butyrate (4-carbons) is produced when two acetate molecules are combined. Although butyrate does not produce CO2 which can be converted into methane directly, the CO2 was produced when the acetate molecules were formed. Propionate is the only VFA that does not release an extra carbon that can be used for generation of methane. Because of these differences in carbon, the energy values for the VFA are approximately 109% for propionate, 78% for butyrate, and 62% for acetate. Therefore, increasing propionate within the rumen will decrease methane production and increase animal performance.
A good illustration of this is comparing feed efficiency and average daily gain of forage-fed or grain-fed animals. Grain-fed animals often have improved gains and efficiencies compared with forage-fed animals. One reason for this is the differences in amount of methane produced from the loss of carbon based on the ratios listed above.
After the VFAs are produced in the rumen, the efficiency of energy utilization is not different among acetate, butyrate, and propionate in animal tissues. However, the energy contained in the feedstuff was lost during ruminal fermentation. This is one of the main reasons producers often feed forages that are more digestible or rations higher in grain content to improve efficiency.
Panhandle Research and Extension Center
University of Nebraska-Lincoln
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- Remond, D., I. Ortigues, and J.P. Jouany. 1995. Energy substrates for the rumen epithelium. Proceedings of the Nutrition Society 54:95-105.