Rumen Fermentation In Vitro as Influenced by Long Chain Fatty Acids

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Abstract
Responses of rumen microbes to fatty acids were evaluated by production of total volatile fatty acid and ratio of acetate to propionate. Fermentations were under CO2 for 20 h in 50-ml Erlenmeyer flasks in a Dubnoff metabolic shaking incubator. Flasks contained 20 ml medium, 1 ml reducing solution, 750 mg substrate (450 mg hay plus 300 mg grain) and varying amounts of long-chain fatty acids supplied as free acids, Ca salts or triglycerides. They were inoculated with 5 ml rumen fluid obtained from a cow fed 3.6 kg grass hay, 2.3 kg grain and 0.2 kg tallow daily. Volatile fatty acid production was decreased by long-chain fatty acids that contained < 18 C atoms and by unsaturated long-chain fatty acids with 18 C atoms. Lauric acid decreased volatile fatty acid production by 69% and induced unusual acetate/propionate ratio (40:1). Stearic acid did not affect volatile fatty acid production or acetate/propionate ratio. Within 2 series of long chain fatty acids (myristic, palmitic, stearic, arachidic; stearic, oleic, linolenic), melting point accounted for 93-95% of the variation of volatile fatty acid production and acetate/propionate. As Ca salts, long chain fatty acids caused small changes of fermentation. These data support the proposition that hard fats and Ca salts of long-chain fatty acids do not interfere with ruminal fermentation.