Nitrogen fixation in ruminants.

The amount of nitrogen gas (N2) available for microbial protein synthesis via nitrogen fixation in the rumen was estimated. Measurements were made of nitrogen fixation by ruminal ingesta from goats fed low protein rations for a prolonged period of time, and by ingesta from sheep fed high protein rations. Nitrogen fixation by rumen microorganisms incubated for 30 or 180 min in vitro was measured with the acetylene-reduction technique. Results suggest that goats fixed approximately .2 to .3 mg of N2 daily. Results from determinations made with rumen ingesta obtained from two sheep fed a high protein diet (17% crude protein) suggested that up to 2.5 mg N2 could be fixed daily. Three mature castrated goats were fed a 5.4% crude protein diet for 16 weeks. A nitrogen balance trial conducted during the eighth week of the experiment indicated that the goats were each losing about 1 g of nitrogen daily. This amount of nitrogen loss was about what would have been expected if there was no nitrogen fixation occurring in the gastrointestinal tract. Based upon results from the acetylene-reduction test and the nitrogen balance trial, it was concluded that nitrogen fixation by rumen microorganisms is of negligible quantity. (