Methylamine is often found as a naturally‐occurring metabolite in the rumen of sheep and cattle fed a variety of diets. The objective of this study was to examine the effect of methylamine on nitrogen‐limited growth of Selenomonas ruminantium strain D grown on a nitrogen‐free basal medium that would not support bacterial growth without the addition of a nitrogen source (ammonia, urea, glutamine, cysteine, or glutamate). In this medium S. ruminantium growth rates were significantly faster (P<0.05) with 11 mM ammonia‐nitrogen than any other nitrogen source and were slowest on glutamate‐nitrogen (1 and 11 mM). Maximum optical density was greater (P<0.05) for all nitrogen sources when the respective nitrogen source was increased from 1 mM to 11 mM nitrogen. Addition of 10 mM methylamine significantly decreased (P<0.05) maximum optical density compared to the respective nitrogen source and concentration without added methylamine for cells grown on 1 mM glutamine‐, 1 mM cysteine‐, 11 mM ammonia‐, 11 mM glutamine‐, 11 mM cysteine‐ or 11 mM glutamate‐nitrogen. It appears that ruminal methylamine could decrease extent of growth of ruminal selenomonads depending on the nitrogen source(s) available for
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