Effect of Dietary Nitrogen on the Rumen Microbial Population in Lactating Dairy Cattle*

Rumen bacteria populations were compared in four groups of six lactating Holstein cows maintained on a basal diet (9.4% crude protein) of maize silage plus concentrate, either without supplement, or supplemented to bring the crude protein content of the diet to approximately 12.5% with soybean meal, urea, or maize silage with urea added at the time of ensiling. The response to increased dietary nitrogen was dramatic, with a 33‐36% increase in total dietary nitrogen level resulting in a 90% (urea) to 230% (urea silage or soybean meal) increase in bacterial numbers. Essentially all identified bacterial species responded to the dietary supplements. The greatest increases wereobserved among those bacterial species known to require branched chain volatile fatty acids for optimum growth in vitro , which included the predominant cellulose digesting organisms ( Ruminococcus spp.) and Selenomonas spp., the dominant propionate producing species.