Influence of Starch and Nitrogen Sources on Ruminal Microorganisms of Steers Fed High Fiber Purified Diets

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Abstract
The ruminal microbial population was determined in steers fed high fiber purified diets with or without 13% starch and with either urea, biuret or isolated soy protein as the dietary nitrogen source. The same major groups of ruminal bacteria and protozoa were present in the steers, regardless of dietary carbohydrate or nitrogen source. Of the bacteria isolated from the rumen 17 to 41% were cellulolytic, 13 to 45% were amylolytic and 12 to 61% were acid producers from glucose. When starch was included in the diet the numbers of cellulolytic bacteria decreased whereas the total number of bacteria increased. Of 826 strains of bacteria isolated, Ruminococci and noncellulolytic Bacteroides sp. were the two most predominant groups of bacteria isolated. Between 51 and 69% of the bacteria isolated were not identified, among which were cellulolytic bacteria including gram-positive rods and nonmotile gram-negative rods in chains.