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Bacterial direct-fed microbials in ruminant diets: Performance response and mode of action^1,2

C. R. Krehbiel^3,*, S. R. Rust, G. Zhang^* and S. E. Gilliland^*

^* Department of Animal Science, Oklahoma State University, Stillwater 74078 and and Department of Animal Science, Michigan State University, East Lansing 48824

³ Correspondence: 208 Anim. Sci. Bldg. (phone: 405-744-8857; fax: 405-744-7390; E-mail: kclinto{at}okstate.edu).

Direct-fed microbials (DFM) have been shown to increase daily gain and feed efficiency in feedlot cattle, enhance milk production in dairy cows, and improve health and performance of young calves. However, their effects on performance have been mixed, and the mode of action remains unclear. Bacteria used as DFM have been defined as single or mixed cultures of live organisms, which, when fed to animals, beneficially affect the host. The original concept of feeding DFM to man and livestock was based primarily on the potential for beneficial intestinal effects, including the establishment of a desirable gut microflora and/or prevention of the establishment of pathogenic organisms. More recently, however, there has been some indication that certain bacterial DFM might have beneficial effects in the rumen, such as decreasing the potential for ruminal acidosis. In several experiments, supplementing feedlot cattle with lactate-utilizing and/or lactate-producing bacteria has been shown to improve feed efficiency and daily gain (approximately 2.5%), with little change in DMI. In addition, increased milk yield (0.75 to 2.0 kg/d) has been reported in studies using dairy cows fed DFM, with little change in milk composition. Few attempts have been made to determine the mechanisms responsible for the beneficial effects of DFM, but the potential for a decrease in subacute acidosis has been evaluated. Responses to bacterial DFM have included a decrease in the area below subacute ruminal pH, increases in ruminal propionate concentrations, increased protozoal numbers, and changes in viable bacterial counts. Effects on some blood variables (lower CO₂ and LDH) also suggest a reduced risk of metabolic acidosis. Recent research has shown that DFM decreased fecal shedding of Escherichia coli O157:H7 from infected calves. Therefore, a possible application for DFM might be to reduce shedding of this pathogen from cattle. Overall, data indicate that DFM have the potential to decrease ruminal acidosis in feedlot cattle and dairy cows, and improve immune response in stressed calves. More research is needed to describe the mode of action, and thereby improve the efficiency of DFM use.

Key Words: Feed Additives • Probiotics • Ruminants • Rumen Fermentation

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