Bacterial direct-fed microbials in ruminant diets: Performance response and mode of action

ABSTRACT 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.

Implications

Bacterial direct-fed microbials fed to ruminant livestock have been shown to decrease scours in neonatal calves, increase milk yield in dairy cows, decrease morbidity in newly weaned calves and/or calves newly received in the feedlot, and increase daily gain and carcass weight in feedlot cattle. Moreover, strains of Lactobacillus acidophilus were shown to reduce fecal shedding of Escherichia coli O157:H7 by feedlot cattle at harvest. Although the mode of action is not fully understood, it seems that adhesion, colonization, inhibitory action, and stimulation of immune function are all important for direct-fed microbials to improve health. Some bacterial direct-fed microbials also seem able to function in the rumen. Depending on the species or combination of species, they can increase ruminal propionate concentration and decrease area below subacute ruminal pH, suggesting the potential for more efficient energy utilization and a reduction in acidosis exists.

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

© 2003, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2003. 81(E. Suppl. 2):E120-E132