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Recently, several letters have expressed concern about the use of antibiotics in domestic animal production and the potential for selection of antibiotic-resistant bacteria. Because limited information is available detailing alternatives to antimicrobial compounds in animal production systems, one focus of my laboratory for several years has been to examine alternatives to antibiotics for ruminant animals. Under some dietary conditions lactate can accumulate in the rumen and decrease ruminal pH. Because the animals do not feel well (i.e., acid indigestion), they eat less and consequently produce less efficiently. For years, ionophore antibiotics have been used to minimize lactate accumulation within the rumen and maintain production efficiency.

Organic acids (i.e., malate, fumarate, and aspartate) potentially provide an alternative to ionophore antibiotics by stimulating rather than inhibiting specific ruminal microbial populations (S. A. Martin, J. Anim. Sci. 76: 3123--3132, 1998). Initial research characterized the effects of organic acids (aspartate, fumarate, malate) on lactate utilization by the predominant ruminal bacterium, Selenomonas ruminantium. All three organic acids stimulated lactate utilization by S. ruminantium between 4- and 10-fold. This bacterium uses the succinate-propionate pathway; malate and fumarate are intermediates of this pathway, while aspartate can feed into this pathway. Based on this pure-culture research, my laboratory demonstrated that some of these organic acids, primarily malate, altered the in vitro mixed ruminal microorganism fermentation in a manner that is similar to ionophore antibiotics (i.e., decreased lactate, increased pH). When animal studies were conducted, malate treatment was effective in reducing the drop in ruminal pH normally seen 1 to 2 h after being fed a high-grain diet and improved cattle performance in two of three feedlot studies (S. A. Martin et al., J. Anim. Sci. 77:1008--1015, 1999). Therefore, supplementing feedlot cattle diets or high-producing dairy cattle diets with malate might be effective in reducing ruminal lactate accumulation.

Malate can be purchased in bulk quantities, and at current prices, the use of malate as a feed additive (80 g/day per head) is estimated to cost $0.09 to $0.19/day per head under feedlot conditions. At present, this cost may prohibit the inclusion of malate in the diets of feedlot or dairy cattle. However, we have shown that forages commonly fed to cattle contain significant concentrations of malate (T. R. Callaway et al., J. Dairy Sci. 80: 1651--1655, 1997). Unfortunately, the malate associated with forages appears to be fermented very rapidly once exposed to the ruminal environment (S. A. Martin et al., J. Dairy Sci. 83:308--312, 2000).

While the results of our research do not address all of the concerns about using antimicrobial compounds in animal feedstuffs, it does demonstrate how understanding the details of the ruminal fermentation can be used to successfully manipulate the fermentation. An organic acid, like malate, would be a desirable feed additive because there is no risk of developing antibiotic resistance or having unwanted residues appearing in either meat or milk products. It should be noted that malate is routinely used in food products consumed by humans and is recognized as safe by the U.S. Food and Drug Administration.

Scott A. Martin
Department of Animal and Dairy Science
University of Georgia, Athens