Manipulation of ruminal fermentation with organic acids: a review.

The dicarboxylic acids aspartate, fumarate, and malate stimulate lactate utilization by the predominant ruminal bacterium, Selenomonas ruminantium. Malate stimulates lactate uptake by S. ruminantium more than does aspartate or fumarate, and it seems that malate and sodium are involved in stimulating lactate utilization by this bacterium. Based on the ability of S. ruminantium to grow on malate in the presence of extracellular hydrogen and produce succinate, malate may be acting as an electron sink for hydrogen in the succinate-propionate pathway used by S. ruminantium. Incorporation of DL-malate into soluble starch and cracked corn fermentations with mixed ruminal microorganisms changed final pH, CH4, and VFA in a manner analogous to ionophore effects. When compared with either dicarboxylic acids or monensin alone, dicarboxylic acid plus monensin addition to cracked corn incubations stimulated the mixed ruminal microorganism fermentation to produce more propionate, less lactate, and increased final pH. Reduced lactate concentrations in dicarboxylic acid- and monensin-treated incubations most likely represents an additive effect of decreased lactate production by monensin-sensitive bacteria (i.e., Streptococcus bovis) and increased lactate utilization by the monensin-resistant S. ruminantium. The inclusion of malate as a feed additive into the diets of ruminants is currently not economically feasible; however, forages rich in organic acids might serve as vehicles for providing malate to ruminants. When five alfalfa varieties and three bermudagrass hay varieties were surveyed for malate content, the concentration of malate in both plant species declined as maturity increased. However, after 42 d of maturity, the concentration of malate in both forages ranged between 1.9 and 4.5% of the DM. These results suggest that the incorporation of forage varieties that are high in malate may include malate economically into the diet and reduce losses associated with ruminal acidosis.