Utilization of Organic Acids to Manipulate Ruminal Fermentation and Improve Ruminant Productivity

Abstract

Much research has been conducted to develop feed additives that can improve the efficiency of ruminal fermentation and increase the productivity of ruminants. Ionophore antibiotics have been banned in some countries, and organic acids (OA) have been proposed as an alternative. Most research on the use of OA in ruminant feeding has focused on malate and fumarate, which are intermediates in the tricarboxylic acid cycle and in the randomizing pathway of propionate formation in the rumen. The effects of OA addition on ruminal fermentation and animal performance can be based on more than one mechanism of action. The simplest effect of OA can be thought as the supplementation with a rapidly fermentable substrate, but calculations from in vitro rate of disappearance of fumarate and malate suggest that a considerable proportion of these OA may outflow the rumen unfermented. Stabilization of ruminal pH through a stimulation of lactate utilization by Selenomonas ruminantium by malate has been proposed as a beneficial effect of this OA. Ruminal pH stabilization as a result of malate addition has been confirmed in most batch and continuous mixed-culture experiments and in vivo. It has also been proposed that OA can decrease methane production in the rumen by competing for metabolic hydrogen. In most batch culture experiments, the decrease in methane production caused by OA has been small, which was explained by simultaneous release of metabolic hydrogen in the conversion of part of added OA to acetate, although substantial decreases were achieved using a slow-release form of fumarate. In vivo effects of OA on methane production have been variable and difficult to explain. With regard to animal performance, OA have sometimes been shown to increase daily gain and feed efficiency in beef cattle and lambs and to enhance milk production in dairy cows, but no effects have been observed in other studies. Inconsistency in productive responses has been attributed to the dose of OA, characteristics of the diet (i.e., forage-to-concentrate ratio, forage type, cereal grains, etc.), chemical form in which OA were fed (i.e., free acid, salts, or encapsulated acids), and characteristics of the experimental animals (physiological state, level of production, ruminal populations, etc.). The use of malate and fumarate in animal feeding is considered safe for the animal, the consumer, and the environment, but the main limitation to their use is currently economical. Future research should aim to identify the conditions in which OA have optimal efficacy.