Malic acid is a four-carbon dicarboxylic acid intermediate in the succinate–propionate pathway of ruminal bacteria. Research has shown that malic acid can stimulate the growth of Selenomonas ruminantium, improving the ruminal environment and increasing propionate production. This microorganism is the predominant specie in the rumen, representing from 21 to 51% of the total viable rumen bacteria. In vitro, malic acid has increased concentrations of propionate and total volatile fatty acids, increased pH, decreased methane production and lactate concentration, and increased digestibility of dry matter (DM), organic matter, neutral detergent fibre (NDF). Malic acid can be added to the diet either as the free acid, or as a salt (for example, calcium malate or sodium malate). Studies conducted in vitro showed that the effect of the free malic acid in the rumen is similar to the effect of disodium malate, although malic acid supplies the ruminal fluid with protons, which contributes to decrease ruminal pH.
Martin et al. (1999) found when forage samples were incubated in rumen fluid malate solubilised and -disappeared very rapidly (within 30 minutes). The authors suggested that dietary supplementation with malate may be necessary to ensure adequate rumen concentrations throughout the feeding cycle. Although in vitro studies have shown positive effects of malic acid on rumen fermentation, in vivo studies that evaluate the effects of malic acid on dairy cow performance are inconclusive.
Studies in lactating dairy cows
The evaluation of malic acid in dairy cow diets is very limited and their results are variable. Stallcup (1979) fed cows a diet with 0, 28, or 70 g supplemental malic acid per cow per day. This researcher reported that cows fed 70 g of malic acid had higher milk yield than cows fed 0 g malic acid.
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