Influence of disodium malate on microbial growth and fermentation in rumen-simulation technique fermenters receiving medium- and high-concentrate diets.

Two incubation trials were carried out with the rumen-simulation technique (RUSITEC). In each trial, four vessels received a diet of grass hay and concentrate (600 and 400 g/kg DM, respectively; diet F), and the other four were fed a diet composed of concentrate and barley straw (900 and 100 g/kg DM, respectively; Diet C). Vessels were given 20 g of the corresponding diet daily, and half of them were supplemented with disodium malate to achieve a final concentration of 6.55 mM. There were no effects (P>0.05) of malate either on pH or on the daily production of NH3-N, but malate treatment increased (P<0.05) DM, neutral detergent and acid detergent fibre disappearance after 48 h incubation. The daily production of propionate and butyrate increased (P<0.001), and the ratio CH4:volatile fatty acids decreased (P<0.001) by supplementing both diets with malate. Whereas adding malate to the F diet produced an increase in acetate production (P=0.011) and the growth of solid-associated micro-organisms (P=0.037), no effects (P>0.05) were observed for diet C. For both diets, there were no differences (P>0.05) between treatments in the daily flow of liquid-associated micro-organisms measured using (15)N as a microbial marker. These results indicate that malate stimulated the in vitro fermentation of both diets by increasing the apparent disappearance of the diet and decreasing the ratio of CH4:volatile fatty acids, but a greater response was observed with diet F. If these results are confirmed in vivo, malate could be used as a feed additive for ruminants fed diets containing medium proportions of forage (i.e. dairy animals) and not only in animals fed high-concentrate diets, as has so far been proposed.