Effects of allicin and diallyl disulfide on in vitro rumen fermentation of a mixed diet

The objective of this study was to determine effects of four doses (i.e. 0, 0.5, 5 and 10 mg/l incubation medium) of allicin and diallyl disulfide on in vitro fermentation of a 1:1 alfalfa hay:concentrate diet in batch cultures of mixed rumen micro-organisms from the rumens of sheep fed either a high forage (HF inoculum; 700:300 alfalfa hay:concentrate; 4 sheep) or a high concentrate (HC inoculum, 300:700 alfalfa hay:concentrate; 4 sheep) diet. It was hypothesised that additive effects would depend on the rumen microbial populations, and thus on the type of diet fed to the host sheep. Initial pH in the incubation medium was 6.98 and 6.39 for HF and HC inoculum, respectively. Since antimicrobial effects of both compounds seemed to be time-dependent, 6, 12 and 24 h incubations, representing 5 replicates on non-consecutive days, were chosen. Allicin at the lowest concentration (i.e. 0.5 mg/l) increased (P<0.05) production of acetate, and the acetate:propionate ratio, at 6 h of incubation, and decreased (P<0.05) hydrogen recovery in batch cultures incoulated with HC rumen fluid. However, these effects did not occur at 12 and 24 h. Diallyl disulfide had similar patterns to allicin, and these were maintained at 12 h of incubation. Only a few interactions (P<0.05) between additives and inoculum type occurred and, in general, effects of allicin were more pronounced with HC inoculum, and those of diallyl disulfide with the HF inoculum. Results show the potential of these secondary metabolites to modify rumen fermentation, but little is known about the stability of these phytocompounds and how this is related to their effectiveness in the rumen.