BACKGROUND: Garlic extracts have been reported to be effective in reducing methanogenesis. Related mechanisms are not well illustrated, however, and most studies have been conducted in vitro. This study investigates the effects of supplementary allicin (AL) in sheep diet on in vivo digestibility, rumen fermentation, and shifts of microbial flora. METHODS: Two experiments were conducted using Dorper x thin-tailed Han crossbred ewes. In experiment 1, eighteen ewes (60.0 +/- 1.73 kg BW) were randomly assigned for 29 days to either of two dietary treatments: a basal diet or the basal diet supplemented with 2.0 g AL/head.day to investigate supplementary AL on nutrient digestibility and methane emissions. In experiment 2, six ewes (65.2 +/- 2.0 kg BW) with ruminal canulas were assigned to the same two dietary treatments as in experiment 1 for 42 days to investigate supplementary AL on ruminal fermentation and microbial flora. The methane emissions were determined using an open-circuit respirometry system and microbial assessment was done by qPCR of 16S rRNA genes. RESULTS: Supplementary AL increased the apparent digestibility of organic matter (P < 0.001), nitrogen (P = 0.006), neutral detergent fiber (P < 0.001), and acid detergent fiber (P = 0.002). Fecal nitrogen output was reduced (P = 0.001) but urinary nitrogen output was unaffected (P = 0.691), while nitrogen retention (P = 0.077) and nitrogen retention/nitrogen intake (P = 0.077) tended to increase. Supplementary AL decreased methane emissions scaled to metabolic bodyweight by 5.95 % (P = 0.007) and to digestible organic matter intake by 8.36 % (P = 0.009). Ruminal pH was unaffected (P = 0.601) while ammonia decreased (P = 0.024) and total volatile fatty acids increased (P = 0.024) in response to supplementary AL. Supplementary AL decreased the population of methanogens (P = 0.001) and tended to decrease that of protozoans (P = 0.097), but increased the populations of F. succinogenes (P < 0.001), R. flavefaciens (P = 0.001), and B. fibrisolvens (P = 0.001). CONCLUSIONS: Supplementation of AL at 2.0 g/head.day effectively enhanced OM, N, NDF, and ADF digestibility and reduced daily methane emissions (L/kg BW(0.75)) in ewes, probably by decreasing the population of ruminal protozoans and methanogens.
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