Effects of dietary allicin supplementation on nutrient digestion and gastrointestinal health of Guizhou black goats

This study aimed to reveal the effects of allicin on nutrient digestion, gastrointestinal enzyme activity, rumen fermentation parameters, gastrointestinal morphology, intestinal barrier function, and gastrointestinal microbial ecology of Guizhou black goats. Thirty-two male Guizhou black goats, each aged five months, with an initial body weight of 18.28 ± 0.41 kg, were divided into one of four groups in a completely randomized design: control (CON, without allicin), low allicin (L, 0.5 g/d per head), medium allicin (M, 0.75 g/d per head), and high allicin (H, 1 g/d per head), respectively. Each group consisted of eight replicates with one growing goat per replicate. The experiment lasted for 75 d, including a 15 d acclimation period and a 60 d experimental period. The results showed that the apparent digestibility of crude protein (CP), dry matter (DM), and neutral detergent fiber (NDF) was highest in the M group, being significantly higher than those in the CON group ( P < 0.05). Moreover, ruminal cellulase and cellobiase activities, jejunal trypsin activity, cecal cellulase activity, ruminal total volatile fatty acids (TVFA), acetate, propionate, and butyrate concentrations were all highest in the M group ( P < 0.05). In contrast, ruminal ammoniacal nitrogen (NH 3 -N) concentration was the opposite ( P = 0.015). As allicin inclusion increased, the jejunal trypsin activity and ruminal butyrate concentration exhibited linear responses ( P < 0.05), while the apparent digestibility of DM and NDF, rumen TVFA, acetate, and propionate concentrations, as well as cellulase and cellobiase activities were affected quadratically ( P < 0.05). In the jejunal mucosa, the protein expression levels and the relative mRNA expression levels of claudin 1, claudin 4, and ZO-1 were higher in the M group than in the CON group ( P < 0.05). As allicin supplementation increased, claudin 1, claudin 4, and ZO-1 protein expressions exhibited both linear and quadratic responses ( P < 0.001), whereas the relative mRNA expression levels were modulated solely by the quadratic term ( P < 0.05). Moreover, the L and M groups had significantly greater papillae height and muscle layer thickness than the CON group ( P < 0.05), and papillae width increased in a quadratic manner ( P = 0.029). However, the M group showed increased villus density compared with the CON group ( P = 0.026), with significant quadratic effects on papillae density and villus height ( P < 0.05). Importantly, allicin also improved the gastrointestinal microecological balance by reconstructing the gastrointestinal microbial composition. In conclusion, allicin improves gastrointestinal health by regulating gastrointestinal microbial composition, promoting nutrient absorption and tissue development, enhancing rumen fermentation and gastrointestinal enzyme activities, and strengthening the intestinal barrier. The optimal supplementation level of allicin is 0.75 g/d per head.