Dose–response effects of chitosans on in vitro rumen digestion and fermentation of mixtures differing in forage-to-concentrate ratios

The in vitro gas production technique was used to study the dose–response effects of three chitosans on rumen fermentation of three mixtures differing in their forage-to-concentrate ratio (80:20, 50:50 and 20:80). Four concentrations (0, 325, 750 and 1500 mg/l of culture fluid) for each chitosan (CHI1, CHI2, and CHI3) were incubated for 24 h in diluted ruminal fluid with three different mixtures. Samples were collected to determine pH, volatile fatty acid (VFA) and ammonia–N concentrations. Methane concentration was estimated stoichiometrically, and in vitro true organic matter digestibility (IVOMD) was calculated. In the high-forage mixture, no differences were found between additives except for propionate molar proportion, for which CHI1 showed a lower proportion than CHI2 or CHI3. Increasing doses of chitosans did not affect total VFA or acetate molar proportions, but decreased IVOMD as well as butyrate and branch-chained volatile fatty acid (BCVFA) molar proportions, and increased propionate molar proportion as well as ratios of C3:C2 and VFA to truly degraded substrate (VFA:TDS). In the medium-forage mixture, no differences were found between additives. Increasing doses of chitosan did not affect total VFA or acetate molar proportions or methane production compared to control, but decreased IVOMD as well as butyrate and BCVFA molar proportions, and increased propionate molar proportion as well as C3:C2 and VFA:TDS ratios. In the low-forage mixture, increasing doses did not reduce total VFA, but decreased IVOMD, acetate, butyrate, and BCVFA molar proportions and methane production, and increased propionate molar proportion as well as C3:C2 and total VFA:TDS ratios compared to control. No differences were found between additives in IVOMD or butyrate molar proportion, but differences were found in acetate molar proportion, with CHI1 showing a higher proportion than CHI2 or CHI3. Differences between additives were also found for propionate molar proportion, C3:C2 ratio, and methane production, and an additive × dose effect was found, with CHI1 showing a weaker response than CHI2 or CHI3 at all tested doses. Differences were found between additives for BCVFA molar proportion, with CHI1 and CHI2 showing greater production than CHI3, and for VFA:TDS ratio, with CHI1 showing a higher ratio than CHI2 or CHI3. These results indicate that chitosans affect rumen microbial fermentation in a dose-dependent manner and that the optimum dose depends on the nature of the incubated substrate and the chitosanʹs characteristics.