Background: Aflatoxins are fungal secondary metabolites negatively affecting ruminant performance; however, little information is available on their impact on rumen fermentation. Aims: This study aimed at determining the effects of different concentrations of aflatoxin B1 (AFB1) from Aspergillus flavus on in vitro gas production and ruminal fermentation parameters using two experiments (Exp.). Methods: In Exp. 1, two concentration ranges (0, 0.5, 1, and 1.5 µg/ml of rumen inoculum as low and 0, 5, and 10 µg/ml as high concentration ranges) were used to evaluate AFB1 effect on gas production kinetics using 96-h incubations. In Exp. 2, only the high concentration range was used to investigate AFB1 effects on ruminal fermentation parameters using 24-h incubations. Results: In the low concentration range, the half-time of asymptotic gas production (T 1/2 ) increased and the fractional rate of gas production (µ) decreased linearly with AFB1 dosage (P<0.05). However, in the high concentration range, the asymptotic gas production (A) and T 1/2 decreased; and the lag time (L) and “ µ ” increased linearly (P<0.001) by increasing the concentrations of AFB1. In Exp. 2, dry matter (DM) and organic matter (OM) disappearance, microbial biomass (MB) and total volatile fatty acids (TVFA) concentrations were depressed, but pH and ammonia-N concentration increased (P<0.01) by increasing the concentrations of AFB1. The pattern of rumen volatile fatty acids (VFAs) was also modified by AFB1, as the propionate proportion increased at the expense of acetate. Conclusion: Aflatoxin B1 had an adverse effect on in vitro ruminal fermentation parameters in high concentration ranges (5 and 10 µg/ml).
