| Summary: | Aflatoxin B<sub>1</sub> (AFB<sub>1</sub>) is a mycotoxin known to impair human and animal health. It is also believed to have a deleterious effect on ruminal nutrient digestibility under in vitro batch culture systems. The objective of this study was to evaluate the effects of increasing the dose of AFB<sub>1</sub> on ruminal dry matter and nutrient digestibility, fermentation profile, and N flows using a dual-flow continuous culture system fed a diet formulated for lactating dairy cows. Eight fermenter vessels were used in a replicated 4 × 4 Latin square design with 10 d periods (7 d adaptation and 3 d sample collection). Treatments were randomly applied to fermenters on diet DM basis: (1) 0 μg of AFB<sub>1</sub>/kg of DM (Control); (2) 50 μg of AFB<sub>1</sub>/kg of DM (AF50); (3) 100 μg of AFB<sub>1</sub>/kg of DM (AF100); and (4) 150 μg of AFB<sub>1</sub>/kg of DM (AF150). Treatments did not affect nutrient digestibility, fermentation, and N flows. Aflatoxin B<sub>1</sub> concentration in ruminal fluid increased with dose but decreased to undetectable levels after 4 h post-dosing. In conclusion, adding incremental doses of AFB<sub>1</sub> did not affect ruminal fermentation, digestibility of nutrients, and N flows in a dual-flow continuous culture system fed diets formulated for lactating dairy cows.
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