Effects of adaptation to lasalocid or monensin on in vitro fermentation of prairie hay

Effects of adaptation to lasalocid (L) or monensin (M) on in vitro fermentation of prairie hay were measured in a replicated in vitro experiment with a 3 × 3 factorial arrangement of treatments. Treatments were adaptation of ruminal fluid donor steers (ADAPT; no ionophore, 200 mg steer−1 d−1 of L, or 200 mg steer−1 d−1 of M) and culture ionophores (IONO; no ionophore, 4 μg mL−1 of L, or 4 μg mL−1 of M). At 12 and 96 h of incubation, in vitro dry matter disappearance (IVDMD) was increased (P < 0.10 and P < 0.05, respectively) by M-ADAPT compared with L-ADAPT, and IVDMD was increased (P < 0.10) by ionophore adaptation treatments compared with control at 96 h of incubation. In contrast to effects of adaptation treatments, culture ionophore treatments decreased (P < 0.01) IVDMD at 12 and 96 h compared with control; furthermore, at 12 h, M in culture decreased (P < 0.05) IVDMD compared with L in culture. At 24, 48, and 72 h addition of both L and M in culture to unadapted inoculum decreased (P < 0.10) IVDMD compared with control. Addition of M in culture decreased (P < 0.10) the rate of IVDMD when added to unadapted and L-adapted inoculum compared with C and L in culture. Acetate was decreased (P < 0.10), propionate increased (P < 0.10), and the acetate:propionate ratio decreased (P < 0.05) by ionophore adaptation compared with control at 24 h of incubation and by culture ionophore treatments compared with control at 48 h of incubation. Results indicate that effects of ionophores on in vitro fermentation of prairie hay depended on adaptation of the donor animals to the ionophores. In addition, monensin, when added to the culture, seemed to be more detrimental to in vitro fermentation of a low-quality forage than lasalocid. Key words: In vitro, fermentation, ionophores, monensin, lasalocid