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 fer mentation of prairie hay were measured in a replicated in vitro experi ment with a 3 x 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 ion ophores (IONO; no ionophore, 4 mu g mL(-1) of L, or 4 mu g mL(-1) of M ). At 12 and 96 h of incubation, in vitro dry matter disappearance (IV DMD) was increased (P < 0.10 and P < 0.05, respectively) by M-ADAPT co mpared with L-ADAPT, and IVDMD was increased (P < 0.10) by ionophore a daptation treatments compared with control at 96 h of incubation. In c ontrast to effects of adaptation treatments, culture ionophore treatme nts decreased (P < 0.01) IVDMD at 12 and 96 h compared with control; f urthermore, 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 cul ture to unadapted inoculum decreased (P < 0.10) IVDMD compared with co ntrol. 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 iono phore adaptation compared with control at 24 h of incubation and by cu lture 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 ion ophores. In addition, monensin, when added to the culture, seemed to b e more detrimental to in vitro fermentation of a low-quality forage th an lasalocid.