EFFECTS OF ADAPTATION TO LASALOCID, MONENSIN OR A DAILY ROTATION OF LASALOCID AND MONENSIN ON IN-VITRO FERMENTATION OF A 90-PERCENT CONCENTRATE DIET

Effects of adaptation to L, M or a daily rotation of L and M (R) on in vitro fermentation were measured in a replicated in vitro experiment with a 4 x 4 factorial arrangement of treatments. Treatments were adap tation of ruminal fluid donor steers (0 or ZOO mg steer(-1) d(-1) of L , M or R) and culture ionophore treatment (0 or 4 mu g mL(-1) of L, M or a 50:50 mixture of L and M). At 12 and 24 h, IVDMD was increased (P < 0.05) by adaptation to L compared with M, and decreased at 12 h (P < 0.05) by adaptation to R compared with the average of L and M adapta tion. At 6 and 12 h, culture ionophore treatments increased (P < 0.10) IVDMD, compared with control. After 24 and 48 h of in vitro incubatio n, L adaptation increased (P < 0.01) acetate, decreased (P < 0.01) pro pionate, increased the acetate/propionate ratio (P < 0.01), and increa sed total VFA (P < 0.05), compared with M adaptation. Total VFA was in creased (P < 0.10) for ionophore adaptation treatments compared with c ontrol at 48 h of incubation. Culture ionophore treatments decreased ( P < 0.10) acetate and increased (P < 0.05) propionate at 24 and 48 h a nd decreased the acetate/propionate ratio (P < 0.10) at 24 h of incuba tion versus control cultures, with no measurable effects on total VFA. Adaptation treatments did not interact with culture treatments, sugge sting that animals used as inoculum donors for in vitro experiments in volving ionophores need not be adapted to an ionophore.