The effects of short-term and long-term monensin supplementation, and its subsequent withdrawal on digestion in sheep

Eight Texel adult wethers fitted with ruminal and duodenal simple cannulas were divided into two groups and used for four periods to ascertain whether rumen microbes can adapt to monensin. The animals were fed on a control diet to establish control values during period 1. During period 2 one group remained on this diet while the other group received a monensin supplemented diet. Period 3 began 96 days after the animals first received the supplemented diet to study any effect of adaptation to monensin. The antibiotic was removed during period 4 to study any lasting effect. s received a pelleted diet composed of prairie hay (43.3%), maize grain (34.4%), lupin grain (20.7%) and mineral mixture given at 45 g kg−1 BW0.75 (1000 g dry matter (DM) day−1) together with 100 g wheat straw. DM and organic matter (OM) digestibility were increased (P < 0.05) during both periods of monensin supplementation. These effects disappeared after ionophore withdrawal. Monensin had no effect on postruminal digestion of OM. Starch digestion was not altered by monensin treatment. The ionophore antibiotic decreased (P < 0.05) total volatile fatty acid (VFA) concentrations after the 96 day period of supplementation. It decreased (P < 0.05) ruminal acetate and butyrate molar proportions while increasing (P < 0.05) that of propionate in the VFA mixture. Samples taken at weekly intervals throughout the experiment showed that monensin established its effects on acetate, propionate and butyrate within 7 days of administration. After its withdrawal, VFA returned to control values within 24 h. Monensin supplementation caused a greater decrease in the concentration of rumen protozoa in the short term than in the long-term period of treatment. nophore increased (P < 0.05) the apparent digestibility of N only during the short-term period, but did not alter non-ammonia nitrogen (NAN) flow in the duodenum whilst that of NH3-N decreased (P < 0.05). The flow of bacterial N increased (P < 0.05) only after the long-term period of treatment. There were no treatment effects on the mean retention time of solid particles nor on the turnover rate and the volume of ruminal liquid phase. ering overall results, we can conclude that there is no real adaptation of rumen microbes after a period of 96–146 days of monensin treatment and that most effects disappear within a few hours following monensin withdrawal.