THE EFFECTS OF SHORT-TERM AND LONG-TERM MONENSIN SUPPLEMENTATION, ANDITS SUBSEQUENT WITHDRAWAL ON DIGESTION IN SHEEP

Eight Texel adult wethers fitted with ruminal and duodenal simple cann ulas were divided into two groups and used for four periods to ascerta in 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 receive d a monensin supplemented diet, Period 3 began 96 days after the anima ls first received the supplemented diet to study any effect of adaptat ion to monensin, The antibiotic was removed during period 4 to study a ny lasting effect. Animals received a pelleted diet composed of prairi e hay (43.3%), maize grain (34.4%), lupin grain (20.7%) and mineral mi xture given at 45 g kg(-1) BW0.75 (1000 g dry matter (DM) day(-1)) tog ether with 100 g wheat straw. DM and organic matter (OM) digestibility were increased (P < 0.05) during both periods of monensin supplementa tion. These effects disappeared after ionophore withdrawal. Monensin h ad 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 an d butyrate molar proportions while increasing (P < 0.05) that of propi onate in the VFA mixture, Samples taken at weekly intervals throughout the experiment showed that monensin established its effects on acetat e, propionate and butyrate within 7 days of administration, After its withdrawal, VFA returned to control values within 24 h. Monensin suppl ementation caused a greater decrease in the concentration of rumen pro tozoa in the short term than in the long-term period of treatment. The ionophore increased (P < 0.05) the apparent digestibility of N only d uring the short-term period, but did not alter non-ammonia nitrogen (N AN) now 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 per iod of treatment. Then were no treatment effects on the mean retention time of solid particles nor on the turnover rate and the volume of ru minal liquid phase. Considering 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 f ew hours following monensin withdrawal. (C) 1997 Published by Elsevier Science B.V.