Lasting effects of monensin on rumen and caecal fermentation in sheep fed a high grain diet

Eventual adaptation of rumen fermentation to monensin feeding has been studied in two rumen and caecal fistulated sheep fed 150 g of hay and 500 g of concentrates at 9.00 h and 16.00 h. After a control period of 23 days, a dose of 60 mg of monensin was infused for 6 days. This period was followed by a 29 day period during which 50 mg were administered. Monensin was introduced into the rumen as an aqueous suspension, just prior to feeding (with the 60 mg dose: before the morning feeding; with the 50 mg dose: 25 mg twice daily before each feeding). The following rumen fermentation parameters were determined in vivo: methane expelled through the fistula for 6 h per day, pH, total volatile fatty acids (VFA) concentration, molar proportions of VFA, lactate and ammonia concentration in rumen fluid sampled 2 h after monensin administration, and in sacco degradability of hay (24 h). In vitro incubations of 24 h were carried out with caecal contents sampled continuously from one sheep between 9.30 h and 10.00 h. Methane production in the rumen (in vivo) and in the caecum (in vitro) was decreased by monensin feeding. The molar proportion of propionate in the rumen was increased, while acetate and butyrate percentages were sometimes lowered. Total VFA concentration was not significantly decreased while ammonia concentration was lowered. Production of propionate in the caecum in vitro was stimulated by monensin administration while methanogenesis and acetate production were inhibited. The in sacco degradability of hay (dry matter) was not affected by monensin administration. These modifications of rumen and caecum fermentation persisted as long as monensin was given, indicating that on a high grain diet, no adaptation of rumen fermentation to the ionophore was observed. On the other hand, the incubations with caecal contents showed clearly that monensin caused analogous and persistent changes in fermentation pattern in the lower gastro-intestinal tract also.