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Summary ― Rumen and caecal contents, obtained from slaughterhouse cattle and rumen contents obtained from a fistulated wether were incubated in vitro with ground hay in the presence and absence of, respectively, casein hydrolysate and mucin. Differences in stoichiometry of rumen and caecal fermentations, indicative of reductive acetogenesis in the caecum, were confirmed, except for incubations with free amino acids. Net fermentation end product production was determined after correction for amounts formed in incubations without the substrate. These determined amounts of hay fermentation end products were compared with the amounts calculated from incubations of hay with added casein hydrolysate or mucin, corrected for amounts formed from the latter added substrates incubated alone. With casein hydrolysate, no differences between the determined and calculated amounts were observed, excluding the occurrence of reductive acetogenesis from hay in the presence of free amino acids. With mucin, the calculated amounts indicated an inhibition of methanogenesis, accompanied by increased amounts of proprionate, butyrate and valerate production. This finding was probably related to the greater availability of easily fermented carbohydrates in the presence of mucins. The absence of an increased acetate production in the incubations with added head space hydrogen gas, also indicate the absence of reductive acetogenesis from hay in the presence of mucin.

Summary ― Adaptation of rumen fermentation to monensin administration. Adaptation of rumen fermentation to monensin feeding has been studied with rumen-fistulated sheep receiving a daily dose of 30 mg of monensin for a period of 21 d followed by a 28 d period during which 60 mg doses were administered. The ration consisted of 300 g of hay and 300 g of concentrates, fed at 9.00 hand 16.00 h. Monensin was placed in the rumen as an aqueous suspension, just prior to the moming feeding. Monensin infusion was preceded and followed by a period during which no monensin was infused. The following rumen fermentation parameters were determined: methane production, pH, volatile fatty acids (VFA) molar proportions, total volatile fatty acid concentration, lactate and ammonia concentrations and in sacco degradability of hay. Rumen gas expelled through the fistula was collected for 6 h per day and analysed. Total VFA concentration, molar proportions of individual VFA, pH, lactate and ammonia concentration were determined on rumen contents, sampled just prior the administration of monensin and 2 and 6 h later. In vitro incubations of 3 h were carried out with rumen fluid, sampled 1 h after feeding. In vivo and in vitro methane production was decreased by monensin feeding. The molar proportion of propionate in the rumen was increased, while acetate and butyrate percentages were lowered. The total VFA and ammonia concentrations were also decreased by monensin, but pH values were increased. In vitro production of propionate was stimulated by monensin administration and methanogenesis decreased. The organic matter in sacco degradability was not affected, probably because of the time difference between the introduction of bags and monensin in the rumen. These modifications of rumen fermentation persisted as long as monensin was given, indicating that in this experiment, there was no adaptation to the ionophore. rumen fermentation / sneep / fnonens/n / adapfaf/on INTRODUCTION

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