[467][468][469][470][471][472][473][474][475][476][477][478][479]. Five ruminally cannulated crossbred steers (474 ± 30 kg) were fed diets containing 70% barley straw and 30% concentrate in an unbalanced 5 × 5 Latin square design experiment to investigate the effects of frequency of feeding concentrate (daily, alternate days or every third day) with different dietary protein concentrations (7.9 and 11.5%) on ruminal liquid and particulate dilution rates, pH and metabolite concentrations. Dilution rates of cobalt-ethylene diamine tetraacetic acid (Co-EDTA) and chromium-mordanted fibre did not differ when low-and high-protein concentrates were fed. Supplemental protein increased (P < 0.05) mean ruminal ammonia concentrations (3.3 vs. 1.6 mM), but had no influence on ruminal concentrations of organic acids. Frequency of concentrate feeding had no influence on dilution rate of digesta markers, pH or on mean ruminal concentrations of ammonia, lactic acid, propionic acid, isobutyric acid, valeric acid and isovaleric acid. Mean concentrations of total volatile fatty acids, acetic acid and butyric acid were, however, reduced (P < 0.05) when steers were fed concentrates on alternate days. The reduced (P < 0.01) ruminal acetic:propionic acid ratios observed in steers fed concentrates on alternate days were consistent with reduced heat productions. Positive relationships (P < 0.05) were obtained between heat production of steers and ruminal concentrations of isobutyric and valeric acids. It was concluded that the low-protein diet did not provide sufficient protein to meet microbial requirements and that under controlled feeding conditions cattle can be fed concentrate on alternate days or every third day without the occurrence of lactic acidosis. Additional research is warranted to examine the relationships between reduced heat production of steers fed concentrates on alternate days and ruminal concentration and production of organic acids. expérience en carré latin 5 × 5 non équilibré qui devait préciser les effets de la fréquence à laquelle un concentré à teneur variable en protéines (7,9 et 11,5 %) était servi (tous les jours, un jour sur deux ou aux trois jours) sur les fluides, le taux de dilution des particules, le pH et la concentration des métabolites dans le rumen. Le taux de dilution du cobalt-EDTA et des fibres mordancées au chrome ne varie pas avec la teneur en protéines du concentré. Le supplément protéique accroît (P < 0,05) la concentration moyenne d'ammoniaque dans le rumen (3,3 c. 1,6 mM), mais n'influe pas sur la concentration des acides organiques. La fréquence à laquelle on sert le concentré n'agit pas sur le taux de dilution des marqueurs des digest, sur le pH ni sur la concentration moyenne d'ammoniaque, d'acide lactique, d'acide propionique, d'acide isobutyrique, d'acide valérique et d'acide isovalérique dans le rumen. Toutefois, on remarque une baisse (P < 0,05) de la concentration moyenne des acides gras volatils, de l'acide acétique et de l'acide butyrique quand on donne le concentré aux bouvillo...