Frequency of concentrate supplementation for cattle fed barley straw. 2. Ruminal dilution rates, pH and metabolite concentrations

Abstract

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 in vestigate 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. Key words: Cattle, straw, protein, feeding frequency, rumen metabolites, lactic acid