Patterns of rumen fermentation in bulls fed barley/molasses diets

Four ruminally fistulated bulls were used in a Latin square design to determine the effect of 0, 20, 40, or 60% sugar beet molasses (dry matter (DM) basis) on rumen fermentation. The basal diet consisted of straw (12%), barley (respectively 74, 54, 34 or 14%) and sunflower meal (9%). All diets were balanced for protein and minerals and supplemented with monensin (4 ppm) and sodium bicarbonate (1%). The cattle were fed twice daily, at 08:00 and 16:00. Ruminal fluid samples were collected at 08:00, 10:00, 12:00, 16:00, 18:00, 20:00, 00:00 and 04:00. The pH increased (P<0.05) from 6.23 to 6.69 hours below pH 6.25 decreased (P<0.05) from 12.0 to 0.0 and area below pH 6.25 decreased (P<0.05) from 0.30 to 0.0 h with increasing quantities of molasses in the diet. Total volatile fatty acid (VFA) concentration was 60.3, 66.0, 61.8 and 57.4 mmol/l for 0, 20, 40, and 60% molasses diets, respectively. Propionate, isobutyrate and isovalerate proportions decreased (P<0.05) and those of butyrate increased (P<0.05) with molasses level. The acetate:propionate ratio increased (P<0.05) from 2.8 to 3.1 and to 3.7 as the level of molasses in the diet increased from 0 to 20 and 40%, but amounted to 3.3 at the level of 60%. Ammonia-N decreased (P<0.05) from 20.2 to 17.8, 14.4 and 12.1 mmol/l with increasing molasses content of the diet. The molasses effect, while consistent with other research on rumen fermentation, was less marked than expected, reflecting fermentation moderation through monensin and stabilization with bicarbonate buffer.