Effects of nonstructural carbohydrate level and starch:sugar ratio on microbial metabolism in continuous culture of rumen contents

A dual flow continuous culture system was used to examine the effects of varying sugar as a proportion of sugar plus starch (NSC) at three concentrations of NSC on nutrient digestibility, ruminal fermentation and microbial growth. Diets contained 240, 280 and 330 g NSC/kg DM. At each NSC level, diets contained 29, 63 and 95 g sugar/kg DM. Corn grain and corn silage were the major starch sources. Liquid sugar blend (LSB) was the supplemental sugar source. Negative responses with interactions were seen at the 63 g sugar/kg DM level compared to no added sugar or 95 g sugar/kg DM in microbial growth, dry matter digested, protein digested, and total carbohydrate digested. Interactions resulted mostly because the negative responses to the 63 g sugar/kg DM were with the 240 and 280 g NSC/kg DM levels, but not with the 330 g NSC/kg DM level. Sugar addition depressed fiber digestion only with the 240 g NSC/kg DM level. At 330 g NSC/kg DM, all sugar substitutions were beneficial. Microbial responses were highest on the 330 g NSC/kg DM diet with 95 g sugar/kg DM, in which sugar made up 290 g/kg of the total NSC. Sugar additions at 95 g sugar/kg DM resulted in large increases in butyrate production. All sugar levels decreased acetate production but had little effect on propionate production.