Impact of Carbohydrate and Protein Levels on Bacterial Metabolism in Continuous Culture

Abstract

Diets formulated with three levels of nonstructural carbohydrate (54, 37, and 25% of DM), with various concentrations of degradable intake protein ranging from 19 to 4% of DM, were fermented in continuous cultures to ascertain the effects of ratio of nonstructural carbohydrate to degradable intake protein on bacterial metabolism. Fermenters were maintained at a dilution rate of 12%/h with a solids retention time of 24h. Regardless of degradable intake protein level, bacterial efficiency (g of bacterial N/kg of DM digested) and VFA production (mM/d) were lower for diets with 25% nonstructural carbohydrate compared with the 37 and 54% nonstructural carbohydrate diets. In response to widening nonstructural carbohydrate:degradable intake protein ratios, bacterial efficiencies at all nonstructural carbohydrate levels declined quadratically from 34.2 to 10.3 with the lowest efficiencies on the 25% nonstructural carbohydrate diets. Bacterial protein production, DM digestion, NDF digestion, and VFA production (mM/d) increased linearly in response to dietary protein. The enhanced NDF and DM digestion, VFA production, and bacterial efficiencies observed with the narrower ratios of nonstructural carbohydrate: degradable intake protein support the theory that level of both degradable intake protein and nonstructural carbohydrate should be considered in order to enhance ruminal digestion and bacterial N production.