Sites of Digestion and Bacterial Protein Synthesis in Dairy Heifers Fed Fresh Oats with or Without Corn or Barley Grain

Abstract

Six Holstein-Friesian heifers fitted with ruminal, duodenal, and ileal cannulas were used in a replicated 3 × 3 Latin square to study the effects of partial replacement [1:1 dry matter (DM) basis] of fresh winter oats (WO) by ground corn (C) or barley (B) on digestion and bacterial protein synthesis. Supplemented diets contained 24% starch, and all diets were fed indoors at 2.5% of body weight (DM basis). Ruminal and total tract digestibilities of organic matter and neutral detergent fiber were similar for all treatments. Ruminal and total tract starch digestibility was similar for C and B diets. Nitrogen intake was greater for WO than for supplemented diets. However, duodenal flows of nonammonia N and bacterial N did not differ among treatments. The efficiency of bacterial protein synthesis was similar for the three diets, suggesting that the fermentation of high quality fresh forage DM provided sufficient energy for the microorganisms in the rumen. Ruminal ammonia-N concentration was greater for WO than for supplemented diets, and for C than for B diet. Ruminal fluid pH and concentrations of total volatile fatty acid were not different among diets, but concentration of acetate was higher and that of propionate was lower for WO than for supplemented diets and for C than for B diet. Supplementation of WO with barley rather than with corn decreased C2:C3 ratio without affecting fiber digestion. Supplements increased N utilization relative to N intake but did not increase duodenal nonammonia N flow.