Effect of protein source on microbial protein synthesis and nutrient digestion in beef cattle fed barley grain-based diets

Four British cross heifers fitted with ruminal and duodenal cannulas were used in an experiment designed as a 4 × 4 Latin square. The basal diet was composed of 90% barley grain concentrate and 10% barley silage (DM basis) with either no protein supplementation (13.6% CP), or an additional 1.2% CP (% of DM) in the form of urea, canola meal (CM) or blood meal (BM). Ruminal ammonia N concentration was highest (P < 0.05) for the urea-supplemented diet (111 ± 18 mg N L-1), but no differences were observed among the control, CM- or BM-supplemented diets (59 to 78 mg N L-1; P > 0.05). Ruminal pH averaged 5.78 and was not affected by protein source (P = 0.97). Canola meal and BM tended (P < 0.10) to increase microbial N flow by 31 g N d-1 or 21% above the control diet. The response of microbial N flow to urea supplementation was intermediate between the control and true protein sources (P > 0.10). Ruminal OM and starch digestion were not affected by the dietary treatments (P > 0.10). In conclusion, barley grain-based finishing diets supplemented with protein sources of varying ruminal degradabilities increased microbial protein supply, but the improvement in microbial protein synthesis had no effect on diet fermentability. Key words: Protein supplements, microbial proteins, barley, beef cattle