The use of endogenous nitrogen for microbial crude protein synthesis in the rumen of growing bulls

The objective of this study was to quantify endogenous nitrogen (N) recycle d for microbial protein synthesis in the rumen. Four growing bulls (Schwarz buntes Milchrind; bodyweight: 240-310 kg) with duodenal T-shaped cannulas w ere fed diets containing four levels of crude protein content (200, 156, 10 2 and 63 g/kg dry matter, respectively). The diets were based on wheat, bar ley, tapioca meal, soybean extracted meal, dried beet pulp, meadow hay and straw. The diet; had an energy level of 11.1, 10.9, 10.2 and 9.6 MJ metabol izable energy/kg dry matter. Faeces and urine were collected in four 7-day balance periods. Duodenal flow rate was estimated by TiO2, pelleted with gr ain, as a marker. The relationship between urine N excretion, the amount of microbial N reaching the duodenum, ruminal N balance and N retention were examined and the amount of endogenous N available for microbial protein syn thesis without negative effects on the N retention was determined. It can b e concluded that up to 16% of the microbial N supply could be covered by re cycled endogenous N, but N retention should not be decreased by more than 1 .5 residual standard deviations of maximal N retention.