Synthesis of Microbial Protein in Ruminally Cannulated Cows Fed Alfalfa Silage, Alfalfa Hay, or Corn Silage

Six ruminally cannulated cows were used in an experiment with a 3 × 3 Latin square design. Three all forage diets - alfalfa silage, alfalfa hay, or corn silage plus 2.2% urea (DM basis) - were fed for ad libitum intake four times daily. The microbial protein marker ¹⁵NH₃ and the liquid marker Cr-EDTA were infused continuously into the rumen for 72 and 48 h, respectively; the solid marker, Yb-labeled forage, was dosed into the rumen twice daily for 60 h. Pool sizes of ruminal NAN were determined by emptying the rumen. Proportions of bacterial N formed from NH₃ were 57, 46, and 82% for the alfalfa silage, alfalfa hay, and corn silage diets, respectively. For all diets, flows of microbial NAN with the liquid and solid phases were about equal. Although feed NAN in the liquid pool was only 12% of ruminal feed NAN, 30% of feed NAN that escaped the rumen flowed with the liquids. Flow of microbial NAN was highest for corn silage (243 g/d) and lowest for alfalfa hay (212 g/d); microbial NAN represented 50% (alfalfa silage and hay) and 76% (corn silage) of total NAN flow. Proportions of NAN intake that were degraded in the rumen were 61, 56, and 57% for alfalfa silage, alfalfa hay, and corn silage (without urea N), respectively; these values were lower than those reported by the NRC. Total flows of NAN from the rumen were 472, 424, and 321 g/d for the alfalfa silage, alfalfa hay, and corn silage diets, respectively. Use of liquid (Cr- EDTA) and solid (Yb) markers to compute the rate of passage of microbial protein proved to be less variable than regression of ¹⁵N enrichment of bacterial NAN over time.