Effect of level and origin of rumen degradable nitrogen on rumen microbial growth and nitrogen utilisation efficiency of animals fed maize silage at maintenance

Isoenergetic maize silage diets, fed at maintenance to 24 suckling cows (Exp. 1) and to 3 (Exp. 2) or 4 (Exp. 3) rumen fistulated sheep, were compared with a urea and controlled release NPN diet (Exps. 1 and 2) and with a protein-N supplemented diet (Exp. 3). Supplementation increased blood urea concentrations (44.7 22.3 vs. 97.6 23.7 mg urea-N . L -1 ) (Exp. 1) and renal urea excretion (2.5 1.1 vs. 7.6 1.8 g urea-N . d -1 ) (Exps. 2 and 3), whereas blood allantoin concentrations (286.7 77.0 mol . L -1 ) (Exp. 1) and renal excretion of purine derivatives (357.6 90.7 mg purine-N . d -1 ) (Exps. 2 and 3) were not affected, indicating additional N supplementation did not improve rumen microbial growth. However, some deficiency of rumen degradable N might occur in non supplemented diets as suggested by the reduced rumen NH 3 -N and RNA concentrations (868 270 vs. 1466 466 mg RNA . kg -1 rumen contents). maize silage / rumen / microbial growth / maintenance / degradable nitrogen Rsum -Effet du niveau et de la nature de l'azote dgradable sur la croissance microbienne et l'efficacit de l'utilisation d'azote dans le rumen d'animaux nourris d'ensilage de mas l'entretien. Des rgimes isonergtiques base d'ensilage de mas ont t distribus 24 vaches (Exp. 1), et 3 (Exp. 2) ou 4 (Exp. 3) moutons fistuls l'entretien. Ils ont t compars des rgimes supplments en ure et en une source de libration contrle d'ure (Exps. 1 et 2) ou en azote protique (Exp. 3). La supplmentation augmente les concentrations sanguines d'ure (44,7 22,3 vs. 97,6 23,7 mg N . L -1 ) (Exp. 1) ainsi que les rejets urinaires d'ure (2,5 1,1 vs. 7,6 1,8 g N . j -1 ) (Exps. 2 et 3) tandis que la concentration sanguine d'allantone (286,7 77,0 mol . L -1 ) (Exp. 1) et