Colonization and source of N substrates used by microorganisms digesting forages incubated in synthetic fibre bags in the rumen

Six mature sheep were fed restricted amounts of either a medium quality roughage or a 1 : 1 mixture of the roughage and barley grain. Disappearance of DM of three roughages (barley straw, oat hay and lucerne hay) from synthetic fibre bags incubated in the rumen for 6 and 24 h was determined. Also, during intraruminal infusions of 15NH4C1, synthetic fibre bags containing each of the three roughages were incubated in the rumen for 6 and 24 h. The origins and amounts of adherent microbial N associated with the bag residues after incubation and washing were estimated from the 15N enrichments of rumen ammonia, adherent microbial N and bag residue total N. The proportion of adherent microbial N derived from the rumen ammonia pool was not affected (p > 0.05) by diet, but was lower (p < 0.05 or p < 0.01) for microorganisms adherent to lucerne hay bag residues (26 and 33% at 6 and 24 h, respectively) than microorganisms adherent to barley straw (47 and 77% at 6 and 24 h, respectively) or oat hay bag residues (44 and 80% at 6 and 24 h, respectively). The proportion of bag residue N consisting of microbial N was not affected (p > 0.05) by the diet, but was lower (p < 0.01) in lucerne hay bag residues (54 and 69% at 6 and 24 h, respectively) than in barley straw or oat hay bag residues (75–76% at 6 h and 81% at 24 h). Microbial N remaining associated with bag residues ranged from 3.7 to 7.6 mg microbial N/g residual DM. Because of this microbial N associated with bag residues, rumen degradability of lucerne hay N was underestimated by ca. 12 and 4% at 6 and 24 h, respectively. The underestimation of the rumen degradability of oat hay N was more than 26% units, and that of barley straw N was more than 75% units. In conclusion, this experiment indicated that the microorganisms digesting low N forages are much more dependent on rumen ammonia as a N substrate than those digesting high N forages, and that microbial N associated with the residues remaining in synthetic fibre bag residues following incubation and washing was substantial.