Incubations were carried out with a batch culture system to study the effects of different N sources on the fermentation by ruminal micro-organisms from Merino sheep of two fibre substrates derived from feedstuffs that differed in their fermentation rate. The substrates were neutral-detergent fibre (NDF) from barley straw and sugarbeet pulp. N sources were ammonia (NH4Cl) and peptides (Trypticase). Three treatments were made by replacing ammonia-N with peptide-N at levels of 0 (AMMO), 33 (PEPLOW) and 66 % (PEPHIGH) of total N. There were no differences (P>0·05) between treatments in NDF degradation for both the barley straw and the sugarbeet pulp. Peptides increased (P<0·05) total volatile fatty acids daily production for both substrates, with greater values (P<0·001) for PEPHIGH than for PEPLOW for the sugarbeet pulp. The presence of peptides also increased (P<0·05) microbial N synthesis compared with AMMO, with PEPHIGH supporting more growth (P<0·001) than PEPLOW when the sugarbeet pulp NDF was fermented. The presence of peptides increased (P<0·01) the amount of solids-associated micro-organisms (SAM)-N for both the barley straw and the sugarbeet pulp fibres, values in the PEPHIGH treatment being higher (P<0·001) than those in PEPLOW. The proportion of SAM-N in the total microbial N was not affected (P>0·05) by the presence of peptides compared with the AMMO treatment, but values were greater for the PEPHIGH compared with the PEPLOW N source, reaching statistical significance (P<0·05) only for the sugarbeet pulp. For liquid-associated micro-organisms, the AMMO treatment resulted in the greatest (P<0·05) proportion of N derived from ammonia for both substrates, with a further decrease (P<0·01) for the PEPHIGH treatment compared with the PEPLOW for the sugarbeet pulp, indicating preferential uptake of peptides when they were available. Microbial growth efficiency (g microbial N/kg NDF degraded) was not affected (P>0·05) by N source. These results indicate that N forms other than ammonia are needed for maximal growth of fibre-digesting ruminal micro-organisms.
