Effect of supplementing a fibre basal diet with different nitrogen forms on ruminal fermentation and microbial growth in an in vitro semi-continuous culture system (RUSITEC).

Incubation trials were carried out with the rumen simulation technique (RUSITEC) to study the effects of four forms of N on the growth of ruminal micro-organisms and the fermentation variables when an all-fibre basal diet was incubated. The basal diet consisted of 10 g neutral-detergent fibre (NDF) from grass hay plus 2 g NDF from sugarbeet pulp. N forms were isolated soyabean protein, soyabean peptides, amino acids blended to profile soyabean protein and NH3 as NH4Cl. Half of the daily N supply was infused as NH4Cl and the other half was infused as each of the four treatments described. Non-NH3 N (NAN) forms increased NDF (P = 0.006), acid-detergent fibre (P = 0.003) and cellulose (P = 0.015) disappearance after 48 h incubation, CO2 (P < 0.001), CH4 (P = 0.002) and total volatile fatty acids production (P < 0.001), as well as the molar percentages of isobutyrate, isovalerate and valerate, which reflected the fermentation of amino acid C skeletons. NAN treatments also increased microbial N flow (P < 0.001) compared with NH3, with peptides and protein supporting more (P = 0.036) than amino acids. The proportion of microbial N derived from NH3 decreased successively (P < 0.05) with NH3 > amino acids > peptides > protein treatments, indicating preferential uptake of peptides without passage through the NH3 pool. Microbial efficiency (g microbial N/kg organic matter apparent disappearance) was greater (P = 0.002) for the NAN forms than for the NH3 treatment, with peptides and protein treatments supporting higher (P = 0.009) values than amino acid treatment. These results indicate that N forms other than NH3 are required for optimal fibre digestion and microbial growth.