Effect of supplementing a fibre basal diet with different nitrogen forms on ruminal fermentation and microbial growth in an in vitro semicontinuous 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-NH3N (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 how (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.