The effects of different sources of nitrogen supplementation on the digestion of fibre components in the rumen of steers

Abstract

Four protozoa-free steers were given diets consisting of approximately equal proportions of a concentrate mixture of rolled barley plus tapioca and ground and pelleted alkali-treated straw. The diets were supplemented with urea and casein (UC), soya bean meal (SBM), ‘normal’ white-fish meal (FMN) or white-fish meal designated as being of ‘low’ rumen degradability (FML). The diets were iso-energetic and were given in amounts calculated to provide sufficient metabolisable energy (ME) to support a growth rate of 0.5 kg day −1. Rumen degradable nitrogen (RDN): ME values (g MJ −1) were estimated to be greater than 1.40 for all diets. The diets were given in a 4 × 4 Latin square design. Polyethylene glycol and chromic oxide were given as flow markers and flows (g 24 h −1) at the abomasum of structural carbohydrate fractions were calculated. Carbohydrate fractions were defined by detergent extraction procedures. Digestibility coefficients for the ash-free NDF-ADF (hemicellulose) fractions were 0.72, 0.80, 0.85 and 0.83 for diets UC, SBM, FMN and FML, respectively. Values obtained with fishmeal-containing diets were significantly different ( P < 0.05) from those with UC diets. Digestibility coefficients of the ash-free ADF-lignin (cellulose) fractions were 0.63, 0.68, 0.76 and 0.74 for diets UC, SBM, FMN and FML, respectively. Significant differences were observed between UC and SBM diets ( P < 0.05) and between UC and FM diets ( P < 0.02). It was concluded that the type as well as the amount of RDN provided in ruminant diets is important in optimizing the digestion of dietary fibre in the rumen.