Utilisation of Barley Straw by Steers: The Effect of Quantity and Source of Nitrogen on the Degradation of Straw Fractions, Particle Outflow Rate and Intake

Abstract

Nsahlai, I.V., Bryant, M.J. and Umunna, N.N. 1998. Utilisation of barley straw by steers: The effect of quantity and source of nitrogen on the degradation of straw fractions, particle outflow rate and intake. J. Appl. Anim. Res., 14: 33–50. This study evaluated the effect of quantity of rumen degradable nitrogen (RDN; 1.37 vs 1.87 g RDN/MJ ME) and of source of nitrogen (N; Fish vs rapeseed meal) on the degradability of straw fractions, rumen metabolities, particle passage rate, intake and digestibility of barley straw using six ruminally fistulated Friesian steers in a replicated 3x3 latin square experimental design. The degradability of leaves of spring (LS) and unknown (LU)varieties and stems of spring (SS) and unknown (SU) varieties of barley straw was determined. The three test protein supplements were designated low nitrogen with rapeseed meal (LNR; 1.37 g RDN/MJ ME; 14.25 g N/kg DM) and high nitrogen with either rapeseed meal (HNR; 1.86 g RDN/MJ ME; 18.80 g N/kg DM) or fish meal (HNF; 1.36 g RDN/MJ ME; 18.82 g N/kg DM). For both the DM and ADF, leaves degraded to a greater extent (P<0.01) and faster (P<0.01) than stems. Increased RDN improved the rate of degradation of straw fraction by proportionately 0.082 to 0.289, but the difference was significant (P<0.05) for the LS fraction only. With the exception of LU, HNF increased (P<0.05) the rate of degradation by proportionately 0.139 to 0.153 compared with LNR. Both HNR and HNF increased straw intake relative to the LNR. HNR also increased the duration over which rumen concentration of ammonia-N was above 14 mg/l compared to HNF (P<0.05) and LNR (P<0.05). Meanwhile, HNF sustained higher ruminal concentrations of iso-valerate than either HNR or LNR. The HNR and HNF diets were respectively associated with 7% and 10% increase in the flow of indigestible matter through the gut. It was concluded that the quantity of RDN but not the source of N is an important factor affecting the degradation of leaves of barley straw.