Optimum nitrogen : sulphur ratio in supplements for cattle consuming tropical pasture hay.

Abstract

OPTIMUM NITROGEN:SULPHUR RATIO IN SUPPLEMENTS FOR CATTLE CONSUMING TROPICAL PASTURE HAY M. MORRISON, A.N. BONIFACE and R.M. MURRAY It is well recognised that the low nitrogen content of tropical pastures is a major constraint to cattle production. Further, it is recognised that responses to nitrogen supplementation depend on a concurrent supply of sulphur (Kennedy and Siebert 1973). There is, however, much argument as to the optimum ratio of nitrogen to sulphur in dry season supplements and the present experiment was undertaken to shed some light on this. Four rumen fistulated Brahman cross steers were fed tropical pasture hay (>90% Heteropogon contortus) and infused with mixtures of urea and sodium sulphate to supply N:S ratios varying from 30:1 to 5:1. The supplements were infused at rates to maintain rumen ammonia levels in excess of 45 mg N/l (Boniface et al. 1986). A mineral mix, complete except for N and S was also fed. Measurements were made of the disappearance of pasture hay dry matter (92.8% OM, 0.55% N, 0.08% S) from nylon bags (250 x 100 mm, 45 pm pore size) held for 48 hours within the rumen of the steers. Figure 1. Disappearance of feed dry matter (%) from nylon bags held for 48 hours in the rumen of steers infused with urea and sodium sulphate at different N:S ratios. As shown in Figure 1, nylon bag digestibilities for unsupplemented steers was 28.5% and increased with all levels of supplementation (P >0.05). The highest digestibility (36.8%) was achieved at a supplemental N:S ratio of 15:1 although this was not significant. These observations, although not in supplements for cattle grazing dry be optimal. Further, they confirm the pastures and the benefit from nitrogen work is continuing. conclusive, suggest that an N:S ratio season tropical pastures of 15:l would low protein content of these tropical plus sulphur supplementation. The BONIFACE, A-N,, MURRAY, R.M. and HOaN, J-P, (1986) Proc. Aust- Soc. AnimProd. 16: (in press). KENNEDY, P.Mrand SIEBERT, B.D. (1972) Aust. J. Agric. Res. 23: 45. 6 Graduate School of Tropical Veterinary Science, James Cook University, Townsville, Queensland 4811.