Effect of nitrogen, sulphur and phosphorus fertilizer on the production, mineral content and cyanide potential of forage sorghum

Abstract

An experiment to determine the effect of nitrogen, sulphur and phosphorus fertilizer, applied at sowing, on the growth and chemical composition of a hybrid sorghum x sudangrass forage, is reported. The factorial design was repeated at six sites in northern New South Wales in one year, 1976-77. Urea (50 kg nitrogen ha-1) substantially reduced plant populations at three sites. At the first harvest, dry matter production, after adjustment for plant population, was more than doubled by 20 kg phosphorus ha-1; yields were depressed by nitrogen where phosphorus was withheld. Hydrocyanic acid (HCN) potential (mean 0.04%), was increased 28% by nitrogen fertilizer, and reduced 34% by phosphorus fertilizer. Sulphur content was increased from 0.15 to 0.17% by nitrogen fertilizer, and reduced from 0.175 to 0.142% by the phosphorus treatment; it was unaffected by 20 or 50 kg sulphur ha-1 applied at sowing. At two sites, phosphorus increased the sodium concentration but the enhanced levels were still below recommended dietary requirements. The ratio of nitrogen : sulphur (after adjustment for potential loss of sulphur in the detoxication of HCN), was increased from 25:1 to 29:1 by nitrogen fertilizer. The response to sulphur was slight, and the effect of phosphorus was inconsistent. Similar but reduced effects on growth and composition occurred in the second harvest. Hydrocyanic acid levels were much higher, and the adjusted nitrogen:sulphur ratios were exceptionally wide. On the evidence available it appears that the direct supplementation of animals is the most feasible means of ensuring adequate levels of sulphur and sodium in the diet.