Some factors affecting the oxalate content of the tropical grass Setaria sphacelata

Abstract

The effect of urea on the oxalate concentration of Setaria sphacelata CV. Kazungula was studied in three field experiments. In the first experiment conducted in autumn, urea increased oxalate and cation concentration, and the relation between oxalate and cation concentration was linear (r = 0.90). In the second experiment conducted over autumn and spring, there was a trend of increasing oxalate concentration with urea rate and a decline with age of plant material. However, in autumn regrowths, urea reduced the oxalate in young regrowth (to two weeks) but in older regrowths urea increased oxalate to a peak of 6.0 per cent in the dry matter. A third experiment confirmed the increased oxalate concentration following urea fertilization and also an effect of potassium at a high rate, especially when applied as KCl. The ranking of the plant parts for oxalate was-leaf blades > leaf sheaths > stems. No oxalate was detected in the seed heads. Urea increased the oxalate content of leaf sheaths and particularly of stems, but had less effect on the content in leaf blades. A diurnal trend in oxalate content of tillers occurred in both the high oxalate cultivar Kazungula and in the low oxalate cultivar Nandi. Of the total oxalate present, 90 per cent was water soluble. No ammonium oxalate was detected but both sodium oxalate and potassium oxalate were isolated from setaria. The high excess cation concentration in Setaria is probably the reason for the high levels of oxalate encountered with this grass.