Tropical Grasslands (1992) Volume 26, 165–170

A sap test for the early diagnosis of toxic levels of nitrate in ryegrass

D.J. LYONS1, G.M. HAWLEY2 and A. J. JEFFREY1

1Agricultural Chemistry Branch, Queensland Department of Primary Industries, Indooroopilly, Queensland
2formerly Queensland Department of Primary Industries, Beaudesert, Queensland

Abstract

Concentrations of nitrate (NO3) and total N in dried ryegrass, and nitrate in cell sap expressed from ryegrass, were monitored in samples collected from 1987 to 1991 from sites in south-east Queensland. Several varieties of ryegrass were studied over a wide range of plant moisture status (82–94%).
A significant linear relationship (P < 0.05, R2 = 0.73) was established between sap nitrate and nitrate in dried grass. A range of 1095–1445 mg/L NO3 in sap was predicted as equivalent to 8850–13 300 mg/kg NO3 which covers the range of levels of nitrate in dried grass reported elsewhere as the maximum safe level of nitrate in feeds.
Results show that grazing ryegrass during the 20-day interval after fertiliser application (125 kg urea/ha/35 days) in May to mid July would have been dangerous as nitrate levels in the grass were high. From late July on, nitrate levels were sufficiently low to allow ryegrass to be grazed 14 days after fertiliser application.
Procedures and interpretative guidelines for the sap nitrate test are described.

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