Recropping intervals for sulfonylurea herbicides are short in the semi-arid subtropics of Australia

Abstract

Chlorsulfuron, triasulfuron and metsulfuron methyl were applied at recommended and double recommended rates in 5 trials (1990–96) in winter (May–July) or late spring (October) to a heavy textured soil in the Central Highlands of the north-east grain region of Australia. Treated areas were sown back to sorghum in the following spring, to sorghum and sunflower in summer, and to chickpea in the following autumn. Shoot dry matter at 3 weeks after emergence and grain yields were measured. Soil residue levels of chlorsulfuron were measured by bioassay following herbicide application and at sowing of sorghum and chickpea. Rainfall and temperature were recorded for each fallow period. The recropping intervals ranged from 2.5 to 4.1 months for spring sorghum, 2.5 to 9.3 months for summer sorghum and sunflower, and 5.7 to 11.7 months for chickpea. The shorter intervals were a consequence of the late spring applications. The spring sorghum crops were significantly injured by all herbicides, when chlorsulfuron residues at sowing ranged from 1.67 to 2.1 ng/g in the surface 15 cm. Summer sorghum, sunflower and chickpea were unaffected by any herbicide treatment, except the covered (rainfall excluded) metsulfuron methyl treatment. Despite the differences in recropping intervals and fallow rain, all summer sorghum crops were sown into low chlorsulfuron residues (ranged from non-detectable to 0.2 ng/g). Similarly, residues measured at chickpea sowing ranged from non-detectable to 0.1 ng/g. The lack of crop response and the low levels of detected herbicide residue indicate that in this environment these herbicides dissipate rapidly, and the safe recropping intervals are much shorter than currently recommended on herbicide labels.