Decreased phytotoxicity of diuron applied over ash of recently burned kangaroo grass (Themeda australis (R.Br.) Stapf)

John TothA, Paul MilhamB and C. Jill kaldorC

A 4/10 Elizabeth Street, Parramatta, New South Wales 2150, Australia.

B New South Wales Agriculture, Agricultural Institute, Forest Road, Orange, New South Wales 2800, Australia.

C New South Wales Health, LMB 961, North Sydney, New South Wales 2059, Australia.


It is not uncommon for soil-active herbicides to be applied soon after crop residues are burned. This can reduce herbicide efficacy. To test the hypothesis that a similar effect might occur with the higher rates of herbicide used for total vegetation control, we burned a grass sward (≈4000 kg dry matter ha-1) that was dominated by kangaroo grass (Themeda australis (R.Br.) Stapf) and within six hours applied diuron (3-(3,4-dichlorophenyl)-1,1-dimethyl urea) at 32 kg a.i. ha-1. There were three other treatments, namely: diuron (32 kg a.i. ha-1) applied after mowing the sward and two controls, i.e. one without diuron or mowing and the other, where the sward was mowed and activated carbon broadcast at 320 kg ha-1, before diuron was applied at 32 kg a.i. ha-1. The latter is a de facto (nil diuron) mowed control.

Measurements of dry matter yield during the following year showed that the burned pre-treatment had reduced the phytotoxicity of diuron; however, the effect on dry matter did not persist. The treatments affected plant populations in the sward throughout the experiment. Population responses also showed that the burned pre-treatment had reduced the phytotoxicity of diuron. The results of this study and others, indicate that the phytotoxicity of soil-active herbicides is likely to be reduced if they are applied soon after vegetation has been burned.


Plant Protection Quarterly (1999) 14 (4) 151-154.