Effects of glyphosate, flupropanate and 2,2-DPA on Hyparrhenia hirta (L.) Stapf (Coolatai grass)

Abstract

Hyparrhenia hirta (L.) Stapf (Coolatai or Tambookie grass) has invaded roadside areas, travelling stock routes and grazing lands in northern New South Wales and southern Queensland. In these situations it is regarded as an undesirable species. There are no herbicides registered for use on this grass and little information is available on the long-term effectiveness of chemical control. Plots consisted of: 3 treatments imposed before herbicide application (untreated, mown and burnt); 5 herbicides [glyphosate, flupropanate (formerly tetrapion), 2,2-DPA, mixtures of glyphosatel flupropanate, and an untreated control]; 3 application rates for each herbicide; and 2 times of herbicide application (spring 1990 and autumn 1991). Herbicide effectiveness on Coolatai grass was measured by comparing reductions in dry matter and phytotoxicity up to 20 months after application. Six months after the spring application, yields were depressed (P<0.05) by herbicides (flupropanate applied at 4.4 kg a.i./ha and 2,2-DPA at 14.8 kg a.i./ha) and preapplication treatments (burnt plots sprayed with flupropanate at 1.1 and 2.2 kg a.i./ha), but their overall interaction was not significant. Ten months after autumn application, Coolatai grass yields were reduced (P<0.05) by herbicide, pre-application treatment and their interaction. Mowing and burning reduced (P<0.05) yields to 40 % of those in undefoliated plots. Yields were lowest (P<0,05) in mown (flupropanate 4.4 kg a.i./ha) and burnt (glyphosate, 2.16 and 4.32 kg a.i./ha, and flupropanate, 4.4 kg a.i./ha) treatments, where mean yields were <30% of the control. Seventeen and 20 months after application, none of the herbicide treatments gave 100% control of Coolatai grass. Glyphosate applied at 2.16 and 4.32 kg a.i./ha in autumn and flupropanate applied at 4.4 kg a.i./ha in autumn or spring were the most effective in the long term, reducing Coolatai grass dry matter yields by 70-80%.