Pre- and post-emergent herbicides tested as alternatives to glyphosate for vegetation management during pine and eucalypt establishment, South Africa

Abstract

Pre-emergent, selective and desiccant herbicides were tested for the control of competing vegetation in a pine and eucalypt trial and, if successful, to reduce the reliance on manual ring-weeding and/or glyphosate (current practice). Both trials consisted of ten treatments replicated three times and laid out in a randomised complete block design. The treatments included an untreated weedy control, a weedfree control (manual ring weed + glyphosate), two pre-emergent herbicides (isoxaflutole and/or indaziflam), a desiccant herbicide (pelargonic acid) and selective post-emergent herbicides (clethodim and clopyralid as a tank mix). The timing, frequency and quantity of herbicide applied per treatment were recorded after each spraying event, with vegetation cover, tree growth and phytotoxicity also assessed on these occasions. In comparison to the weedy control, all the herbicide treatments resulted in the suppression of competing vegetation, together with significantly improved tree growth (similar to that of the weedfree control). Of the herbicides tested, indaziflam and isoxaflutole provided effective and long-term suppression of the target vegetation, but with sedges and ferns not as well controlled as the other vegetation types. Although clethodim and clopyralid were effective for the control of younger, more susceptible vegetation types, they were not as effective on woody perennials. Pelargonic acid proved effective for the control of young weeds in the immediate area around the trees, but leaf scorch occurred where the herbicide came into contact with the lower needles/leaves. In the pine trial, some discolouration and abnormal needle growth was observed following application of clethodim and clopyralid, but this was transient and did not have an impact on overall tree performance. In addition to demonstrating the viability of desiccant, selective and pre-emergent herbicides as alternatives to glyphosate for vegetation management, the study showed that pre-emergent herbicides resulted in an overall reduction in the total active ingredient used to achieve this control.