A glyphosateresistant perennial ryegrass population is also resistant to amitrole

A doseresponse experiment was conducted on a previously confirmed glyphosate resistant perennial ryegrass (Lolium perenne) population from a Marlborough vineyard to examine if it is also resistant to amitrole after an initial experiment suggested amitrole is less effective on this population The plants of two populations of perennial ryegrass Population O (glyphosate resistant) and Population SP (glyphosate susceptible) were multiplied up by splitting out tillers and planting them into pots The plants were sprayed with rates of amitrole from 0 to 9600 g ai/ha and each treatment consisted of five replicates (one plant per replicate) The dry weight of plant material was measured 8 weeks after herbicide application The data were fitted to a three parameter logistic model and the herbicide rate giving 50 reduction in growth (GR50) was calculated The GR50 value of Population SP for amitrole was 523 g ai/ha whereas the GR50 for Population O was found to be 131 times greater This is the first confirmed case of amitrole resistance evolving within New Zealand and further work is currently underway to study this resistance