WILD OAT (AVENA-FATUA) POPULATIONS RESISTANT TO TRIALLATE ARE ALSO RESISTANT TO DIFENZOQUAT

In response to farmer complaints of poor triallate performance, wild o at seed was collected from 34 fields in Alberta in the fall of 1990. S creening trials in the greenhouse indicated that 15 of the populations were highly resistant to triallate applied at the equivalent of the r ecommended field rate (1.7 kg ha-1), whereas the other 19 populations were adequately controlled. All triallate-resistant populations were a lso highly resistant to difenzoquat applied at 1.7 kg ha-1 (equivalent to twice the recommended field rate). The effect of increasing rates of both herbicides on dry weight of five of the resistant and two of t he susceptible populations was determined in greenhouse experiments. T riallate applied up to 3.4 kg ha-1 had little or no effect on the resi stant populations, whereas the susceptible populations were controlled at 1.7 kg ha-1. At rates of 6.8 kg ha-1 or higher, there were differe nces among the resistant populations and among individuals within the populations in the response to triallate. Response of the resistant po pulations to increasing difenzoquat rates was variable between experim ents, but in all cases the effect of difenzoquat on wild oat dry weigh t was considerably less in triallate-resistant than triallate-suscepti ble populations. Effects of increasing rates of triallate and difenzoq uat on resistant and susceptible wild oat populations growing with bar ley in field experiments were generally similar to the responses in th e greenhouse.