LAUSR

Echinochloa colona (L.) Link is one of the most troublesome weed species in rice (Oryza sativa L.) crops. Despite numerous cases of herbicide resistance in E. colona worldwide, in Colombia the reports are scarce and most of them dating from over two decades ago. To screen the resistance of E. colona to bispyribac-sodium, cyhalofop-butyl and quinclorac, in the Saldaña and Purificación counties, a survey was carried out in 23 rice fields, through a grid of 26 squares of 2.56 km2 each. Seedlings from 23 populations were treated with commercial formulations from these respective herbicides at their recommended (1x) dose and twice the dose (2x) under controlled conditions. Relative fresh weight and percent control were evaluated. Populations with relative fresh weight greater than 20% and control below 80%, were categorized as resistant. For statistical analysis, a mixed model was used with populations as a random effect. The distribution of resistance was evaluated by a spatial autocorrelation analysis. It was established that 91% of populations were resistant to bispyribac-sodium, 48% to cyhalofop-butyl and 43% to quinclorac. Sixty-five percent had multiple resistance to two herbicides and 22% to three herbicides. Resistance was randomly distributed, according to the results obtained from the mixed model that showed a homogeneous response of populations within fields and heterogeneous among fields. This indicates that management strategies at field level generate a local selection pressure that determines the evolution of the resistance independently in each field.