Rapid increase of herbicide resistance in Echinochloa spp. consequent to repeated applications of the same herbicides over time

Weed control in rice fields has become particularly difficult because of the increased occurrence of herbicide resistance. The objective of the study was to assess in field conditions, in Italy, if repeated applications of the same herbicides on Echinochloa spp. populations already showing an initial level of resistance would increase their level of resistance over a short time. Repeated applications of penoxsulam and cyhalofop-butyl were performed at different rates, either alone, in combination, or in sequence for one, two or three consecutive years (2010–2012) in the same plots. Confirmation of resistance was performed in greenhouse on plants derived from untreated plots and from those that survived the field treatments. Penoxsulam efficacy was lower than that obtained with cyhalofop-butyl. The average efficacy across herbicide treatments declined over time as it was 52%, 32%, and 13% at 28 days after treatments in plots treated for one, two and three years, respectively. The treatments performed in greenhouse confirmed the low herbicide efficacy observed in the field. Study results highlighted that repeated applications of ALS and ACCase inhibitor herbicides can accelerate resistance selection in Echinochloa, and even herbicide rotation, sequencing, or mixing might not be effective if applied at time intervals of insufficient length.