Does the Inclusion of Atrazine in S-Metolachlor/Mesotrione/Bicyclopyrone Improve Glyphosate-Resistant Canada Fleabane Control in Corn?

A total of seven field studies were conducted in southwestern Ontario from 2018 to 2020 to evaluate S-metolachlor/mesotrione/bicyclopyrone (Acuron Flexi®) and S-metolachlor/atrazine/mesotrione/bicyclopyrone (Acuron®) for the control of glyphosate-resistant (GR) Canada fleabane in corn. Acuron Flexi® and Acuron® applied preplant (PP) controlled GR Canada fleabane 90-97% and 99-100%, respectively. Commonly used PP herbicides in Ontario, dicamba/atrazine + glyphosate and tolpyralate + atrazine + glyphosate controlled GR Canada fleabane 97-99%. Acuron Flexi® and Acuron® applied postemergence (POST) provided 75-92% and 92-99% control of GR Canada fleabane, respectively. Dicamba/atrazine + glyphosate and tolpyralate + atrazine + glyphosate applied POST provided 84-97% and 94-98% control of GR Canada fleabane, respectively. Glyphosate applied POST at 900 ae ha-1 provided only 35-38% control of Canada fleabane in corn. Orthogonal contrasts indicated that herbicides applied PP provided up to 10% greater control of GR Canada fleabane than POST. There was no significant difference between the low and high rates of Acuron® or Acuron Flexi® in controlling GR Canada fleabane. Acuron® controlled GR Canada fleabane 14, 9 and 7% greater than Acuron Flexi® at 4, 8, and 12 WAA, respectively. GR Canada fleabane interference with corn reduced yield up to 42%. However, there was no difference in corn yield between herbicides treatments evaluated. Orthogonal contrasts indicated that reduced GR Canada fleabane interference with PP herbicides evaluated provided 0.9 t ha-1 greater corn yield than the POST herbicides evaluated. There was no significant difference in corn yield with Acuron® or Acuron Flexi® at the lower rate or higher rates. Additionally, Acuron Flexi® applied PP provided a similar corn yield as when applied POST. Based on these results, the use of Acuron® results in improved GR Canada fleabane control than Acuron Flexi® but this did not result in an increase in corn yield. The study concludes that there is potential for Acuron Flexi® for GR Canada fleabane control in corn especially in areas where atrazine use is restricted, however additional weed control tactics may be required for acceptable control.