Information from this research will be used to further develop and demonstrate recommendations for control of herbicide-resistant horseweed to Michigan soybean growers. We will continue to hold meetings throughout the state to make growers aware of the issues and the options for control of this weed. The awareness from our outreach efforts will continue to be important to identify and manage this troublesome weed.

Update:

View uploaded report

Shiawassee County:
• There was a high population of glyphosate-resistant horseweed at this location.
• Fall applications of Sharpen at 2 fl oz/A + MSO, 2,4-D ester at 1 pt + glyphosate, Sharpen at 1 fl oz + 2,4-D ester + MSO, and Clarity at 1 pt all provided good to excellent control of horseweed at the time of planting.
• Following each of these treatments with an effective spring burndown treatment, resulted in 99-100% control of horseweed within 21 d after the spring application.
• Spring only treatments provided greater than 90% control within 21 DAT.
• By 47 DAT, several of the herbicide treatments started to break and there were only 5 of 20 treatments that resulted in greater than 90% control. These treatments still provided greater than 80% horseweed control at 60 DAT. All but one of these had a fall burndown herbicide application.
• A subsection of treatments were examined to determine the residual activity of several PRE non-ALS herbicides (Table 1). Authority MTZ at the high rate of 18 oz/A provided 90% control 61 DAT. None of these treatments were above 90% by 96 DAT.
• By 96 DAT, only one treatment had enough residual activity to have 90% horseweed control. The residual control of this treatment combine three sites of action and one of these was the ALS-inhibitor chlorimuron, which probably helped with the longer activity since this population was not ALS-resistant.

Table 1 is attached.

• The POST ALS-treatments worked best with the spring treatments that had provided some good initial control, FirstRate at 0.45 oz and Classic at 0.67 oz worked the best.
• Due to the continued emergence of horseweed at this location (including later emergence), even with a good fall and spring treatment growers planting a soybean that would allow them to apply an effective POST would be beneficial for control (LibertyLink or Roundup Ready 2 Xtend). Issues will arise without a POST option.

Ionia County:
• Overall treatments worked well in this trial.
• All fall treatment with the exception of Sharpen 1 oz/A + MSO (81%) provided greater than 90% horseweed control at the spring application timing.
• At 80 d after the spring application, treatments that generally looked the best had a fall followed by spring treatment. Residual herbicide treatment selection was similar for most of the treatments.