Montana State University

Preharvest herbicide applications can help wheat growers control weedy fields at harvest

July 21, 2009 -- By Fabian Menalled, MSU Extension Cropland Weed Specialist

Preharvest herbicide applications can help wheat growers control weedy fields at harvest. (Photo courtesy of Fabian Menallad.)   High-Res Available

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If producers face serious weed problems as they prepare to harvest wheat, they may consider applying a preharvest herbicide. Such late herbicide applications will probably not be as effective in controlling weeds as most spring applications, but can help to dry out and suppress weeds that escaped spring spraying. This is particularly useful in small grain fields containing weeds that cannot be easily separated from crop seeds.

Few herbicides are cleared for preharvest applications in wheat. Among them are dicamba (Clarity), glyphosate (Roundup Original, Ultramax, UltraMax II, and WeatherMax), and several formulations of 2,4-D Ester or Amine.

Because these herbicides can damage wheat seeds, producers should carefully read the product label before spraying. Also they should conduct a germination test prior to planting the seed from fields where a preharvest treatment was applied.

Finally, care should be taken as vapors from 2,4-D or dicamba may drift into neighboring fields causing significant damage. Herbicide volatilization (vapor drift) is particularly risky when using ester formulations and when temperatures are over 85F.

Prior to applying a preharvest herbicide, producers should consider the relative difficulty of harvesting a weedy field versus the potential of injuring the crop. Research conducted in South Dakota indicated that the greatest risk of yield reduction is during the period five to seven days before heading until seven to 10 days after heading. Furthermore, weed scientists in North Dakota determined that preharvest applications may impact flour and the bread making quality of hard red spring wheat.

Simple tips can help maximize the benefit of a preharvest herbicide application. For example, it is recommended to spot-treat as close to bloom as possible to prevent weed seed production. In addition, weeds at harvest time are generally large and it may take up to 15 days for herbicides to dry them out. Therefore, using full-labeled rates and tank mixes can help manage these large weeds.

Disclosure: Common chemical and trade names are used in this publication for clarity by the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval.

Contact: Fabian Menalled, MSU Extension Cropland Weed Specialist; phone 406-994-4783 e-mail: