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Vegetable IPM Updates Archive
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Preharvest Weed Control in Wheat grown in Rotation with Vegetables (March 21, 2018)
This is the time of year when people start to regret not controlling weeds earlier in wheat. Uncontrolled weeds begin to become more visible and both the crop and weeds are too big for most herbicides. The weeds not only produce millions of little time bombs (seeds) but can contaminate the crop and make harvest difficult. There are some herbicide options at this point but none are great and all of them can only be applied when the wheat is in the hard dough stage and the crop is essentially done. If the wheat isn’t too far along, both Simplicity and Osprey will control some big broadleaf and grass weeds. They may not be killed but they likely will stop growing. These herbicides can only be applied up to the jointing stage and most vegetables cannot be planted for 5 to 9 months following simplicity or 10 months following Osprey. The growth regulators( 2,4-D,dicamba,MCPA and others) can only be applied up to tillering or the heads will be distorted and yield reduced. All of the preharvest herbicide options can only be applied when the wheat is in the hard dough stage and the crop is essentially finished. Therefore, if the wheat is too far along now, you likely will have to wait. A few of the options include:

Glyphosate: Do not use on wheat grown for seed. There is a zero tolerance for this herbicide in wheat for export to some countries and it also has the potential to reduce seed germ. This may be an option, however, for grain intended for other uses that is seriously contaminated with weeds. Applying glyphosate too early will damage the crop. There is a preharvest interval of 7 days and this treatment is likely to be slow. Application will have to be by air and drift should be avoided as glyphosate is none selective.
Aim/Shark: This is a contact herbicide and will work as a desiccant without systemic activity. There is a preharvest interval of 3 days and results can be slow. Systemic activity is not necessary since this will be used as a burndown harvest aid treatment. Paraquat is not registered for wheat.
Growth Regulators(2,4-D,Clarity,Dicamba): These are highly systemic and volatile. They will need to applied by air and will be difficult to use after march. These may be an option in isolated areas. There is a 7 to 14 day preharvest interval and the plantback interval can be from 1 to 6 months to many vegetable crops.
Weed Control

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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.

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