Kansas Wheat: Pre-Harvest Weed Control

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Recent hail storms and other problems have affected wheat stands in some areas of Kansas. The resulting thin stands in some areas, along with the abundant rains in May, have caused weeds to start showing up in many wheat fields — especially in fields not treated earlier.

Herbicide Resistance Info


When broadleaf weeds are given the opportunity to grow rapidly in wheat fields because of wet weather and open canopies at the end of the growing season, these weeds flourish and often grow above the wheat canopy.

This raises several potential concerns, including harvest difficulties, dockage problems, weed seed production, and soil water depletion. No one wants to spend extra money on a below-average crop, but it may be necessary.

Figure 1. Weeds in wheat near harvest time. Photo by Dallas Peterson, K-State Research and Extension.

Unfortunately, there aren’t many good options at this point in time. There are also a lot of questions about which herbicides are approved and the “use guidelines and restrictions” for pre-harvest treatments in wheat. Listed below are the various herbicide options producers can use as pre-harvest aids in wheat.

There are differences in how quickly they act to control the weeds, the interval requirement between application and grain harvest, and the level or length of control achieved. All of them will require good thorough spray coverage to be most effective.

Please note that the 2,4-D rate approved for pre-harvest weed control in wheat has been reduced to a maximum of 0.5 lb/acre, which is equal to 1 pt of a 4-lb formulation or 2/3 pt of a 6-lb material. 2,4-D also has a 14-day pre-harvest requirement. 

Another herbicide that is sometimes mentioned as a possible pre-harvest treatment is paraquat. Paraquat is not labeled for pre-harvest treatment in wheat. Application of paraquat to wheat is an illegal treatment and can result in a quarantine and destruction of the harvested grain, along with severe fines. 

Product and rate

Advantages

Disadvantages

Comments

Aim EC (1 to 2 oz)

Acts quickly, usually within 3 days.

Short waiting interval before harvest – 3 days.

Controls only broadleaf weeds.

Regrowth of weeds may occur after 2-3 weeks or more, depending on the rate used.

Apply after wheat is mature. Always apply with 1% v/v crop oil concentrate in a minimum spray volume of 5 gal/acre for aerial application and 10 gal/acre for ground applications.

Do not apply more than 2 oz of Aim during the growing season.

Dicamba (0.5 pt)

Controls many broadleaf weeds. 

A waiting period of 7 days is required before harvest.

Acts slowly to kill the weeds.

Controls only broadleaf weeds.

High potential for spray drift to susceptible crops.

Apply when the wheat is in the hard dough stage and green color is gone from the nodes of the stem.

Do not use treated wheat for seed unless a germination test results in 95% or greater seed germination.

Glyphosate (1 qt  of 3 lb ae/gal product, or 22 fl oz of Roundup PowerMax or WeatherMax)

Provides control of both grasses and susceptible broadleaf weeds.

Acts slowly. May take up to 2 weeks to completely kill weeds and grasses.

Cannot harvest grain until 7 days after application.  Kochia, pigweeds, and marestail may be resistant.

Apply when wheat is in the hard dough stage (30% or less grain moisture).

Consult label for recommended adjuvants.

Not recommended for wheat being harvested for use as seed.

Metsulfuron (0.1 oz)

Provides control of susceptible broadleaf weeds.

Acts slowly.

Cannot harvest grain until 10 days after application.

Controls only susceptible broadleaf weeds.  Kochia, pigweeds, and marestail may be resistant.

Apply when wheat is in the dough stage.

Always apply with a nonionic surfactant at 0.25 to 0.5% v/v.

Generally recommended in combination with glyphosate or 2,4-D.

Do not use on soils with a pH greater than 7.9.

Weeds growing under limited moisture may not be controlled.

Do not use treated straw for livestock feed.

2,4-D LVE (1 pt of 4lb/gal product or 2/3 pt 6 lb/gal product)

Provides control of susceptible broadleaf weeds.

Acts slowly. Weak on kochia and wild buckwheat.

Cannot harvest grain until 14 days after application.

Apply when wheat is in the hard dough stage to control large, actively growing broadleaf weeds.

Weeds under drought stress may not be controlled.

Do not use treated straw for livestock feed.

It is very difficult to estimate the value of preharvest weed treatments as it will depend in part on the differences a treatment would have on harvest efficiency and dockage.

It may not pay to treat wheat with lower weed densities unless harvest is delayed. If the weeds are about to set seed, a preharvest treatment can go a long way toward reducing weed problems in future years by preventing seed production.

In the coming weeks, we will address the issues of controlling weeds and volunteer wheat shortly after harvest this year. This will be important in reducing the incidence of wheat streak mosaic, which was a major problem across western Kansas in 2017.