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Corn+Soybean Digest

Are Fall-Applied Herbicides Effective?

A Worth County, MO, producer was interested recently in getting more information about fall-applied herbicides, so I sent out a call for help to weed scientists in the North Central Region. The following are some of the responses I got back.

Bill Johnson, University of Missouri weed scientist, stated that he didn’t think there was a straightforward answer as to whether producers should make fall applications. He thinks the issue is field and site specific.

Part of the problem in Missouri is the spread of chickweed and henbit throughout the central and northern parts of the state. Last November, Johnson applied some fall treatments for control of these two weeds in fields that would be planted to soybeans this spring. The early March ratings showed that both chickweed and henbit were controlled well by a number of herbicides tank mixed with 2,4-D at 1 pint/acre. The tank mix partners included Sencor (10 oz/acre); Canopy XL (4.5 oz/acre) plus Express (0.17 oz/acre); Valor (2 oz/acre) plus FirstRate (0.6 oz/acre); or Backdraft (4 pt/acre).

Chad Lee, University of Michigan, says that fall-applied programs will work to prevent a spring burndown application, but typically will not last long enough to prevent the need for a postemergence application. He thinks that fall applications might be the way to go for farmers who want to split their workload or who are targeting weeds such as quackgrass.

Dave Regehr and Gary Kilgore, Kansas State University, both indicated that there was a lot of interest in fall atrazine applications to fields going to corn or milo the following spring. Kilgore says that he customarily recommends 1.5 lbs of atrazine for henbit control, but he goes on to say that control of that weed has not resulted in increased crop yields in the tests he has run.

Regehr is somewhat concerned about the environmental effects of fall applied atrazine in areas with mild, wet winters. There could be too much atrazine runoff on sloping land.

According to Missouri’s Johnson, making a fall atrazine application here is really a moot issue since it is illegal to apply that compound more than 45 days ahead of planting corn in Missouri.

Bob Hartzler, Iowa State University, states that fall application of pre-emergence herbicides has been a growing trend in the northern Corn Belt. This practice spreads the workload and reduces the risk of herbicide failures due to lack of rainfall following at-planting applications. The primary disadvantage of this practice is that herbicides are placed in the environment several months prior to when they are needed to control weeds. Although degradation of the products is greatly slowed by cold soils during the winter and early spring, significant losses can still occur between the time of application and the time when the herbicide is needed to control weeds.

I think that all of the North Central weed scientists would agree that fall herbicide applications will not provide full-season annual weed control during the growing season that follows the fall application

Several weed scientists are concerned about the environmental impact of fall applications. Chris Boerboom of Wisconsin said that they advise against making fall applications and part of the reason is environmental.

Bob Wilson, University of Nebraska, thinks that fall is an excellent time to apply herbicides for control of perennial weeds like Canada thistle, field bindweed and leafy spurge.

Arnold Appleby of Oregon State agrees with this. He suggests that application timing be about 1 week prior to the first frost. That would be around October 1 in Northwest Missouri.

Rene Van Acker, University of Manitoba, also reminded me that fall applications of 2,4-D is the way to go if you are after dandelion.

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