Pre-emergence herbicides hard to resist for weeds

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With resistance to glyphosate increasing in weed species, the importance of using pre-emergence products as part of an integrated herbicide program is becoming more evident.

Ohio State University Extension weed specialists said that pre-emergence herbicide applications can drastically improve the control of lambsquarter, giant ragweed and marestail in corn and soybean fields, but their effectiveness is contingent upon how they are incorporated into management practices.

Marestail, giant ragweed and lambsquarter remain some of the most challenging weeds to control for several reasons:

* They are some of the first weeds to emerge in the spring, and marestail grows quickly in size, making proper burndown treatments a must to control them.

* They can emerge well into the growing season -- giant ragweed most often -- which makes it difficult to time a single post-emergence application.

* They become more difficult to control with increasing size and age.

* Their increasing resistance to herbicides, both glyphosate and acetolactate synthase (ALS), reduces the number of control options.

Pre-emergence herbicides are meant to be applied prior to weed and crop emergence. The benefits of pre-emergence herbicide applications go beyond getting a head start on weed control. The management practice maintains yield at a minimum and, in many cases, improves yield; allows for greater flexibility of post-emergence herbicides; reduces early season competition between weeds and the crop; and aids in more efficient nitrogen management in corn, which can save money.

The key to capitalizing on these benefits is more timely weed control, something that growers, both in no-till and conventional tillage production systems, really need to focus on.

Growers practicing conventional tillage, especially for soybeans, need to use pre-emergence herbicides, because these growers are simply having a harder time controlling weeds with glyphosate. Growers who omit preplant burndown treatments in no-tillage make applications when weeds are large and old, and use rates too low for the weed size and age are placing themselves at risk for control failures.

No-till soybean growers also are at risk for ineffective weed control. They tend to practice delayed burndown, where they plant soybeans and then let the weeds grow to between 12 inches and 24 inches tall before they make a one-time spray application intended to last the entire season.

This scenario is exacerbating the problem of glyphosate resistance more than any other practice, and it has got to stop. No-till growers should be spraying when weeds are much smaller and younger.

The best method for controlling marestail, giant ragweed and lambsquarter is to incorporate pre-emergence herbicides into other herbicide programs, ones that make use of several herbicide application timings and a diversity of herbicides to compensate for existing or developing resistance issues.

Such management techniques include preplant burndown treatments in no-tillage, use of residual herbicides in conventional and no-tillage, and proper management of post-emergence herbicide applications, including glyphosate.

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