I wrote an article in the August 1994 Today's Farmer that addressed weed resistance to the ALS family of herbicides used in soybean and corn production.

Many of the ALS herbicides that faced resistance problems came to market during the mid to late 1980s. They were so effective that many fields received nothing but ALS chemistries for several consecutive seasons. Weed resistance was reported in the early 1990s. That time frame coincides with the estimated 5- to 8-year period it takes for one resistant weed to evolve into a herbicide resistant weed population.

It has been 7 years since Roundup Ready (glyphosate) technology burst onto the scene. Guess what folks? We are starting to hear rumblings of glyphosate-resistant weeds and/or weed population shifts toward species that are naturally tolerant to glyphosate. Weed species that have developed resistance to glyphosate include marestail (horseweed) in Maryland and Tennessee, and waterhemp in Iowa and Missouri. These two species demonstrate different scenarios concerning glyphosate tolerance. Marestail has always had some natural "tolerance" to glyphosate. It can also act as a winter annual with fall/early spring germination, or as an early summer annual, germinating into June, thus making it hard to control with a non-residual herbicide like glyphosate. Similarly, waterhemp produces multiple germination flushes from mid-spring through late summer. It is also a diecious (sexual) species. With individual male and female plants, the crossing and exchange of a rare genetic trait such as resistance to glyphosate is greatly diversified, accelerated and complicated.

How does a weed population become resistant to a given family of herbicides? Weeds possess many of the same metabolic processes and genetic makeup as do the crops that contain resistance to a group of herbicides. When millions of weed seed per acre exist, the chance that one or two individual biotypes possess genetic resistance to a given group of herbicides occurs naturally. By eliminating those few resistant plants through cultivation and/or exposure to herbicides that have different modes of action, herbicide-resistant weed populations can be reduced or eliminated.

The fastest way to develop resistance is to use the same herbicide mode of action annually, sequentially or both. The flexibility, convenience, efficacy and cost effectiveness of glyphosate resistant weed control systems has led to widespread use. An estimated 80 percent of soybean acreage will be planted to glyphosate-tolerant varieties in 2003. Increases in glyphosate-tolerant corn (estimated 10 to 20 percent of the 2003 corn acreage), the potential release of other glyphosate-tolerant crops, the extensive use of glyphosate as a burndown, combined with non-crop uses of the material, provide even more weed resistance pressure.

I believe the resistance and/or weed species shifts we are currently seeing in glyphosate-resistant cropping systems will continue. Weed scientists have always recommended that we should rotate crops, herbicides and herbicide modes of action. Don't use the same herbicide in sequential applications. Tank mixing herbicides with varying modes of action and using cultivation when possible will also help prevent resistance. Even though glyphosate may possess tougher anti-resistant characteristics compared to the ALS chemistries, the possibility of resistance, weed species shifts and the parallels to 1990s ALS resistance patterns is a cause for concern.

If you expect glyphosate resistance is occurring, keep in mind that weed escapes are rarely the result of herbicide resistance. Weather conditions, soil environments and application errors should be thoroughly investigated before pushing the glyphosate resistance panic button.

An example of misdiagnosed glyphosate resistance was recently reported at the University of Illinois. A stalk borer tunneling in giant ragweed prevented glyphosate from translocating through the plant.

There have been tens of millions of crop acres where glyphosate has been used effectively, while only a handful of potential resistant weed populations have developed. Keep using the glyphosate resistant cropping systems that have worked so well over the past several field seasons. However, remember, if you suspect glyphosate resistance, look for the same "red flag" factors discussed with ALS resistance in the early '90s. They apply to the resistance issues of today.

Glyphosate resistance red flags

• Use of glyphosate year after year in multiple or sequential applications per year.
• Failure to control one weed species (which should have been susceptible) while other weed species are controlled
• Build up or spread of an uncontrolled weed species.
• Very healthy weeds mixed with weeds of the same species that were controlled.

If suspected glyphosate resistant weeds are occurring in your production fields, please contact an MFA or university agronomist to evaluate the situation.