While plant breeders have been busy selecting crops for herbicide resistance, Mother Nature has been doing the same thing with weeds.

Once-dependable weed-killers no longer do the job - or do it as well.

"It's a nit-picky point in a way, but resistant weeds haven't really evolved. They were always there," points out Andy Kendig, a weed scientist with the University of Missouri.

"A few weeds - maybe only one or two in a million - are resistant to a particular chemical," says Kendig. "But if they survive spraying with that herbicide and farmers keep using it year after year, those weeds keep producing seed and keep multiplying."

Growers developed the habit of using some herbicides season after season, primarily because the products were effective, convenient and worked well. As tolerant weeds survived long enough to bear seed, their resistant progeny made up a bigger percentage of weeds growers had to fight.

The growing list of weeds that have been "selected" for resistance include these:

* Cockleburs that resist herbicides such as Classic, Pursuit, Beacon and Scepter.

* Tall waterhemp with resistance to ALS herbicides.

* Barnyardgrass that has resistance to Propanil in rice.

* Johnsongrass with resistance to Poast, Fusilade and Assure in soybeans and cotton.

New herbicides show promise, but there's no reason to believe they won't be susceptible to weed resistance at some point.

"Roundup was a little weak on some weeds from the beginning," says Kendig. "For example, we see not-quite-dead morningglories when we spray Roundup. It varies from year to year, but the potential is there.

"Still, I like Roundup Ready soybeans," he adds. "The simplicity is good; it controls a lot of tough weeds. But I'd use a tank-mix if I had something like morningglory or hemp sesbania."

Roundup Ready corn looks good for the same reasons. But Kendig is cautious about using Roundup on both corn and beans in a rotation.

"I'd still use an atrazine-based program with corn," he states. "We haven't come up with a corn herbicide that replaces the simplicity and effectiveness of atrazine."

Ford Baldwin, University of Arkansas extension weed specialist, shares Kendig's concerns about weed resistance.

"It's an issue every grower will have to face head-on," says Baldwin. "Failure to implement resistance management programs could make good herbicides useless, and could eventually result in weed species that cannot be controlled with chemicals."

Baldwin and Kendig suggest these management strategies:

* Rotate crops wherever possible.

* Rotate herbicides having different modes of action, or use tankmixes of herbicides with different modes of action.

* Avoid making sequential applications of herbicides with the same or similar weed-control activity.

"I would urge producers to get together with their county extension agents, crop suppliers or chemical dealers to lay out their programs," says Kendig.

"At least have your chemical dealer help you keep an eye on what you're doing. New herbicides do not lessen the need for management details. If you grow Roundup-resistant crops and spray nothing but Roundup, you are setting up the same kind of pattern that gave us herbicide-resistant weeds in the first place."