Post-emerge herbicides coupled with herbicide resistant seed varieties offer convenience and good control on most weeds. But a shift in weed population proves it's best to vary herbicide chemistry and modes of action.

I don't like to let a single weed go to seed if I can help it," said David Kramer. "But we're seeing a shift in the weeds that give us the most trouble and that complicates our weed-control program."

Kramer farms in partnership with his mother, Marjorie, and brother, Steve, just west of Iola, Kan. The Kramers typically rotate corn and soybeans and have gone to less and less tillage, although they still do some pre-plant tillage on most of their land.

"I'm less than satisfied with the pre-emerge residual herbicide packages we've used," Kramer added. "I have tried a Lariat-Bullet tank-mix, and this year, we went to Magnum and Bicep as a pre-emerge. I haven't been happy with anything I've used as a residual herbicide for weeds in corn."

Kramer's observation about weed control is shared by many other growers. Management changes, especially in corn, have made new problems out of some weeds that formerly were mostly a nuisance.

"The advent of no-till has caused some weed shifts," said Paul Tracy, MFA staff agronomist. "No-till leaves more residue on the soil surface, so larger weed seeds--such as cocklebur and velvetleaf--cannot get in contact with the soil. However, smaller-seeded weeds, such as pigweed, can filter down through the residue and be ready to germinate when the soil warms up.

"Most farmers have also gone to earlier corn planting," Tracy added. "We're planting corn in mid-April, rather than up in May. If cool, wet weather holds back the corn, it gives more time for weeds to get going. Most residual herbicides have to work longer, and many of them play out before corn is well established."

That jibes with Kramer's experience.

"Pigweed is hardest to control, in both corn and soybeans," he said. "We have a lot of Palmer pigweed and some waterhemp. Both of them germinate over a long period of time.

"We grow Roundup Ready soybeans, and that takes care of most weeds on the soybean side of the rotation," Kramer added. "A couple of years ago, we tried Roundup Ready corn. We put down a residual pre-emerge package, but we got a period of rainy weather and the corn (and weeds) got too tall before we could come over the top with Roundup. With our environment, corn grows in spurts--it grows fast after a rain, then slows down as the soil dries out. It's hard to make Roundup Ready corn work.

"Still, I wanted something safe to spray over the top, so this year we went with IMI corn and used Lightning herbicide post-emergence," he continued. "The IMI corn gives me a wider window to make a post-emergence spray, and the Lightning has done a real good job on most weeds. The seed cost is less than with Roundup Ready corn, too."

Roundup Ready technology has created much of the excitement where herbicide-tolerant crops are concerned. But biotechnology and conventional plant breeding have produced other herbicide-resistant hybrids. Back in the early 1990s, American Cyanamid developed the IMI herbicide-resistant gene that was deployed by several companies in their hybrids. The hybrids are variously designated IMI, IR or IT, depending on the company selling them, and are resistant to imidazolinone, the active ingredient in Lightning, Pursuit and other ALS-inhibiting chemicals. Shortly afterward, STS hybrids were developed, with resistance to sulfonylurea herbicides (such as Synchrony). Since, other herbicide-tolerant hybrids, such as Liberty-Link, have come on the market.

"I'll plant IMI hybrids on more acres next year, and use Lightning," said Kramer. "Lightning kills a broad spectrum of weeds, and it's more reasonably priced than some over-the-top herbicides. But I'd still like to find a pre-emerge with longer residual activity."

But Kramer may be wishing for more than most pre-emergent sprays can deliver. With late-germinating weeds, most pre-emerge chemicals just don't have the staying power. Kramer generally uses the old corn standby, atrazine, in his pre-emergent package.

"Atrazine hasn't lost its effectiveness, but the legal rate has been cut from 3 pounds per acre to just over 2 pounds, because of water quality concerns," said Dave Comstock, manager of AgChoice at Iola, Kan. "As a result, we list atrazine for weed suppression, rather than for total weed control."

"On the other side, we have a lot more post-emergence chemicals now and they work, although they don't provide much residual activity," said Tracy. "Any grower who depends on over-the-top chemicals needs to get into a two-pass mentality. If you want the best, most effective weed control in corn, plan on spraying the field twice.

"For one thing, the bigger weeds are, the harder they are to kill," he added. "For consistency, you need to get a post-emergent spray on the weeds while they are still small and tender. Weeds such as Palmer pigweed not only sprout later in the season, they are hard to kill. If you spray early to get quicker germinating weeds, you will usually need to come back again to wipe out the later-germinating weeds."

And, you may need to learn to live with a few weeds that escape the sprays.

"While 100 percent weed control is a good goal, it may not be cost effective in every case," said Tracy. "That last 5 percent of the weeds can be very expensive to get."