Doug Buhler is as concerned about weeds as any no-tiller. The weed specialist from Ames, Iowa, is apprehensive about the increased resistance to herbicides that weeds are showing in an ever-increasing frequency.

“I think it’s alarming,” says the U.S. Department of Agriculture researcher. “It’s a serious issue. Weed populations in the fields really are a function of the production practices we use. Weeds respond very rapidly to tillage, they respond to the herbicides we use and they respond to rotations.”

Many Weed Species

Buhler says most no-till fields have 20 or 30 weed species and because of their ability to invade crops and substantially decrease yield, a more comprehensive weed management program is needed.

“Go back to the 1940s when 2,4-D came along and was very effective on many broadleaf species, but 2,4-D didn’t kill grasses,” he says. “Then we shifted to weeds that can tolerate tillage and the herbicides we were using. Then we moved into the weeds that were coming along like velvetleaf and cocklebur, large-seeded weeds that came up from deeper in the soil and are more herbicide tolerant.”

The next era brought no-tillage to the forefront and weed species with an ability to survive in undisturbed habitats, such as winter annuals and small-seeded summer annuals, that lay on the surface and give no-tillers fits. Buhler says waterhemp is an example of a weed that became resistant to certain herbicides in a no-till situation.

“Waterhemp is very well-adapted to no-tillage,” he says. “It doesn’t come up until June and we’ve seen flushes of waterhemp into August in cool, wet summers. What’s happening is it doesn’t make any difference what herbicide you use because if it is a post-emergence without residual, waterhemp comes up after you spray. If you use a pre-emergence, the herbicide is already broken down in the soil and dissipated when the weed emerges.

“That’s a current hole in the system which are the reduced tillage and post-emergence materials without residuals. Late emergers are the weeds no-tillers will have to fight on an increasing basis.”

More Options Needed

As weed populations shift, no-tillers need more options to combat weeds in a specific manner without creating more resistance.

“What we need to do is stop the first weeds,” says Buhler. “If you missed the lambsquarters and the smartweed that came up early, it doesn’t make any difference if you get the late ones. It’s the first ones that come up that are the most competitive.”

Buhler is trying to develop ways to predict when certain weeds will emerge—much like entomologists are identifying key times for insect infestations.

“I think weed control is becoming more and more difficult,” he says. “We’ve killed all the easy weeds. Now we have the tolerant ones. Woolly cupgrass is a good example. It is tolerant to a wide range of herbicides, its seed has a fairly long life in the soil and it germinates over a wide period of time. The next weed like that will be a really nasty one.”

Alerting growers to the next big weed problem is what Buhler hopes to accomplish as weed population shifts bring the next challenge to the surface.

“Certainly, we know some of the weeds that we’ve seen over time cause us problems as tillage systems changed,” says Buhler. “But for those of us who do this type of work, hopefully we’ll understand weeds better and be able to predict the next problem before it arises.”

Economic considerations and herbicide-application levels are among the management decisions no-tillers face in trying to put together an effective weed management program.

“But every field is different,” says Buhler. “No-tillers must understand the strengths and weaknesses of the management options they have available.”

Alternate Control Methods

If no-tillers incorporate a minimal level of in-furrow cultivation, Buhler believes the perennial weed problems will be easier to overcome by introducing a weed control mechanism other than herbicides.

“Changes to soil surface, changes in cover, moisture and temperature all impact the weeds and the environment where these wild species survive,” he says. “Certainly, this issue of perennial weeds changes when we go to no-till, and you would expect that with less tillage there will be more problems. A modest level of in-furrow cultivation disturbs perennial weeds and helps keep them from becoming established.”

Dogbane Results Promising

A 14-year study of dogbane and various tillage practices in a no-till corn-and-a soybean rotation showed promising response methods. A big difference, though, came in wicking the dogbane every other year in the rotation.

“What you’ve done is take the tillage out of the system and that is very suppressive to a weed like dogbane,” says Buhler. “Wicking was a fairly low-cost option of going in and taking advantage of the height differential with the weeds and the crops.”

Among annual weed species, redroot pigweed and giant foxtail were found to thrive on the surface under crop residue, where germination and root establishment are possible.

Marestail A Problem. Marestail is an example of an invading species that moves in with the wind and quickly establishes in no-till fields. The weed can cause big problems because it begins to grow in the spring and prospers in undisturbed soil conditions with no-till. Marestail can be controlled with a burndown the previous fall, but it’s not always that easy.

“If you go farther South, sometimes it gets too big before planting,” reports Buhler. “If you get into Kentucky and Tennessee, you see a lot of problems because marestail gets knee-high before planting. In the North, where it’s not uncommon to plant without a burndown in cool, spring planting, marestail may be established but difficult to detect.”

Rotate Herbicides

The key, Buhler says, is to integrate control strategies for all weeds by rotating herbicides to properly attack different weed species. Combined with different modes of weed control, such as cultivation and differing crop rotations, no-tillers can see improved weed-control management.

“We have to be able to fight weeds in no-till,” Buhler says. “We want to put the best weed management systems in place. We need to rotate herbicides to make sure we’re matching things up as best as possible.”