Herbicide-tolerant crops have changed the way many farmers control weeds.

Roundup Ready soybeans, for example, have turned the days of walking beans and physically removing weeds into a thing of the past. Roundup is one brand of glyphosate, a popular non-selective herbicide. Glyphosate use has become widespread since soybeans genetically engineered to tolerate glyphosate were introduced about eight years ago and herbicide-tolerant corn was commercialized more recently. Glyphosate now is used on roughly 80 percent of U.S. soybean acres and nearly 20 percent of corn acres, according to U.S. Department of Agriculture figures.

However, improper use of glyphosate can cause problems. As with any chemical, relying solely on glyphosate-tolerant crops for weed control or incorrectly applying the herbicide could lead to herbicide-tolerant weeds and shifts in the kinds of weeds in fields, University of Nebraska weed scientists say.

Proper management is key to preserving the herbicide's effectiveness. "We're seeing a slow shift in weed species occurring" since glyphosate has become widely used, said Stevan Knezevic, integrated weed management specialist at the university's Haskell Agricultural Laboratory near Concord. "Farmers need to be on the lookout for this."

Roundup controls some species better than others, which leaves fewer vulnerable weeds in many crop fields while tougher-to-control species are increasing, Institute Agriculture and Natural Resources weeds scientists said.

Corn and soybean growers need to watch what types of weeds are in their fields and see if species are shifting, Knezevic said. New or different weeds that withstand glyphosate treatment could be a clue that glyphosate-tolerance is developing.

Growers may need to change weed control tactics. Applying glyphosate at recommended rates and at the optimal time of day are good strategies. "Rate is important," said Bob Wilson, weeds specialist at the university's Panhandle Research and Extension Center at Scottsbluff. "Cutting the rate of Roundup puts a great amount of selection pressure on weed species."

Some producers tend to cut back on the amount of glyphosate they apply to cut costs. However, IANR research found that weeds that already are hard to control with Roundup tend to survive and their numbers increase if treated with less than the recommended concentration of the herbicide. That contributes to a shift to harder-to-control weeds, Wilson said.

"If you use the high rate, or recommended rate, you kill the weed and it doesn't build up over time," he said.

Shifts in species have happened before, Wilson said.

"When 2, 4-D came out in 1948, it was used to control broadleaf weeds, but not grassy weeds," he said. "We saw a shift from broadleaf to grassy weeds."

Glyphosate-tolerant crops should be only one component of an overall weed management program to ensure the long-term effectiveness of herbicide-tolerant crops, Knezevic said.

Growers should rotate crops and use different herbicides as part of overall weed management, IANR weed scientists agree.

"A good integrated weed management program advocates the use of a combination of preventive, cultural, mechanical and chemical tools to keep weed pressure below the threshold levels that reduces yields and profits," Knezevic said.

Corn and soybean growers who rotate glyphosate-tolerant corn and soybeans may want to limit glyphosate to one application per year and use other herbicides, such as soil-applied herbicides, Knezevic said.

In addition, crop rotations could include cultivation to control weeds and herbicides that are not glyphosate-based products.

Since glyphosate use has expanded, horseweed, or marestail, has become resistant to glyphosate in eight U.S. states, he said.

"There hasn't been any confirmed cases in Nebraska, but now with Roundup Ready corn coming on board, we could be pushing the envelope further," Knezevic said.

Other weeds species that are becoming harder to control with glyphosate include: water hemp, wild buckwheat, Pennsylvania smartweed, lady's thumb, ivyleaf morning glory, venice mallow, yellow sweetclover and field bindweed.

Application timing also is important, IANR research shows. Farmers also should apply glyphosate products at the right time of day and temperature. During the growing season, glyphosate works best from 7 a.m.-7 p.m. and when temperatures are above 70 degrees.

"It's best to wait until an hour after sunrise and to cease application about an hour before sunset," Alex Martin, IANR weeds scientist who studied application timing.

In addition, it's important for producers to know what is in their glyphosate products, said Brady Kappler, IANR weed science educator. Nebraska research compared glyphosate brands and found that while they perform about the same, there are differences in how they should be prepared.

Some brands are "fully loaded" and ready to use while others need additional surfactant to work properly. It also is important to add ammonium sulfite (AMS) to the tank when spraying glyphosate to counteract the effects of hard water and to improve weed control. AMS should be added at a rate of 8.5 to 17 pounds per 100 gallons of water.

"All brands are similar, but it's important they are mixed properly for the best weed control," Kappler said.

For more information about herbicide-tolerant crops and weed control, consult Nebraska Cooperative Extension NebGuide G02-1484-A, Use of Herbicide Tolerant Crops as a Component of an Integrated Weed Management Program, available from a local extension office or on the Web at http://ianrpubs.unl.edu/weeds/g1484.htm.

Source: Bob Wilson, Ph.D., professor, agronomy and horticulture, (308) 632-1263
Stevan Knezevic, Ph.D., associate professor, agronomy and horticulture, (402) 584-2808
Alex Martin, Ph.D., professor, agronomy and horticulture, (402) 472-1527
Brady Kappler, associate extension educator, agronomy and horticulture, (402) 472-1544
Writer: Sandi S. Alswager, IANR News Service, (402) 472-3030