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Weed problems get harder to resist

A survey found that 57% of farmers are not concerned about herbicide resistant weeds. If you're not alternating herbicides properly, your complaint against herbicide failure may be easily dismissed.

It's easy to ignore the problem of herbicide resistant weeds for a while. But ignore this problem long enough and it's likely to get worse before it gets better. >From triazine resistance through ALS resistance, weed scientists have bombarded farmers with the threat of uncontrollable super weeds for so long, it's getting difficult to take them seriously. Now, they caution, the problem is reaching a critical juncture.

"For some weeds we already face the risk of running out of effective modes of action for our herbicides," warns Ian Heap, weed scientist, WeedSmart, Corvallis, OR. As proof, Heap points to his native Australia, where a worst-case scenario exists. Some 4,000 small-grain farmers confront a biotype of annual ryegrass that is resistant to virtually all available herbicides. Scientific tests confirmed that some populations of this weed carry resistance to nine different chemical classes, all with unique modes of action. In two separate cases, they identified ryegrass biotypes with resistance to glyphosate, the active ingredient in Roundup herbicide.

"Many of these farmers no longer have effective herbicide treatments for their crops and have had to move away from cropping. They are either raising sheep or leaving fields fallow for several years and using cultivation to try to control the weeds," Heap relates. "It is very possible that once resistance develops to one herbicide, it will take out at least two or three of the herbicides that are available for controlling that weed in that crop."

Increased threat of resistance. The weed scientist warns of several conditions that may prevent U.S. farmers from keeping up with the evolving rate of herbicide resistance. Heap notes that, for starters, herbicide-resistant weeds are increasing rapidly. Every year since 1978, scientists identified an average of nine new cases of herbicide-resistant weed biotypes. His official survey reports 188 different confirmed cases of herbicide resistant weed biotypes in 42 countries.

Another reason herbicide resistance may catch up with U.S. farmers, Heap says, has a political basis. Heavy workloads at the EPA are plugging the product registration system. "The EPA is so busy reviewing many older chemistries that they have a huge backlog of work, and that probably means that, for a while, very few new registrations will get through."

The types of resistance scientists now find, and an increasing reliance on chemical weed control, add to the renewed resistance threat. In the last 10 years, more weeds were identified with resistance to the ALS-inhibitor herbicides (Glean, Pursuit and Accent) than to any other herbicide mode-of-action. To date, 38 weed species have evolved resistance to ALS-inhibitor herbicides in 11 countries. Some 17 of these species are found in the United States. The ALS-inhibitor chemicals account for 17% of global herbicide sales, more than any other single chemical group. For more information, check out

Donn Thill, weed management specialist at the University of Idaho, was the first to confirm ALS resistant weeds in U.S. fields. He explains that the genetic makeup of the weeds and herbicide use patterns made development of ALS resistance relatively easy. "This mutation happens naturally and frequently in native species, unlike triazine resistance. Add to that the facts that there are some 20 different ALS-inhibitor herbicides out there, with one or more registered in every major crop worldwide, and you can see how it became a problem," Thill says.

Waterhemp. No weed in the United States has been identified with the type of multiple resistance found in Australia's annual ryegrass. Waterhemp, however, comes close. Bob Hartzler, weed management specialist, Iowa State University, said that he and other weed researchers at the university identified waterhemp populations with resistance to both the triazine herbicides (atrazine) and the ALS inhibitors. This weed is now a major threat to corn and soybean fields across Iowa, Indiana, Illinois, Ohio and Kansas.

"Waterhemp is a tough weed to begin with. So when you lose some of the tools you had to manage it in the first place, it becomes a real problem. The development and implementation of integrated weed management systems are the only means of minimizing the problem," Hartzler says.

Scientists also are worrying about the transfer of genes from engineered crops into field weeds. Researchers in Denmark, for instance, found that a transgenic herbicide-resistant gene moved from Canola to a weedy relative, field mustard.

Weed scientists are in general agreement, however, that gene transference is unlikely to affect corn or soybean production in this country. They credit this to modern breeding systems and to the fact that neither corn nor soybean plants have any closely related weed relatives growing in this country.

Glyphosate, glufosinate resistance. Although Roundup Ready genes are unlikely to move from soybeans to waterhemp or foxtail, many people are concerned about the potential spread of Roundup resistant weeds in crop land.

"There is always the possibility that we could select for resistant biotypes," Thill says, "but it is more likely we will see shifts to weed species that Roundup does not control. Volunteer crops growing in Roundup Ready crops aren't going to be a big problem to control."

Roundup, or glyphosate, is considered a low-risk herbicide for the evolution of herbicide resistance, according to Heap. The chemical structure, mode-of-action, limited metabolism in plants and lack of residual activity make it less likely that resistance will evolve. Furthermore, Heap suggests that glyphosate resistant and glufosinate resistant (Liberty) crops may help control resistance weeds.

Proper management. What happens next is anybody's guess, but weed scientists warn that proper management is key to avoiding disasters. "The discovery of glyphosate resistant ryegrass in Australia is a timely reminder that sound herbicide resistant management strategies will remain important after the widespread adoption of glyphosate resistant crops," Heap notes.

Hartzler worries that in Iowa the growth of farm size may encourage simpler management programs that encourage more weed problems. A survey conducted by The Leopold Center's Weed Management team at Iowa State University confirmed these concerns. In the survey, 57% of respondents reported they didn't worry enough about herbicide resistance for it to influence their buying habits.

"If we shift to a Roundup-Ready-type system, we will see different weed problems. They may not be resistant weeds," Hartzler says, "but it's more likely we'll see weeds that escape control for other reasons. It's hard to say how big a problem they will be at this point."

"We're in an era of new technology as far as the adoption of herbicide resistant crops and it depends a lot on how judiciously this technology is used," says Thill. "If used as part of an integrated weed management plan, then we will be okay. If not, we could get into serious problems with the new technology.

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