Rapid spread of resistant weeds

Published online: Sep 18, 2013

As ag retailer agronomists and crop consultants try to convince their clients to make changes to their herbicide programs to avoid herbicide-resistant weeds, those agronomists and consultants have had limited ammunition to prove that resistance on a farm is maybe only one growing season away.

"The normal response by many farmers in the past has been to not worry about weed resistance until it is costing them an economic hardship," said Bill Johnson, Ph.D., professor of weed science at Purdue University. "A resistant weed could be in a neighbor's field, and they didn't seem to care. It is my neighbor's problem; I don't have it. Usually, the only way that they are going to put some kind of a management program in place is when they have resistance, and it has caused them some problems."

It is hard to convince many farmers to be proactive in preventing weed resistance to the farming program they have become comfortable using. And if weeds haven't been identified as a problem in their county, then the farmer figures he still has time to react next season.

The search and discovery of herbicide-resistant weeds in the U.S. has been very unsystematic over the years. Methods and resource investment to identify (ID) resistant weeds has mainly been left to individual state funding and private investment. But even announcing that a specific weed is resistant to a specific herbicide mode of action and has been found in a state doesn't seem to impress a lot of farmers.

Indiana has been acknowledged as a state that has taken the lead in identifying resistant weeds so that its farmers could turn the tide on the spread of weeds during the past decade.

Surveying and sampling has occurred as new weed problems have emerged. Johnson noted that from about 2003 through 2008 the focus was on marestail/horseweed. Ragweeds became the focus from about 2005 to 2010. And today the focus is on Palmer amaranth and waterhemp.

"When we got started, I was much more concerned about waterhemp than I was Palmer amaranth, but as we went out and did some scouting of waterhemp, we kind of stumbled on the fact that we had a lot of Palmer amaranth out there," Johnson explained.

"Palmer amaranth is a weed that I think people in the North have misidentified; they have assumed it is waterhemp," said Les Glasgow, Ph.D., chairman of the North American Herbicide Resistance Action Committee and technical product lead in charge of herbicide resistance management strategy for Syngenta. "It well could be that there is a large number of undetected Palmer amaranth populations. It is an amazing weed considering that it started out in the desert Southwest and can be happy all the way into Michigan."

The spread of Palmer amaranth and waterhemp has been astonishing, and it continues at a rapid pace because those surviving are usually resistant to glyphosate and herbicides with other modes of action.

"A Palmer amaranth plant can produce a million seeds and is very competitive. It is a phenomenal species, and waterhemp is not far behind in seed production. Both are spread by seed and pollen. These two weeds have both male and female plants, and, therefore, are obligate outcrossers. Outcrossing allows genetic mixing and increased biodiversity of the species from which resistant individuals can be selected," explained Glasgow.

U.S. tops resistant weeds list

Weed species characteristics along with the use of one herbicide over and over leads to weed resistance.

The U.S. easily tops the list of countries with the most herbicide-resistant weeds at 144, according to the International Survey of Herbicide Resistant Weeds. Next most is Australia with 62 herbicide-resistant weeds followed by Canada.

All three countries have focused attention on herbicide resistance with university Extension or equivalent, commodity crop associations, the crop protection industry and others determined to identify weeds resistant to specific herbicides and then provide solutions to assist growers in combating resistant weeds while economically producing high-yield crops.

We hear much less about the e_ orts to assist growers, but rather more about banning herbicides, by the next level of countries with herbicide-resistant weeds: China, 34; France 35; Germany, 33; Spain, 33; and Brazil 31. And it wouldn't be a farfetched assumption that some countries with many fewer herbicide-resistant weeds either haven't been using as many modern intensive farming techniques, including herbicides, or really haven't focused on identification of resistant weeds.

Worldwide, 24 weeds have been identified as resistant to glyphosate, and 14 of them are in one or more states of the U.S. Also worldwide, glyphosate ties for tenth for the number of weeds that are resistant to it. Atrazine leads the list with 64 resistant weed species identified worldwide.

Second internationally is imazethapyr with 39 resistant species. In descending order, the herbicides with number of species resistant are tribenuron-methyl, 35; imazamox, 33; chlorsulfuron, 31; simazine, 31; fenoxaprop-P-ethyl, 28; metsulfuron-methyl, 26; paraquat 26; bensulfuron-methyl, 24; _ uazifop-P-butyl, 24.

As known in the industry, weeds can become resistant to ALS-inhibitor herbicides quite quickly when they are used continuously. Weed resistance to ALS-inhibitors should be widespread inside and outside the U.S. because this mode of action has 54 registered active herbicides in its group.

Local level discovery

The rapid increase in discovery of herbicide-resistant weeds makes it likely that more weeds are resistant than those included in the International Survey of Herbicide Resistant Weeds, even though weeds are constantly being added, such as the July listing of smallflower umbrella sedge in California being resistant to ureas and amides.

Taking the discovery of herbicide-resistant weeds to the local level, lower than state level in the U.S., is extremely hard. In most states, the onus lands on the individual farmers.

Resistant weeds being discovered often is the result of growers finding weeds they have not been able to control.

"A farmer will find a weed or a patch of weeds in a field that he is not able to control and will call in a epresentative from the company who supplied the product applied or will check with his local university Extension service," noted Glasgow.

"The international survey defines what should be done to prove that it is a resistant weed. There is some pretty stringent testing that needs to go on, especially when a new weed species is suspected to be resistant or the first time that a herbicide is showing lack of control of a previously sensitive species," he added.

Therefore, the listing of weeds is usually behind the curve on what is happening in the real world to some degree. A farmer shouldn't wait until a weed shows up on the list as resistant to the herbicide mode of action before altering a weed management program.

Reasons for resistance

Julian Smith, Ph.D., director of discovery and innovation with Brandt located in central Illinois, provides a concise thought about management programs that are driving weed resistance. "Farming is more intensive than it was in the past. We tend to have more monoculture environments than we ever had before. Although there still are a multitude of rotations, monocultures are more common. And I think another piece is that there are not as many tillage operations carried out on farms as there used to be. Pre and post tillage used to be a pretty valuable mechanical means of weed control."

Smith isn't saying he is favoring more tillage or against continuous corn and soybean cropping. He simply noted that continuous use of one herbicide with minimum tillage sets the stage for weeds to adapt or for those that thrive in specific management programs to thrive.

An example of what Smith noted is why marestail has become a major problem in some places. Glasgow said, "It is a winter annual and can get established in the fall. It starts out as a small rosette on the surface of the soil, and you can probably manage it with some herbicides then, but once it gets any size, it becomes a really difficult one to control. And it is also a prolific seed producer with seed that has characteristics to float long distances in the air."

Researchers point to the major problems and spread of a few herbicide-resistant weeds causing the biggest concern at the moment. In the Midwest, it is Palmer amaranth, waterhemp, common ragweed, giant ragweed, marestail, and in the West, it is kochia.

United effort to inform farmers

As noted earlier, relying on individual states to run their own ID program for resistant weeds has not always worked to help warn farmers as the menace of these weeds and others approach, let alone ways to proactively try to stop the forward movement.

Perhaps the most important program to be initiated recently was one that Johnson has established through the United Soybean Board.

"A year and a half ago, the United Soybean Board (USB), where soybean checkoff money goes, had concern about the spread of herbicide-resistant weeds. So, they approached us about doing a project to encompass as many soybean states as we could include for their funding. So, they've had funding in place last year and they funded us again this year for Extension services to raise awareness about the prevalence of herbicideresistant weeds and management programs," Johnson said.

Only a couple states with at least 2 million acres of soybeans are not participating in this project, he noted.

Part of the project in some states is to get farmers to be aware that resistant weeds are spreading into their farming region and they need to take action before it is too late.

Why aren't there maps with confirmed resistance found in these counties and suspected resistance in these counties by weed and herbicide? As Johnson noted, the challenges are too much to expect every state to search for these situations and have everything verified. It is impossible to make guesses about weeds resistant to various herbicides due to the nature of weeds, the number of herbicides to which weeds could be resistant and the many variables in farming and environment.

Johnson does send graduate students into the fields to collect samples from areas of the state where certain weeds have not  been identified, such as Palmer amaranth, and then also to have them take samples of weeds to see if they are resistant to specific herbicides.

Changes to weed control

Having farmers aware that there are resistant weeds in their fields or their neighbors' fields that will probably invade their fields eventually leads to the switching of different herbicide programs, of which there is at least one per crop protection company offered as a solution or partial solution. Methods can also include mechanical control and hand weeding that went by the wayside in recent years.

Smith is a proponent of helping plants fight for their share of light, nutrients and water in low-level weed infestations and notes that the use of multiple modes of action herbicide programs put stress on the crop plants.

He notes that stress mitigation is important because the plant has to metabolize each herbicide molecule into a non-toxic form.

"In so doing, it takes resources within the plant that may manifest itself in a transient nutrient deficiency such as manganese or zinc . But this can be overcome through the use of micronutrients along with the herbicide spray," Smith said.  

He noted the problem with resistant weeds in a nutshell. "It is not like a resistant weed turns bright red and says here I am. It takes a lot of vigilance on the part of the growers, crop advisors and Extension service to identify resistant weeds."

Source: www.agprofessional.com