Weed Worries Won’t Wane
March 1, 2012 
By all accounts, 2011 was one of the most profitable and yield-plenty years agriculture has ever seen. But most industry observers agree that these same conditions also increased the number of hard-to-control weeds grower-customers and ag retailers had to deal with.
And the problem, say experts, will only get worse in 2012 and the years ahead. According to researchers, the number of weed species with resistance to more than one herbicide mechanism of action has increased dramatically since 1990. Currently, 50 weed species have been confirmed with multiple forms of resistance including giant and common ragweed. Furthermore, new varieties with confirmed or suspected resistance are being discovered with alarming regularity.
“Many weed scientists consider herbicide resistance to be the most serious immediate challenge to our agricultural system,” says the University of Kentucky’s Michael Barrett, president of the Weed Science Society of America (WSSA).
And resistant weeds aren’t being limited to the farm anymore. A recent article in Weed Science magazine reported that golf course owners in Tennessee have found a biotype of annual bluegrass that seems to have developed a resistance to glyphosate application.
According to Barrett, part of the reason for this rapid advancement of resistant weeds ties back to the nature of these plants. “Weeds have challenging biology, competitive ability and the potential for rapid evolution of new resistances,” he says.
But perhaps a bigger reason for the spread of resistance weeds is the widespread popularity of glyphosate and its accompanying tolerant crops as an all-purpose control solution. According to USDA statistics, growers planted approximately 91% glyphosate-tolerant soybeans and 68% glyphosate-tolerant corn in recent years.
“The use of glyphosate in all major row crops this past year was 275 million acres treated,” says Andy Hurst, technical brand manager for Bayer CropScience. “That exceeded the next closest active ingredient applied by nearly six-fold.”
Jason Weirich, a weed scientist at the Delta Research Center for the University of Missouri, agrees that the popularity of glyphosate since 1996 has probably increased the resistance problem. “My generation is known as the glyphosate babies,” says Weirich. “It’s all we grew up with — glyphosate, glyphosate, glyphosate.”
To better understand how this might work, weed scientists Paul Neve from the University of Warwick in England and Ken Smith and Jason Norsworthy from the University of Arkansas recently developed a computer model to look at the evolution of glyphosate resistance in Palmer amaranth (also known as pigweed). When run, this model predicted — and subsequent fieldwork confirmed — that uninterrupted use of glyphosate alone (i.e., five applications per season per year) would result in glyphosate resistance within four years.
“With the computer model, we were able to run thousands of scenarios, and the results were surprising,” says Norsworthy. “We knew utilizing residuals and reducing soil-seed bank were key pieces of the puzzle. What we didn’t know was that early-emerging weeds produce exponentially more seeds than those that emerge with crop competition. With more seeds, there is a much higher probability that one of those seeds could be a resistant mutant.”
Palmer Problems Proliferate
It’s no mystery why these three weed scientists were looking at Palmer amaranth, either. This weed can tolerate high temperatures, dry soil conditions and can display a prolonged emergence pattern with a higher germination rate at shallow seed depths. Also, because it is a dioecious species — meaning plants are either male or female — a great deal of genetic diversity can be introduced into the Palmer population from one year to the next. This is probably how, researchers agree, the weed was able to develop its resistance to herbicides and spread so quickly throughout the Southern tier of states. During 2011, say observers, this pattern of distribution was probably aided by the rampant flooding that occurred in the spring, which likely carried numerous resistant Palmer amaranth seeds several miles from their home fields.
“I see the resistant weed problem today in Arkansas like it was four or five years ago in the Midwest,” says Arkansas Grower Malcolm Haigwood, talking about the rapid spread of Palmer amaranth in his area. “Now, it’s getting to the point where glyphosate application is almost not effective anymore at weed control in our state.”
Furthermore, the Palmer amaranth problem is headed north. According to Aaron Hager, an Extension weed specialist for the University of Illinois, the species is fairly common in the southern third of his state, but it is expanding northward. Recently, he says, at least one population of glyphosate-resistant Palmer amaranth has been confirmed in the mix.
“The growth rate and competitive ability of this species exceed that of other Amaranthus species,” says Hager. “Waterhemp can add close to 1-inch of new growth per day under good growing conditions, whereas Palmer amaranth can add multiple inches. This species has managed to drastically spread across the southern U.S., so what says it won’t spread to Northern Illinois as temperatures increase. This could be a huge problem, especially as glyphosate resistance is identified.”
To attempt to control their Palmer amaranth problem, Les Glasgow, Ph.D, head of weed management strategies for Syngenta Crop Protection, has the following suggestions for grower:
• Plant into clean fields following tillage or use of herbicides to burndown weeds.
• Apply a preplant or preemergence residual herbicide.
• Overlap an early postemergence residual herbicide.
• Remove weed escapes from fields before they seed set.
Waterhemp Won’t Waiver
Of course, Illinois growers — and those across much of the Midwest — are already dealing with their own prolific weed, waterhemp. While native to the U.S., waterhemp didn’t become a major agronomic problem until the 1980s, when reduced tillage systems and simplified weed management programs came into vogue. In fact, according to Dr. Bryan Young, professor of weed science for Southern Illinois University, 90% of Illinois acres from Bloomington to Decatur have waterhemp present.
“Waterhemp is a challenge for growers and has been for the last 20-plus years,” says Young. “Throw in the challenge of herbicide resistance and it’s no wonder that waterhemp has become public enemy No. 1 for growers.”
The big challenge, says Dr. Rick Cole, weed management product manager for Monsanto Co., is once waterhemp becomes established, it has a growth rate 50% to 70% greater than other annual weed species. “When allowed to compete with crops during the growing season, waterhemp can cause dramatic yield losses in both soybeans and corn,” says Cole.
In addition, waterhemp is very adaptive. The weed has the ability to grow during drought and, because of its dioecious nature, can quickly reshuffle its genetic diversity into resistant types. According to Dr. Mike Owen, Extension weed specialist and professor of agronomy for Iowa State University, some types of waterhemp have developed resistance to at least five modes of action. “In Iowa, we have resistance to waterhemp in triazine, ALS inhibitor, PPO inhibitor, glyphosate and HPPD inhibitor herbicides,” says Owen.
And it sounds as if resistance to a sixth mode of action has arrived. According to Dr. Mark Bernards, assistant professor of agronomy, crop science and weed control at Western Illinois University, he has found a population of waterhemp that appears resistant to 2,4-D. “When tested, this population survived doses of 32 quarts per field of 2,4-D,” says Bernards. “These plants also showed some ability to survive dicamba application, although I’m not comfortable saying this is proof of resistance to that herbicide just yet.”
To combat waterhemp, Iowa State’s Owen says growers need to develop their weed management strategies as if all waterhemp is resistant to one or more herbicides. “All herbicide weed management needs to be redundant, including that for waterhemp,” he says. “Growers should focus on five years down the road, not just on the flush in front of them or what they expect from the coming year.”
Beyond this, Owen has these suggestions:
• Do not use only one tactic or herbicide to control weeds.
• Use tank mixes of herbicides with different modes of action.
• Use a soil-applied residual herbicide on all acres, regardless of the crop or trait present.
Scouting Some Solutions
To prevent the spread of resistant weeds, many experts recommend ag retailers and their grower-customers adopt a greater reliance on scouting techniques. This will help not only in determining what weeds are present in the fields, but provide a better read on their intensity and locations. With this information in hand, growers can make very informed decisions on their long-term weed management planning, including whether or not to utilize a fall herbicide application.
“By scouting at harvest for weed escapes, you can go into winter with a specific weed management target for the coming year and have time to develop a plan to reach that target,” says Dr. Bill Johnson, professor of weed science at Purdue University.
Adopting a rotation with different herbicide-tolerant crops is also recommended by some experts. “We try to rotate our crops through glyphosate at the most every other year,” says Al Ludwig, a grower based in North Central Iowa. “Sometimes, we have fields go three years in a row without Roundup on them.”
Besides scouting and rotation practices, WSSA’s Barrett recommends growers apply herbicides at the full labeled dose for each weed species, not exceeding maximum weed size. “Also, growers should identify and promote individual best management practices for specific farming sectors with the greatest potential impact,” he says. “This herbicide sustainability and best management practices adoption should be used by both the public and private sectors.”
Some folks in the agricultural community believe herbicide research will soon discover “the next big active ingredient” to help combat resistant weeds. However, as Barrett points out, no herbicides with new mechanisms of action are in advanced development trials and the last such product was introduced more than 20 years ago. “Therefore, we will have to rely on currently available herbicides for the foreseeable future,” he says.
Despite this fact, there are a few new crop protection options slated to appear into the ag marketplace during the next few years. For instance, Dow AgroSciences is moving forward with its Enlist Weed Control System, expected to hit the market soon. The Enlist system will be a single postemergence product, combining glyphosate with a new 2,4-D choline (instead of an amine or an ester as in traditional 2,4-D formulations), offering users multiple modes of action for their weed control. This will be coupled with herbicide-tolerant traits in elite germplasm.
“If Enlist is not available as the glyphosate-resistant weed problem continues to expand, the use of the alternative herbicide and tillage programs is projected to reduce net farm income by up to $2.5 billion as early as 2017, and then continue at that level every year through 2020,” says Damon Palmer, U.S. commercial leader for Enlist weed control system at Dow AgroSciences.
Also coming in the next few years is Engenia herbicide, claimed to be “the next generation formulation of dicamba” by manufacturer BASF, which will be used in conjunction with dicamba-tolerant crops. “Engenia will be an important tool for soybean growers battling herbicide-resistant weeds such as Palmer amaranth, waterhemp and marestail,” says Paul Rea, vice president, crop protection for the company. “Engenia will provide a valuable new herbicide option as part of a comprehensive weed control system to help growers protect their crops from yield-robbing weeds.”
In addition, Lowell Sandell, a weed scientist at the University of Nebraska, says that agriculture can still rely on glyphosate application for some of its weed control needs. “Just because we have glyphosate-resistant weeds out there doesn’t mean that glyphosate is not a useful herbicide at all,” says Sandell. “We need to maintain its usefulness through active resistance management. Rotation of modes of action and diversification of our weed management programs, both from a herbicide and a cultural practices standpoint is critical.”
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