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Proper management can delay the spread of resistant weeds

FARGO, N.D. -- Glyphosate (Roundup) was made commercially available in 1974 and has since become the world's most important herbicide. Glyphosate is a broad-spectrum systemic herbicide and has a unique mode of action (inhibition of EPSP synthase)...

FARGO, N.D. -- Glyphosate (Roundup) was made commercially available in 1974 and has since become the world's most important herbicide. Glyphosate is a broad-spectrum systemic herbicide and has a unique mode of action (inhibition of EPSP synthase) that sets it apart from other herbicide families.

Until 1996, glyphosate primarily was used as a "burndown" treatment to control weeds in noncrop areas or on agricultural land before seeding. Glyphosate could not be used for in-season weed control on crops because its nonselective nature meant crop species also were susceptible.

However, the scientific breakthrough of transgenic glyphosate-resistant crops in the 1980s completely revamped modern agriculture. Scientists were able to isolate the CP4 EPSPS gene found in a common soil bacterium and insert it into the plant genome to obtain the first glyphosate-resistant plants.

Soybeans were the first glyphosate-resistant crop on the market in 1996. Today, glyphosate-resistant cotton, corn, canola and sugar beets are commercially available. Glyphosate-resistant alfalfa and hard red spring wheat also have been developed but currently are not commercially available.

A popular choice

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Glyphosate-resistant crops became popular for many reasons: Glyphosate is simple to use, highly effective, inexpensive, posed little threat of crop injury and did not limit future crop rotations. These qualities quickly changed the pattern of glyphosate use from being strictly a pre- or post-crop burndown treatment, to include in-crop use as well.

The success of glyphosate resistance technology is obvious. Approximately 96 percent of all soybeans grown in the United States and 64 percent of all soybeans grown globally were glyphosate-resistant in 2006. More than 12 million growers in 23 countries planted 282.4 million acres of biotech crops in 2007 as adoption of glyphosate-resistant crops continued to increase by more than 10 percent per year.

The desire for glyphosate-resistant crops has been evident in North Dakota as well. In 2004, 40 percent of corn, 84 percent of soybeans and 69 percent of canola acres were planted with glyphosate-resistant varieties in North Dakota. It is very likely these numbers have continued to increase in the past few years as well.

Researchers also expect more than 90 percent of sugar beets planted in the Red River Valley region will be glyphosate-resistant in 2009.

Before the introduction of glyphosate-resistant crops, more than 10 different herbicide modes of action were used to combat weeds in soybean. However, now many farmers in the U.S. and around the world grow only glyphosate-resistant crops in their rotations and rely almost exclusively on glyphosate for their weed control. The significant increase in glyphosate use that accompanied these new glyphosate-resistant crops greatly increased the potential for selecting glyphosate-resistant weeds. Producers are going to have to modify to their weed management systems for glyphosate-resistant crops to be viable into the future.

A growing resistance

How does weed resistance occur? Weed resistance begins as a fluke of nature and, in a broader context, is a prime example of how evolution occurs. Over time, reproduction "errors" cause some plants to behave differently from their parents. In the case of herbicide resistance, these mistakes, or mutations, change a few offspring so that they are not susceptible to a normally lethal dose of herbicide. The frequency of these mutations often is very low, but when you consider the millions of acres and the trillions of weeds to which glyphosate is applied, it makes sense that some inherent variation does occur.

Because glyphosate is so effective in controlling such a wide spectrum of weeds, any resistant biotypes that remain in the field will have a great opportunity to produce resistant offspring. Eventually, with continued glyphosate usage, the majority of the weed population will be resistant.

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Delaying the development of herbicide-resistant weeds is the most effective control measure. The greater the selection pressure (i.e. the more often glyphosate is used), the greater the likelihood of resistance occurring. A few herbicidea that increase the likelihood of herbicide resistance are ones that act on a single site of action, herbicides that are applied multiple times throughout the growing season, herbicides that are used in consecutive growing seasons and herbicides that are used without other weed control options.

Does this sound familiar? This is exactly how glyphosate functions and is being used.

It did not take too long for the first weeds to become glyphosate resistant in cropland. Although not the first instance of glyphosate resistance worldwide, the first evolved glyphosate-resistant weed reported in a glyphosate-resistant crop was horseweed in 2001. To date, populations of 15 species have been reported to be resistant to glyphosate worldwide. The most problematic glyphosate-resistant weeds in the United States are Palmer amaranth, common waterhemp, common ragweed, giant ragweed and horseweed.

Resistance has been reported for some species in only small isolated patches, while other species such as Palmer amaranth and horseweed have resistant biotypes on thousands of acres.

Jeff Stachler, extension weed scientist at North Dakota State University in Fargo, says horseweed is particularly troublesome because it is largely self-pollinated (more than 90 percent) and its wind-blown seed can be spread long distances. This means glyphosate-resistant seed can be spread to areas that don't have a long history of glyphosate pressure. If glyphosate is being applied to that area, resistance will be noticeable in that field very quickly; thus shortening management response time.

Glyphosate-resistant weeds

Since 2006, resistant populations of common waterhemp, giant ragweed and common ragweed have been documented in Minnesota. North Dakota has at least one confirmed population of glyphosate-resistant common ragweed as well. The North Dakota site began to show signs of resistance after only 4 to 6 years in a glyphosate-only cropping system. This is particularly scary because many producers in the region plant almost all glyphosate-resistant soybean, corn and now sugar beets. This illustrates that glyphosate resistance can occur and develop rapidly (less than five years) in the Red River Valley.

Stachler is convinced that there are glyphosate-resistant populations of kochia and common lamb's-quarters in Minnesota and North Dakota as well. Local populations of Powell amaranth and redroot pigweed also have the potential to become resistant, since other species within the same plant genus elsewhere in the U.S. have been confirmed resistant. Because these all are common weeds in the area, it is important to pay attention to any sign of reduced glyphosate performance in these weeds especially.

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Be on guard

Scout early and often. Unless scouted for, resistant populations generally are not noticeable until they make up 10 percent to 30 percent of the total population. During the first several years, the proportion of resistant weeds remains very low, but then in one growing season, performance for a particular weed may quickly drop from excellent to poor. Stachler says fields should be scouted comprehensively before and after glyphosate applications to catch early signs of weed resistance.

Stachler asks producers to scout fields 14 to 21 days after treatment and look for plants that have a full range of injury symptoms from uninjured to dead. If all weeds are dead, there likely is not a resistance problem. If resistance is beginning to develop, you will observe control of most weeds, with a few weeds escaping control without apparent justification. The resistant weeds may range in appearance from completely uninjured to severely injured. Once glyphosate resistance is suspected, appropriate actions should be taken to minimize the development of large resistant populations.

Management practices to slow glyphosate resistance. It is important to be proactive rather than reactive with glyphosate-resistance. Once a weed is resistant to glyphosate, the resistance traits likely always will be present in the population. When this occurs, glyphosate may no longer be an effective control option for that particular weed population in that field.

There are a few things that can be done to prevent or slow weed resistance:

n Alternate herbicide modes of action. Use a preemergence soil-applied herbicide first. This will allow a larger application window for the first glyphosate treatment as well as provide another mode of action to slow resistance. Rotate glyphosate-resistant crops with other herbicide-resistant crops and conventional crops. Tank-mixing glyphosate with other herbicides may be another effective option. Other modes of action may be more effective against some hard-to-kill species.

n Utilize cultural and mechanical practices. Spring tillage is an effective way to control early emerging weeds in traditional cropping systems. Don't forget about row crop cultivation. It's not the most popular these days, but it is an effective integrated pest management alternative to herbicide application. Utilize narrower crop rows and denser planting populations to shade out competing weeds sooner.

n Maximize glyphosate efficacy. Don't wait too long before spraying your weeds. It is best to spray glyphosate on small (less than 4 inches) annual weeds. Use effective herbicide rates. Ammonium sulfate at

1 percent to 2 percent should always be used. This is important to condition hard water, but even when not using hard water the addition of AMS has been shown to improve glyphosate absorption and translocation in weeds. Use the lowest water volume allowed on the label.

n Pay attention to changing environments. Drastic temperature changes can shock plants making herbicides less effective. Glyphosate is most effective during warm and humid weather.

The evolution of glyphosate-resistant weeds shows that nature is very dynamic and that there is no one magic bullet for weed control. Therefore, diversification of management practices is important. There is no guarantee that industry will invent another glyphosate. The last herbicide mode of action was discovered more than 20 years ago, so it is essential that we protect the weed control tools that we have to ensure they remain effective into the future. The threat of glyphosate-resistant weeds is real, but with proper management practices we can delay their spread.

Editor's Note: Mikkelson, from Benson, Minn., is a graduate student at North Dakota State University in Fargo. The essay was written for a Principles of Weed Science course taught by Dr. Shane Friesen.

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