Can we prevent glyphosate-resistant weeds in Idaho?

University experts provide answers

Published in the March 2012 Issue Published online: Mar 13, 2012 Don Morishita
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Herbicide-resistant weeds have become a bigger concern to farmers in the past 10 years. Grower awareness of herbicide-resistant weeds in the irrigated regions of southern Idaho and eastern Oregon has been spurred by the spread of ALS resistant kochia throughout this area. In some locales of this area, growers have faced other herbicide-resistant weeds such as metribuzin- resistant pigweed, ACC-ase resistant

wild oats, and isolated pockets of ALS resistant redroot pigweed and Russian thistle. News of glyphosate-resistant weeds in the Midwest and South have also caught growers attention.

 

The introduction of Roundup Ready (RR) crops began with soybeans in 1996 followed by cotton and canola in 1997 and corn in 1998. At that time, there was 145,000 acres of field corn grown in Idaho, and only a small percentage of that was RR. In response to growth in the dairy industry, there were 350,000 acres of field corn in 2011. Nearly 100 percent of that was RR corn. Roundup Ready sugarbeets were made available to growers in 2008 and the adoption rate was more than 90 percent the first year. In 2011, nearly all of the sugarbeets grown in Idaho, Oregon, Montana, Wyoming, Nebraska and Michigan were RR, while in the Red River Valley, over 80 percent were RR. California was the only state that didn't grow RR sugarbeets.

 

First Resistance Reported

The main reason grower adoption of RR sugarbeets was so fast is because weed control was always a challenge in sugarbeets and the herbicides were not very effective and required multiple applications, sometimes as many as five, but almost always at least three. Plus, at least one or two cultivations were needed for weed control. It was not unusual to include hand weeding if you could find the hoeing crews to do the job.

In 2000, horseweed was reported in Delaware as the first glyphosate-resistant (GR) weed in the U.S. due to the introduction of RR crops. Since then, 12 additional weed species, including kochia and common waterhemp have become resistant to glyphosate. In southern Idaho and eastern Oregon, kochia is widespread and there have been unconfirmed reports of common waterhemp. The threat of GR weeds in Idaho and eastern Oregon is real and it is important that farmers, crop consultants and crop advisors are aware of this.

 

Resistance Management Economics

Tank-mixing another herbicide mode of action has been the number one recommendation for glyphosate-resistant weed management. However, this has not been a recommendation used by many growers. One of the primary reasons growers choose not to tank-mix another herbicide mode of action with glyphosate is cost. With glyphosate prices in the neighborhood of $3 to 5 per acre, it's difficult to argue adding another herbicide to the mix, which costs $20 or more per acre. However, when you compare the cost for weed control before RR beets, it was not unusual to spend $100-150 per acre with two cultivations included (see Table).

Considering the technology fee with three glyphosate (Roundup) applications at 22 fluid ounces per acre (fl oz/A) compared to one glyphosate application followed by a tank mixture of glyphosate plus Outlook, the cost is $100.63 versus $116.18; about 16 percent higher. When comparing that to the cost of controlling weeds in conventional beets using Progress, UpBeet and Stinger and two cultivations, the glyphosate and Outlook tank mix is about 12 percent higher. These prices calculated for the herbicides used in conventional beets are based on applying the herbicides in a 7-inch band. Near the end of conventional beets, many growers were using 11-inch bands for more consistent results, which also increased the total cost. Remember, too, in the conventional beets it was not uncommon to follow all of those applications and cultivations with hand labor, which of course added to the cost.

 

Resistance Management Strategies

For a number of years, recommendations for preventing or at least delaying the development of herbicide resistant weeds have included the following key points: 1) rotate herbicide modes of action; 2) rotate crops; 3) use short-residual herbicides; 4) cultivation; 5) keep accurate records of herbicide use; 6) plant weed-free seed; and, 7) practice integrated weed management.

Most sugarbeet producers grow beets in the same field once every three or four years to reduce disease potential, so crop rotation is not a challenge.

Rotating herbicide modes of action was actually a bigger challenge with the conventional beets because a grower could use UpBeet, Matrix, Affinity BroadSpec and Steadfast, all of which are sulfonylurea or ALS inhibitor herbicides registered for use in beets, potatoes, small grains and corn, respectively. This helped lead to the spread of ALS resistant kochia.

With glyphosate, one could argue that since it can only be used in beets, corn and alfalfa, and not that many growers grow all three of those crops; the risk is not that great. However, if a farmer is growing beets and corn, it's possible that a RR crop be grown every year if they're growing corn and beets, although this is unlikely.

A more realistic scenario could have a RR crop in three out of four years if corn is grown in two of the four years. As the demand grows for more corn to meet the dairy industry needs, so grows the potential for corn and beets to be grown in the same field three out of four years. If glyphosate is the only herbicide used, that puts a lot of selection pressure for GR weeds.

Although glyphosate has no soil activity, it is usually applied two to four times during the growing season, which makes it more like a residual herbicide in that multiple flushes of weeds are controlled as they emerge and are sprayed with glyphosate.

 

Expert Weed Scientists Weigh In

In the Great Plains, Midwest and Southeast, weed scientists are in the middle of the GR weed areas. Several of these experts were contacted recently to see what they are recommending for glyphosate resistance management strategies.

From North Dakota State University, Jeff Stachler believes that any glyphosate used during the growing season (including harvest aid applications) sooner than one out of every two years has a significant risk in selecting for GR weeds. Obviously, the more frequently a RR crop is grown in a rotation, the faster the selection. The speed of selection will depend upon what other weed control tools are being used in addition to glyphosate and what glyphosate rate is being applied to what sized weeds.

As the only weed scientist contacted that works in sugarbeets and also battles GR weeds, Stachler strongly feels that farmers growing corn and sugarbeets should include another herbicide mode of action applied pre-emergence and postemergence in corn with glyphosate, especially if the rotation is only RR crops.

He's convinced the best strategies for RR beet growers in the Red River Valley is to use a preplant incorporated or pre-emergence herbicide, like RoNeet or Nortron followed by glyphosate postemergence or use Stinger and/or Nortron (applied postemergence) proactively. They also need to scout their fields during the summer and remove all surviving and/or present plants by hand to eliminate the build-up of any potential GR plants. "Anything less than this will cause failures," he says.

In Kansas, kochia is one of the weeds that is resistant to glyphosate. Dallas Peterson, Kansas State University, feels that frequency of applications and using only glyphosate are key factors leading to glyphosate resistance initially.

Once GR weeds become established, it then spreads to other fields that may be using better resistance management strategies. This point emphasizes the need for everyone to implement resistance management strategies because you can't isolate resistant weeds, especially weeds like kochia.

Peterson didn't think growing only RR beets would be that vulnerable to the development of GR weeds if the producer is using different herbicides and weed control practices when producing the other crops in rotation.

However, even in non-RR crops, glyphosate is often applied as a "clean sweep" after the crop is planted, but before it has emerged. To compound this issue, the glyphosate label allow using rates below the lowest recommended use rate in beets, which is 22 fl oz/A of Roundup Power Max or 0.77 pounds acid equivalent per acre (lb ae/A).

Bill Johnson, Purdue University, has a rule of thumb he uses for Indiana growers.

He says "Once a field has been treated about 30 times with glyphosate, we can definitively observe a shift in predominate species and some resistant species will start to crop up."

He added that for those farmers growing RR beets and corn who rely mostly on glyphosate for weed control to expect things will be okay for about four to seven years.

Then, as he puts it "the wheels will start to fall off. It will be slow at first, but it will happen." As far as recommendations, Johnson said he tells growers to "use at least two modes of action particularly on the difficult to control weeds."

In the Southeast, Tom Mueller and Larry Steckel, University of Tennessee, recognize that because of the simplicity and low cost of RR systems, it is logical for farmers to use only glyphosate because of the weed control and the flexibility it adds for re-cropping. Both of them feel that GR weeds can develop quickly with selection pressure. Mueller believes resistance can become apparent in 5 years continuous use while Steckel tags the number of applications at 12 to 16; these are numbers easily achievable in 5 years. The fact that this is less than Johnson's estimate in Indiana may be due to a number of factors such as weed species, agronomic practices and environment. A point that Mueller makes that sounds familiar to beliefs in Idaho and Oregon is that until GR weeds become a problem, it is almost impossible to get farmers to change their practices. 

He says, "Once even a few farmers get GR weeds, all you have to do is have them speak at a meeting, and put them in your state wide extension bulletins and then "POW" your phone will start ringing off the wall with "How do I prevent this on my farm?"

In Idaho, it's impossible to know which weeds will become resistant to glyphosate first, but it is believed that kochia will likely be the first.

This is based on several reasons: 1) it is widespread and grows in a wide range of conditions; 2) the seed survive for only 1 to 2 years in the soil, so nearly new generations of seed emerge every year; 3) it can produce up to 12,000 seeds or more per plant; 4) seeds can be dispersed over a wide area because of the tumbleweed habit of mature plants that have broken off at ground-level and the light weight of the seed, and, 5) studies have shown that there are many kochia biotypes from natural mutations that make this weed biologically diverse.

 

Resistance Management

Practices A Must

All of these weed scientists believe that if sugarbeet is the only RR crop grown in a rotation with non-RR crops every three or four years, the probability of selecting for GR weeds is low, although nobody knows for certain how many years that may be. The answer also depends on which weed species becomes resistant. What they are all quite certain of is that GR weeds will develop into a big problem if growers don't adopt resistance management practices.

Idaho and eastern Oregon growers have a unique opportunity to prevent or at least delay occurrence for years.

So, what should you do?

First and foremost, do not use glyphosate rates below the recommended 22 fl oz/A rate when using Roundup Power Max, for example. It is imperative to keep in mind that not all glyphosates are formulated the same. Roundup Power Max contains 4.5 lb ae/gal which means that 22 fl oz/A equals 0.77 lb ae/A. Depending on the manufacturer or distributor, other glyphosate brands may contain 3, 3.7, 4, 4.17, 4.72, and 5 lb ae/gal. Using 0.77 lb ae/A with these other formulations, is equivalent to 32, 27, 24, 21 and 20 fl oz/A.

If you ignore this information and apply 22 fl oz/A of a 3 lb ae/gal glyphosate, you will actually apply 0.5 lb ae/A glyphosate, which is 33 percent less than what you intended to apply. Not only can this result is unsatisfactory weed control, this may help select for glyphosate resistant weeds. In the Red River Valley, Jeff Stachler recommends using the highest labeled rate, which is 1.125 lb ae/A or 32 fl oz/A Roundup Power Max, from emergence to the 8-leaf stage.

Secondly, it isn't likely anyone would stop using glyphosate and switch back to the old beet herbicides.

However, tank mixing another mode of action with glyphosate should definitely be considered. As shown in the table comparing weed control costs, a tank mixture of glyphosate with another mode of action like Outlook is not much more than using glyphosate alone and still cheaper than the cost of controlling weeds in conventional beets and more importantly, better than risking the development of glyphosate resistant weeds.

Lastly, if you're not growing another RR crop in the rotation with sugarbeets, then the next best thing to do is eliminate or at least reduce using glyphosate as a "clean sweep" in the other crops grown in rotation.

If you are growing another RR crop in the same rotation with beets, you should strongly consider one or both of these options in these RR crop: 1) apply a preplant incorporated or pre-emergence herbicide like RoNeet or Nortron in sugarbeets; or 2) tank-mixing another mode of action, like Dual Magnum, Eptam, Nortron or Outlook in sugarbeets.

Keep in mind the tank-mix partner needs to be the most effective herbicide for the weeds to be controlled and not the most convenient.

One last recommendation, reduce the number of glyphosate alone applications to only one or two per year.

To paraphrase a common saying, it doesn't take a weed scientist to figure out that the more weed control strategies that are used with glyphosate in a RR crop, the longer glyphosate will stay effective for controlling weeds in your beets and other RR crops.

This is especially true in beets, because there aren't any other herbicides that are as effective as glyphosate.