Preserving a Valuable Tool

Using Best Management Practices to Prevent Roundup Resistance

Published online: Apr 10, 2009 Feature Ray Hollist
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Since man first began to till the ground in an effort to stabilize his food supply, weeds and weed control have been a significant impediment to his success.

By their nature, crop plants are not good competitors. Weeds on the other hand are excellent competitors.
They can survive and produce seed when water and plant nutrients are very scarce.

Crop plants will thrive and produce at very high levels when competition is minimized and adequate levels of fertilizer and water are provided.
Huge sums of money and vast quantities of time have been spent on developing methods and producing products that control weeds.


It goes without saying that the cost of controlling weeds in a cropping system is almost insignificant when compared to the cost of doing nothing.

Not only are weedy fields unattractive to the eye, they can be devastating to the bottom line. Left uncontrolled, weeds can reduce yields 60-70 percent and substantially increase harvesting costs.
The power of selective herbicides was first discovered during World War II, when the herbicidal properties of synthetic growth regulators were discovered.

Since then, time and labor saving crop protection chemicals have become a normal fixture in modern production agriculture.
In 1973, the Monsanto Company began to market the herbicide known as Roundup.

As a young man, I remember Roundup being applied to an area of a field that was overrun with Quack Grass and a variety of other grassy and broad leaf weeds.

I remember being amazed at how quickly Roundup solved a problem that had been nearly impossible to control.


In 1996, the first crop with tolerance to Roundup was developed.
Since Roundup Ready Soybeans were first developed, many other crops have been added to the Roundup Ready list, with one of the most recent being Roundup Ready sugarbeets.

This technology has made sugarbeet production significantly easier and far less time consuming.
Some of the advantages realized initially were savings in cultivation and improved stands.

There also appears to be a significant yield advantage, but it will take a few years of harvest data to determine how big an advantage there really is.
Almost every tool we use in modern agriculture has some advantages and disadvantages.


After observing the effectiveness of Roundup last year, disadvantages are initially hard to find. One topic of significant concern is weeds developing resistance to Roundup over time.

Other areas of the U.S. and many countries throughout the world have documented cases of weed resistance to Roundup.
This problem is appearing initially in areas where multiple Roundup Ready crops are grown or the same Roundup Ready crop is grown year after year.
Since we want to have access to this tool for many years to come, how do we protect its efficacy?

Resistance can develop in many ways. A weeds' primary objective is to reproduce, and many weeds have developed great efficiency in reproduction.
Red Root pigweed for example can produce over 100,000 seeds per plant and over 1,000,000 seeds/pound.

Weeds with high seed production will have a greater potential to develop resistance. Some of the lowest seed producers can produce several thousand seeds per plant.
Our responsibility is to use the herbicide in such a way as to minimize the risk of resistance.

Resistance would be defined as a weed that once was controlled by the herbicide being applied and now is not controlled by that herbicide.
An example would be strains of Kochia resistant to ALS inhibitors.


One advantage that many sugarbeet growers have is their rotation.
If small grains or potatoes are grown in the rotation, then several different herbicides with different modes of action from Roundup are being used over a three-to-four year rotation.

If multiple Roundup Ready crops are grown, then there will need to be a tank-mix partner with a different mode of action used to prevent development of resistance.

When selecting a tank-mix partner be certain that it is effective against the weed species that has developed resistance to your primary herbicide.
It is also very important to apply Roundup at recommended rates.
Cutting rates may save a few dollars in the short term but could cost significantly more long term as Roundups' effectiveness is decreased and additional products have to be used.
Also, be sure that the rate you are using is consistent with the size of weeds in the field.

As weeds get larger, they develop a thick waxy cuticle and it is harder to get chemical into the plant.
Also, the plant is larger and it will take more chemical to trigger the same effect. Additionally, some weeds can begin to produce seed when they are very small.

By waiting to get the most bang for your buck, you can actually increase the weed pressure in the field.
Be sure to follow manufacturer recommendations for surfactants that can be added to improve herbicide efficacy.
When any herbicide is applied, it will be more effective if the weed is healthy and actively growing.

Weeds under drought stress will not absorb and metabolize the chemical as they should.
If plants are dusty, or a significant amount of dust is being generated as you spray, Roundup will be less effective as it binds to soil particles and becomes inactive.

Be sure that sprayers are calibrated correctly and functioning properly before any application is made.


One of the cost savings associated with Roundup has been reduced cultivation passes.
Cultivation should still be considered as an option where applicable, because tillage can be a significant help when trying to prevent resistance.

Fields should still be regularly scouted for weed pressure. If a resistance problem is beginning to develop it will only be found through regular scouting.

One of the indicators that resistance is developing may be green weeds in an area where most weeds of a similar size are dead, or only one species surviving an herbicide application.
Generally resistance problems will not have any specific pattern in the field.

If you have a concern that resistance is developing, invite qualified professionals such as university weed specialists to visit the field and test for resistance.


Another advantage of planting Roundup Ready beet seed has been an improved stand because of the reduction of chemical pressure on the crop.
In years past, we have started planting beets as soon as possible because we knew that we would lose a couple of weeks of growth due to chemical applications.

Now, I think it is reasonable to wait a little and plant when conditions are better and it is likely that the first planted beets will survive spring frosts.

Individual circumstances will dictate when you need to start, but frozen or blown out fields that have to be re-planted usually yield less than fields where the first planting survives.

Occasionally if replanted early enough, for example April 25, yields and sugars may be comparable. But the later the replant date, the more loss in yield.


Optimum spacing requirements will vary by area, field and experience.
Decisions on spacing should be made with the desired stand in mind. Generally speaking, your finished stand will be 70-80 percent of your seeding rate.

On 22-inch rows if you want a finished stand of 150-170 beets/100 ft., you will need to plant at 5.5-inches which is an initial rate of 218 beets/100 ft.

At a 6-inch spacing you could expect a finished stand of 140-160 beets/100 ft, with an initial rate of 200 beets/100 ft.
Plants per acre at 5.5- and 6-inches are 35,000 and 41,000 plants/acre respectively.


Making the decision to replant should only come after scouting and stand counts throughout the field.
If the stand is reasonably uniform and you have 70 or more beets/100 ft., and it's the end of April there isn't likely to be an advantage to replant.

If it's April 20 and you have 50-60 beets/100 ft., it may well be worth replanting.
There are several resources available to help you make the correct decision when it comes to replanting.

Having the right plant population and then protecting them through the growing season with good irrigation management, a solid fertility program and strong weed, pest and disease control programs will result in excellent yield and quality come harvest.

Editor's note: Hollist is a crop consultant in Idaho and can be contacted through email at