University of Vermont

Managing Herbicide Resistant Crops Managing Herbicide Resistant Crops

Sid Bosworth, Associate Extension Professor, Plant & Soil Science Department, University of Vermont

As our ability to manipulate genes becomes more sophisticated, we are seeing more and more herbicide-resistant crops emerging on the market. This technology provides some new weed control tools that may allow crop producers to better manage weeds and even reduce the amounts of herbicides that are often needed to obtain adequate control. Below are some frequently asked questions about the use of these new products.

Do you get better weed control with herbicide resistant crops?
Not necessarily. If you don't pay attention to details, control will be no better with herbicide resistant varieties than your non-resistant varieties. However, the use of herbicide resistant crops may increase your spectrum of herbicides allowing you to better manage those hard-to-control weeds. There may also be less risk of crop injury since the variety or hybrid is resistant to that herbicide. This might give you more flexibility in application timing. However, each weed has its optimum size or growth stage for the most effective control by a herbicide. Check the herbicide label for specific weed sizes.

Can you have effective weed control with only a postemergence application program?
This would certainly cut down and costs and allow you to actually assess weed pressures before any application is made. However, from what I've seen and what other weed specialists tell me, it really varies for each situation. The success of total postemergence programs depend on what weeds are present in the field, their severity, proper application timing of the herbicide, a competitive crop to help suppress weeds, and timely rainfall and good soil moisture prior to and following herbicide application. Fields infested with fewer weeds and less late germinating weeds would have a better chance of succeeding with total postemergence programs. Examples of late germinating weeds include pigweed, fall panicum, witchgrass, crabgrass, black nightshade, and common ragweed. See Table 1.

Table 1. The relative emergence of some common weed species during the planting and early crop development (April - June). The difference in time of emergence can vary from year to year. For instance, if there is an unusually warm spring, all three categories could emerge within a relatively short period of time. Also, These categories only relate to initial emergence and do not indicate the length of time that a particular specie might continue to germinate through the season

Wild mustard 
Black nightshade 
Canada thistle 
Fall panicum 
Giant ragweed 
Foxtail spp. 
Penn. Smartweed 
Common ragweed* 
Redroot pigweed* 

*Medium to late.

How soon should the postemergence application be made?
Again, each situation is different depending on weather conditions, the severity of weeds, and the stage of the crop. As a general rule, weeds should be controlled in corn within 4 to 6 weeks after emergence to avoid any yield losses. However, this "critical weed free period" can begin as early as 3 weeks if weed populations are high or if soils are dry. Therefore, if you are relying on a total post program, it is important that you at least be ready to spray by the third week after emergence.

Your risk of not getting on the field when needed depends on soil type and weather and the availability of an applicator. Will you be in the middle of first cut three to four weeks after corn is planted? Will commercial applicators suddenly be tied up with both preemergence applications of late planted corn as well as early postemergence applications for early-planted corn?

Don’t be fooled by looks alone – a clean field as a result of a one-time application of Roundup applied mid to late post may look good in August, but your yield has already been hurt because of early competition. Figure 1 is an example of Roundup Ready soybeans that were extremely weedy but cleaned up well after one application of Roundup Ultra. Although this field yielded well, one wonders how much yield was lost due to a lack of early postemergence control.


Figure 1. A Roundup Ready soybean field in Grand Isle County, Vermont with before and after treatments, 1998 (photos provided by Craig Altemose).

Can you cut back on rates of preemergence herbicides if combined with a post program?
I think this is one of the most viable options that can save some money but reduce the risks of a total post program. The residual herbicides can provide good control well into the critical weed free period, but then the postemergence herbicide can be applied if needed. However, before you reduce any rates, check with the label and your company representative. Some companies have included reduced rates in their label when combined with a post program. For example, Monsanto is allowing for reduced rates of its preemergece products when used in Roundup Ready corn program that receives at least one pass of Roundup postemergence. Other companies may not back up a reduced rate program.

If you have Roundup Ready or Liberty Link corn, should you tank mix with a residual herbicide?
Again, it depends on the time of application and the severity of the weeds. If you make an early postemergence application and your field is infested with late emerging weeds or perennials, it would probably make sense to include a residual type herbicide unless you are planning to follow up with sequential applications or cultivation. This year, you will be allowed to make a sequential application of Roundup, but that is labeled for grain harvest only. If you are going to harvest silage, you can only make one application and at least 50 days before harvest.

Would narrow row, high population plantings allow you to use only one application of Roundup or Liberty?
It sounds reasonable that early canopy closure would make the crop more competitive. With soybeans, we have seen success with a one-time application but I don't believe this is the case with corn. Studies at Penn State did not show narrow row corn to have any fewer weeds than the typical 30" rows when treated with various herbicide combinations.

Are the corn hybrids with herbicide resistance the best for you farm?
This probably is the bottom line question. So far, I would say many of the herbicide resistant corn hybrids are at maturity ratings more suited for longer growing seasons than we typically have in Vermont. Also, many of the herbicide resistant hybrids may be better suited for grain production rather than for silage. You really need to check with your seed company representative.

What are the risks of developing herbicide resistant weeds or weed population shifts as a result of the use of HR crops?
That depends on how we use these products. One advantage of the newer herbicide resistant crops like Roundy Ready or Liberty Link is that they broaden the spectrum of controls allowing for more opportunities to rotate herbicide families - a good management strategy for reducing the risks of herbicide resistance development.

However, we are likely to see the development of herbicide resistance weeds or at least a shift in tolerant weed populations as a result of the overuse of individual herbicides or herbicide families. I am particularly concerned about Roundup Ready crops like corn and soybeans, which have seen an explosion of acreage increases across the country. Because of its unique mode of action, weed resistance to Roundup is less likely to occur than other herbicides; however, it can happen. In fact, it has. Since 1995, there have been two biotypes of rigid ryegrass reported to have developed Roundup resistance in Australia. Thus far, there are no reports in the U.S.

While it is unlikely that we will see a sudden burst of Roundup resistant weeds, what is more likely is that we could get a shift in populations toward more herbicide tolerant weeds. These are weeds that are more difficult to control and usually require higher rates of Roundup for control. Examples include velvetweed, common ragweed, yellow nutsedge and black nightshade. As rates required for control become higher, there will be a point where it may be more economical to use another management strategy.

Is there really a concern of the development of a “super” weed as a result of bioengineering technology?
This is certainly a valid question being asked both within and outside of the agricultural community. There is a concern that these weeds may be more aggressive and difficult to control. The implications are not just to agriculture but also to parks and natural areas where these new plants could compete and decrease native populations. According to Dr. Allison Snow, an associate professor of plant biology at the Ohio State University, the risk of genes flowing from cultivated crops to wild relatives poses possible risks only under two conditions: 1) if the crop itself can survive without cultivation (ie., it becomes a weed), or 2) if the crop can spontaneously hybridize (cross pollinate) with closely related wild species. According to Snow, crops of higher risk would include canola (which often escapes from cultivation and can persist on it’s own) as well as cultivars of squash, sunflower and radish, and rice (all of which have shown to outcross with wild relatives). Some recent research by Snow and associates demonstrated that weeds that acquired genes from genetically engineered crops were shown to be hardy and aggressive. At the present time, most of the transgenic crops that we have in New England (corn, soybeans and potatoes) do not meet either of the above criteria and would be considered “low risk” transgenic crops in terms of direct gene flow.

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Last modified May 26 2004 01:29 PM

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