The Future of Weed Control In Soybean: How Many Options Will There Be?
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The introduction and commercialization of glyphosate-resistant soybean varieties and corn hybrids has dramatically altered weed management approaches. Estimates place the adoption of GMO soybean (principally glyphosate-resistant varieties) at approximately 75 percent of the U.S. soybean acreage1. The adoption of this technology has, in many respects, simplified weed control for many producers. For example, soybean producers can use a single active ingredient (glyphosate) for postemergence control of many broadleaf and grass weed species. Application rates can be adjusted according to weed spectrum and size. No concerns exist for rotational crop injury from herbicide carryover. Simply stated, this new weed control "system" has worked well for many producers.
But does this system perhaps work too well? Does this system simplify weed management decisions to the extent that integrated weed management consists only of one or more applications of glyphosate? Some might argue that "if the system ain't broke, don't fix it!" But if "problems" of one sort or another develop that reduce the effectiveness of this system, what will soybean producers do then? Will there be new soybean herbicide active ingredients in the near future that could remedy any of these potential "problems"? Will the weed spectrum change somehow in response to the widespread use of glyphosate? Perhaps we should consider another suggestion—that "an ounce of prevention is worth a pound of cure."
New Soybean Herbicides on the Horizon
Some may find a degree of comfort in the idea that new soybean herbicide active ingredients will always come along just in time to save the day/farm if and when problems develop in the "old" system. For example, weed scientists have heard the statement, "I plan to continue to use glyphosate until it's no longer effective. By that time, someone else will have brought out a new herbicide to solve the problem." In years past, this did happen; introduction of new soybean herbicide active ingredients was an almost annual event. Nowadays, however, it may not be as prudent to simply use the system until it breaks, all the while anticipating that a new "cure" will be along shortly.
University researchers have usually evaluated a new herbicide active ingredient for between one to four years before the product is commercialized. While formulations of existing soybean herbicide active ingredients continue to change, novel active ingredients are not finding their way from the laboratory to the field as rapidly as in years past. It is unlikely that many (or perhaps any) new herbicide active ingredients will be introduced into the soybean market anytime during the foreseeable future, perhaps none during the next three to four years. So, if the effectiveness of current soybean active ingredients declines, there may not be a new soybean herbicide that comes along and saves the day/farm. Consider this for a moment: Waterhemp is tough enough to control with the few effective postemergence soybean herbicides on the market today. What would happen if the utility of one or more of these options was lost? Read on!
Changes in the Weed Spectrum
What is meant by "changes in the weed spectrum"? There are, of course, several ways in which the weed spectrum in any given field can change over time. Weed species that were not seen previously in a field can become more prevalent and problematic; repeated applications of specific herbicides may select for weed biotypes resistant to that herbicide; and the biology or growth characteristics of one or more weed species may change in response to changes in agronomic practices. Some recent examples of changes in the weed spectrum by each of these mechanisms may help illustrate the point that weeds today do not necessarily behave like they did years ago.
"New" Weed Species Becoming More Prevalent
Reduced tillage production practices occur on more acres today than they did 25 years ago. When tillage is reduced, it is not uncommon to begin encountering more biennial and perennial weed species such as poison hemlock, hemp dogbane, and common pokeweed. Without sufficient tillage to adequately disturb the root system of perennial weed species, these weeds can often flourish in reduced tillage fields. If tillage is eliminated after crop harvest, winter annual weed species frequently become well established and are able to survive over the winter to cause problems the following spring.
Some may argue, however, that the prevalence of winter annual weed species today may be related not only to reduced tillage but also to the reduced utilization of soil residual herbicides. Think back to the 1980s and early to mid 1990s, a time when few producers were considering applying soil residual herbicides in the fall to control winter annual weed species. Reduced tillage practices were common back then, but so also was the use of soil residual herbicides. Today, reduced tillage is still a common production practice, but soil residual herbicides are not used to near the extent they once were. Many would agree that biennial, perennial, and winter annual weeds are more common today than they were 25 years ago and that these weeds didn't necessarily "change" over time, but rather adapted to changes (reduced tillage and less use of soil residual herbicides) imposed on the production system.
Weed Biotypes Resistant to Herbicides
In 1993, the list of herbicide-resistant weed biotypes in Illinois was a small fraction of today's list. Many Illinois producers have had the unpleasant experience of contending with one or more herbicide-resistant biotypes, and (unfortunately) the weeds continue to thwart many of our management tools. It's become somewhat "old news" that much of the Illinois waterhemp population is resistant to ALS-inhibiting herbicides or that many populations are resistant to triazine herbicides. Foes et al. published research in 1998 on an Illinois waterhemp biotype with resistance to ALS-inhibiting and triazine herbicides, one of the first reported instances of a summer annual weed species with resistance to more than one herbicide family. Unfortunately, the story doesn't end there. Recent research has identified an Illinois waterhemp biotype with resistance to ALS-inhibitors, triazines, and PPO-inhibiting herbicides (yes, three-way resistance has become a reality). Leave it to waterhemp to run up the score!
So, what options remain for postemergence control in soybean of a waterhemp biotype with resistance to three herbicide families? Four soybean herbicides can provide postemergence control of waterhemp; three are no longer effective because the biotype is resistant to PPO inhibitors, so that leaves only glyphosate. It doesn't require much imagination to conclude that if only one viable option remains, that will be the option the producer has to use. Will the selection pressure of using only glyphosate result in a glyphosate-resistant waterhemp biotype? Some have suggested that selecting for weed biotypes with resistance to glyphosate is unlikely to happen (Bradshaw et al. 1997), but glyphosate-resistant horseweed (marestail) biotypes in six states have been reported recently (VanGessel 2001). True, horseweed isn't waterhemp, and to date no waterhemp biotypes with actual resistance to glyphosate have been documented. However, several researchers have reported waterhemp populations with "decreased sensitivity" to glyphosate (Patzoldt et al. 2002; Zelaya and Owen 2000). Should these examples (documented glyphosate-resistant horseweed, waterhemp biotypes with reduced sensitivity to glyphosate) be ignored, or perhaps should they be seen as evidence that weed species have adapted to extensive glyphosate use?
Changes in Weed Biology
A contemporary example of a "change" in weed biology is that of giant ragweed. Surveys of Illinois producers conducted during the 1980s indicated that giant ragweed was not ranked among the top 10 most prevalent weed species. Today, however, only waterhemp ranks above giant ragweed on the most recent producer survey. Weed scientists at the University of Illinois recently have determined the competitive ability of these two weed species with soybeans. Waterhemp, at a density of approximately 89 plants m-2, can reduce soybean yield up to 31 percent (Hager et al. 2002), whereas giant ragweed at 15 plants m-2 can reduce soybean yield up to 87 percent (Wax et al. 2002). These data indicate that giant ragweed should be taken very seriously!
Research conducted by Stoller and Wax (1973) in the late 1960s and early 1970s demonstrated that giant ragweed emergence was essentially complete by the beginning of May. During this period of time, producers were generally able to control most giant ragweed populations with preplant tillage. However, recent research by weed scientists at the University of Illinois has shown that giant ragweed emergence in agronomic production fields can continue well into June and sometimes even into July. Clearly, these results show that giant ragweed has adapted its biology to changes in how producers grow their crops.
Looking to the Future
The examples described above illustrate how weed species have adapted and continue to adapt to changes in production practices. In some instances, weeds adapt in response to a single selection factor, while at other times, the adaptation is due to multiple changes in production practices. Whether single or multiple factors are involved, it is important to remember that weeds will continue to adapt and challenge us. These examples should further illustrate the need for an integrated approach to weed management. Integrated weed management introduces multiple tactics to control weeds and slow the rate at which weeds are able to adapt to a single management approach.
Glyphosate-resistant soybean varieties offer many advantages to soybean producers, but as the previous examples illustrate, over-reliance on a single management option can lead to new weed management challenges. The weed spectrum in many Illinois soybean fields today is such that a singular management strategy (for example, a single postemergence herbicide application) may not always provide consistent control. Introducing an integrated weed management approach into glyphosate-resistant cropping systems may well stave off some of these potential new challenges, enhancing the long-term effectiveness of this valuable weed control strategy. Is an ounce of prevention worth a pound of cure?
Literature Cited
Bradshaw, L. D., S. R. Padgette, S. L. Kimball, and B. H. Wells. 1997. Perspectives on glyphosate resistance. Weed Technology 11:189_198.
Foes, M. J., L. Liu, P. J. Tranel, L. M. Wax, and E. W. Stoller. 1998. A biotype of common waterhemp (Amaranthus rudis) resistant to triazine and ALS herbicides. Weed Science 46:514_520.
Hager, A.G., L.M. Wax, E.W. Stoller, and G.A. Bollero. 2002. Common waterhemp (Amaranthus rudis) interference in soybean. Weed Science 50:607_610.
Patzoldt, W. L., A. G. Hager, and P. J. Tranel. 2002. Variable herbicide responses among Illinois waterhemp (Amaranthus rudis and A. tuberculatus) populations. Crop Protection 21:707_712.
Stoller, E. W. and L. M. Wax. 1973. Periodicity of germination and emergence of some annual weeds. Weed Science 49:224_229.
VanGessel, M. J. 2001. Glyphosate-resistant horseweed from Delaware. Weed Science 49:703_705.
Wax, L., K. Maertens, and C. Sprague. 2002. Giant ragweed: old weed, new problem. Proceedings, Agronomy Day 2002.
Zelaya, I. A. and M.D.K. Owen. 2000. Differential response of common waterhemp (Amaranthus rudis Sauer) to glyphosate in Iowa. Proceedings North Central Weed Science Society 55:68.
Figures and Footnotes
1United States Department of Agriculture National Agricultural
Statistics Service June 2002 report.