Due to the heavy reliance on glyphosate for weed management in the cornbelt, it is generally accepted that the selection of glyphosate resistant weeds is inevitable. In fact, some weed scientists have stated that glyphosate resistant waterhemp has already appeared in Iowa, whereas others aren’t so quick to lay the ‘resistant’ label on this pernicious weed. In this forum, Mike Owen will argue the case for declaring the arrival of glyphosate resistant waterhemp in Iowa, whereas Bob Hartzler will argue against.
Bob: I think to declare waterhemp resistant to glyphosate we need evidence that farmers are no longer able to control the weed with labeled rates of glyphosate. The work done in your lab and at the University of Missouri has confirmed that waterhemp biotypes exist that require higher rates of glyphosate than average, run-of-the-mill biotypes. In addition, you have shown the potential for selecting biotypes with an even higher level of tolerance. But as far as I am aware, no farmers have stopped planting Roundup Ready crops in the fields where these biotypes are present, nor are they adding herbicides to glyphosate to pick up the waterhemp that escapes the glyphosate. Granted, they may use a preemergence product rather than go with a total postemergence glyphosate program, but this is a tactic we suggest farmers use regardless of the type of weed problem they have. Until a population is found in a field that can’t be controlled with glyphosate I don’t think we should talk about glyphosate resistant waterhemp.
Mike: My position has been, and will continue to be that resistant individual common waterhemp plants exist within populations. Thus, resistance to glyphosate in common waterhemp exists. I do not disagree that no widespread shift in resistance profile has occurred in common waterhemp, nor does it appear that common waterhemp populations are more or less difficult to manage with glyphosate. However, I believe it is important to provide growers with information that details not only the benefits of a technology, but also the risks. Thus, I believe it is appropriate to indicate that glyphosate resistance exists in common waterhemp populations, as long as the topics of frequency and economic importance of the resistance are addressed. I also suggest that when research provides specific information about the mechanism(s) of resistance, the genetic basis for the characteristic, and the frequency within populations, weed scientists will be able to predict with reasonable accuracy, the speed at which resistance will increase, and how big a problem glyphosate resistance will ultimately become. At this time, however, I feel justified in suggesting that shifts to glyphosate resistant populations are inevitable and that resistance does exist within common waterhemp populations.
Bob: My problem with this argument is why stop with waterhemp? It’s likely that glyphosate resistant individuals exist in most important weed species found in Iowa, and could be identified if we initiated an extensive selection program. This was done with birdsfoot trefoil and fescue in the 1980’s. I understand the value of your research, but just feel we confuse people by reporting glyphosate resistance in waterhemp when it hasn’t occurred in a field situation as far as we know.
Mike: I believe your point is accurate, but you take things too literally. While I agree with you that there are likely individual plants in all weed populations that are resistant (WSSA definition) to glyphosate, or any herbicide for that matter, common waterhemp deserves “special” attention for several reasons: 1) common waterhemp is economically important and widely distributed across the Midwest, and 2) no special research or extensive selection program (if you ignore 75%+ GMO soybeans and the concomitant use of multiple glyphosate applications in Iowa) was needed to identify the problem. While I admit that the research that has been conducted is far removed from the “field reality”, I argue that it represents the likely evolution that common waterhemp populations will demonstrate if weed management continues along the current path. Yes, we have “artificially” enhanced the glyphosate selection pressure that occurs in commercial agriculture, but that does not diminish the fact that resistance, admittedly at a very low frequency, occurs in commercial fields. It is hoped that when our research is completed, we will be better able to address the appropriate questions you raise. Specifically, how quickly will wide-spread glyphosate resistance in common waterhemp develop and what is the economic implication of this resistance.
Bob: OK, I’m willing to concede that glyphosate resistant waterhemp is a problem in the greenhouses of Midwest Universities. However, since my job responsibilities do not include weed management in greenhouses, I will continue to take the stance that we do not have evidence of glyphosate resistant waterhemp in Iowa’s agronomic fields. I wish I had been clever enough to come up with this habitat differentiation, but I must give credit to Dr. Ian Heep. Ian maintains the world’s repository of herbicide resistant weeds (www.weedscience.com). I find it comforting that at this point of time he has not chosen to list waterhemp as being resistant to glyphosate.
Mike: Bob, first off, you must recognize that Ian’s last name is spelled “Heap” – ruins your credibility… I will concede that the recognized “selected” population of glyphosate-resistant common waterhemp is concentrated in the greenhouse. However, I would like to suggest that the parent plants were selected and rescued from field applications. I would also importune that the population we used as a source for the genetic material was representative of many fields where problems with glyphosate performance have been reported. Recognize that we have data from several Iowa fields that we used to develop the validity of the research program. Not only was efficacy observation used to select plants from the field, but we also used a shikimate assay to support our hypothesis that resistance to glyphosate did exist within field common waterhemp populations.
You must agree that we receive numerous complaints every year about common waterhemp control problems with glyphosate. I suggest that there are likely (rare) individual common waterhemp plants within these complaint fields that would demonstrate resistance to glyphosate if we conducted the experiments. Indeed your own field studies reported on the ISU Weed Science web page support my position. My feeling is that your main complaint is that the glyphose resistance in common waterhemp has not escalated as quickly as other herbicide resistant weed populations. I suggest that there is evidence, albeit incomplete, that resistance in common waterhemp has the potential to increase some time in the future. When our research project is completed, hopefully within a year, we will be able to address your questions about resistance frequency, mechanisms, and heritability. When this information is available, we will then provide a reasonable projection/prediction about when glyphosate resistance will escalate to the levels that are of economic importance.
Bob: EXCUUUSE ME!1 I forgot that Ian is Australian and thus used the U.S. spelling of Heep rather than the Aussie version. I have really run out of any further valid arguments. In closing, I feel that waterhemp simply is a plant that initially was relatively tolerant to glyphosate. Thus, the rates of glyphosate used in RR crops are near the threshold required for commercially acceptable control. In fields where everything is right (timing, environment, spray coverage, etc.) growers will get excellent control. However, in fields where things are less than ideal farmers can expect failures. I think it's identical to the struggles farmers had in the years between the appearance of ALS-resistant waterhemp and the introduction of RR soybeans. Farmers had to rely on the diphenyl ether herbicides to control waterhemp, and sometimes they worked and sometimes they didn't. Now we have diphenyl ether-resistant waterhemp, but back then (mid to late 1990's) resistance didn't have any thing to do with the control problems. People who are as old as you can remember similar scenarios when we used Basagran to control velvetleaf in soybeans (sorry about the cheap shot about age, but you started it with the comment about my spelling abilities). Inconsistent control isn't an indicator of resistance, in fact I would argue it's the opposite. Resistance leads to consistent control failures, something we have not seen.
Mike: Well, at least you did not bring up my hockey ability (or lack thereof – although I am still participating). As I have the final counterpoint (by the initially agreed upon protocol of this point-counterpoint format), I will make several final observations. First, I believe you are arguing semantics – tolerance vs. resistance. My interpretation of resistance is that this represents a weed (population or individual) which will not be controlled by the rate of herbicide that normally controls the population, irrespective of environmental conditions. Thus, by that definition, resistant individual common waterhemp plants exist in fields.
Second, I agree with you about the analogy with the diphenyl-ether herbicides; the occurrence of resistant individuals within the common waterhemp populations was rare indeed until selection pressure provided the opportunity for the trait to evolve more widely. I believe that glyphosate resistance in common waterhemp may follow a similar evolutionary path.
Third, the evidence indicates that the trait for resistance is heritable. Thus I argue that with the continuous and pervasive selection pressure brought about the adoption of RR systems and the use of glyphosate, the ultimate evolution of glyphosate resistance in common waterhemp populations is inevitable.
My fourth and final point is that our research has identified individual common waterhemp plants that were not controlled by glyphosate when used at very high rates. We have demonstrated that this characteristic is heritable and occurs regardless of environment. We have increased the level of resistance in the progeny from these plants several fold. Thus, resistance to glyphosate in common waterhemp populations occurs albeit at an extremely low frequency.
1Since this format is a blatant rip off of the skits involving Dan Ackroid and Jane Curtain on Saturday Night Live, I decided to include a Steve Martin reference. The SNL skit's were parodies of the final segment on pre-Andy Rooney 60 Minutes that featured a debate between James Kilpatrick and Shana Alexander. For those people wondering why we haven't included Dan Ackroid's immortal opening line (Jane, you ignorant _____ ) , please remember this site is hosted by an institution of higher learning.
Prepared by Bob Hartzler, extension weed management specialist