Resistance Run Amok

Published: August 4, 2009

Figure 1. Location of waterhemp and palmer pigweed seed samples collected in 2008 for determination of glyphosate resistance

Last year we conducted a survey, primarily of the northern half of Missouri, in which we looked for weedy soybean fields just before the time of soybean harvest. As we came across these fields we collected some of the surviving weed seedheads, recorded GPS coordinates and other information about the field, then gleaned the seed for subsequent herbicide resistance testing in the greenhouse. Some chemical industry representatives, agricultural retailers, extension agronomists, and crop consultants also sent in for seed for testing. This survey was targeted primarily towards waterhemp, but we did harvest and test several other species as well. In the end, we ended up with 88 waterhemp and 9 palmer pigweed populations to test. This process has taken an incredibly long amount of time to complete and is still ongoing with some of the other species not shown in Figure 1, but we have recently completed our analysis of the waterhemp and palmer pigweed populations.

Figure 2. Confirmed glyphosate-resistant waterhemp and palmer pigweed populations present in Missouri.

In this screening, I classified a population as "resistant" if 60% or more of the plants sprayed with a 2X rate of glyphosate (1.5 lbs a.e.) were alive and clearly capable of reproduction three weeks after treatment. In most cases, we sprayed at least 30 plants from each population for this determination. Based on this classification, we identified 45 separate waterhemp populations across 28 counties in Missouri that were resistant to glyphosate (Figure 2). This represents 51% of the total waterhemp populations collected. Although there weren't many samples taken from the boot heel region, we also identified both glyphosate-resistant waterhemp and glyphosate-resistant palmer amaranth populations from Scott and Mississippi counties, respectively (Figure 2). The good news, if there could be any good news in all of this, is that we also screened for resistance to the PPO-inhibiting herbicides in these same populations and found very few instances of PPO resistance. In fact, we only identified 1 population of the 88 waterhemp populations sampled that was clearly resistant to the PPO-inhibiting herbicides.

As this survey was mostly completely random in nature (i.e., simply looking for weedy soybean fields and harvesting seed), these results are both surprising and concerning to me. It seems clear that we now have quite a significant amount of our acreage with infestations of glyphosate-resistant waterhemp. As part of our ongoing effort to try to determine the extent of this problem in Missouri, we will be continuing this survey in 2009 and will focus our efforts on the southeastern and southwestern edges of the state.

So, if you have or suspect you have a glyphosateresistant pigweed species like waterhemp or palmer pigweed present what can you do about it? First, if you decide to stay with soybeans you must rotate to an alternative herbicide that is effective on your resistant weed species and acts at a site-of-action different from glyphosate. In soybeans, this usually means you will need to use a preemergence herbicide. In our research, we have observed that preemergence soybean herbicide treatments like AuthorityFirst and the other Authority-based products, Sonic, Prefix, Boundary, Dual II Magnum, and Valor will all provide excellent control of glyphosate-resistant waterhemp, although a postemergence follow-up treatment will still usually be required due to the nature of waterhemp germination. In addition to these preemergence herbicide options, the postemergence PPO-inhibiting herbicides like Phoenix, Cobra, Ultra Blazer, Flexstar, etc. should also provide good control of glyphosate-resistant waterhemp and palmer amaranth as long as there is no PPO-resistance present in the population. Another option if you decide to stay with soybeans is to utilize LibertyLink soybeans and Ignite. However, even if a grower chooses o utilize this new mode-of-action in soybeans, I would still start with a preemergence herbicide and follow this with a timely application of Ignite.

Second, if you have a glyphosateresistant weed like waterhemp or palmer pigweed you can rotate away from soybeans altogether. For example, rotate to a conventional corn hybrid and use alternative herbicides in this system for at least one year or perhaps two in an attempt to reduce the glyphosateresistant weed seedbank. In our research with glyphosate-resistant waterhemp, we have found that most prepackaged atrazine and chloroacetamide mixtures will provide excellent control of glyphosate-resistant waterhemp in corn. Additionally, we have observed excellent control of glyphosate-resistant waterhemp in corn with postemergence herbicides like Distinct, Status, Callisto, Impact, Laudis, and others.

Glyphosate and Roundup Ready crops have simplified weed management in soybeans dramatically over the past decade. They have enabled us, for the most part, to achieve excellent weed control at an economical price. In order to preserve the utility of this technology, growers must be willing to adapt and change their practices when situations like glyphosate resistance arise. In fields where glyphosate-resistant weeds are suspected or are present in only small areas, paying a little more now through the use of an alternative herbicide or different cropping system will be much better than allowing these weeds to proliferate and develop into a much bigger problem in the long run.