By Tammy Jones, B.Sc., P.Ag
Tammy Jones completed her B.Sc. in crop protection at the University of Manitoba. She has more than 15 years of experience in the crops industry in Manitoba and Alberta, with a focus on agronomy. Tammy lives near Carman, Man., and spends her time scouting for weeds and working with cattle at the family farm in Napinka.
I’ve heard Dr. Peter Sikkema speak about weed control many times, and the various factors that impact herbicide efficacy. During a War Against Weeds podcast (March 12, 2024), he made the statement: “…it’s irresponsible for herbicide manufacturers, ag retailers, and agronomists to guarantee acceptable weed control with herbicides. And also, I think it’s unreasonable for farmers to expect perfect weed control with herbicides in every field in every year.”
A number of factors impact herbicide activity, ranging from the uncontrollable aspects of weather (including rainfall, temperature, and relative humidity) to the controllable decisions on herbicide selection and application timing. The argument can be made that based on the extensive testing of herbicides, the labeled rate of a herbicide should be guaranteed to control weeds as long as they haven’t developed resistance.
Dr. Sikkema pointed to the tank mix application of dicamba, a synthetic auxin primarily used for broadleaf control, antagonizing any accompanying Group 1 herbicides that provide grass weed control. After much research, the antagonism was found to be due to the dicamba upregulating enzymes that metabolize the Group 1 herbicides. Oddly enough, more antagonism occurs when dicamba was co-applied with quizalofop-p-ethyl than clethodim, even though they are both Group 1 herbicides.
Studies have also shown that glyphosate and dicamba combined will be less effective than when applied separately. In this case, the antagonism is due to a reduction in translocation of the glyphosate as the dicamba quickly impacts the plant to limit the movement of the glyphosate. While unfortunate, the examples of antagonism are somewhat frequent, and the typical method of overcoming this is to adjust the rates of the herbicide in formulated products to achieve consistent control. But there are many instances where a herbicide application involving a single herbicide (therefore no antagonism) has failed to obtain the expected level of control. The weed population is not resistant based on subsequent testing, and the agronomist or herbicide manufacturer is struggling to provide an explanation.
A paper from Kansas State University (KSU) is a fascinating example of how challenging it is to understand the mechanisms that allow a weed to survive normally lethal doses of herbicide. It’s an understatement to say “it’s complicated.” The study, published in 2015 by Dr. Mithila Jugulam’s lab in the KSU department of agronomy, investigated the impact temperature can have on herbicide activity. The research team confirmed that Palmer amaranth sprayed with mesotrione was much more likely to survive under elevated temperatures (approximately 85°F to 95°F or 29°C to 35°C) than under cool conditions (less than 80°F or 27°C). For two main reasons, it seemed counterintuitive (to me) that hot temperatures would favor the survival of Palmer amaranth after mesotrione application:
- Mesotrione is a Group 27 “HPPD-inhibitor” herbicide, which kills plants by blocking the HPPD (4-hydroxyphenylpyruvate dioxygenase) enzyme. Without this enzyme, a sensitive plant will begin to turn white, as the chlorophyll in plants is destroyed by sunlight, which is why the Group 27 herbicides are sometimes referred to as “bleachers.” In my mind, warmer temperatures would equate to sunshine and the Group 27 herbicide should be impacting the plant more, not less.
- Palmer amaranth is a C4 plant, native to the southwest U.S. and northern Mexico, which also led me to believe that the elevated temperatures referenced in the study would be normal and not a stress to the plant.
In order to determine how the Palmer amaranth plants were surviving in warmer temperatures, the KSU team allowed the plants to grow between 8 to 12 inches (20–30 centimeters) under three different temperature regimes. Mesotrione was applied to the plants and then tracked. The weed scientists determined there were two separate processes that allowed the weeds to survive the herbicide:
- At higher temperatures, Palmer amaranth was able to metabolize more mesotrione within the plant before it could cause significant injury; conversely, cooler temperatures slowed metabolism.
- At higher temperatures, HPPD enzyme activity in the Palmer amaranth was increased, making it harder for the HPPD-inhibitor herbicide to block all the activity of the HPPD enzymes in the weed.
The practical implication of this study is that by timing mesotrione application for earlier in the growing season or earlier in the morning versus afternoon, when temperatures are cooler, Palmer amaranth control is more consistent.
Bringing that back to Western Canada, where mesotrione and thankfully Palmer amaranth are not frequently encountered, it makes sense that farmers, with limited time to apply spray, may see differences in the efficacy of herbicides based on the timing of application. This is not due to herbicide resistance or the perceived failure of the herbicide, but rather the diabolical ability of a weed to survive.