Using tank mixes to extend herbicide ‘life’
Before herbicide selection has taken place it is very rare for an individual weed to be resistant to two herbicides. Mixing herbicides at full label rates in a single application takes advantage of this fact.
Armed with this knowledge growers can get in early and hit weed populations hard with multiple modes of action to stave off herbicide resistance. Even weeds that may possess the mutation to allow them to survive either or both modes of action on their own are very unlikely to survive an application of both at the same time.
Look for product combinations of two or more herbicides to safely apply to the crop at full label rates for the target weed. The products must be physically compatible with no antagonism between them and no existing resistance to any of the herbicides in the mix.
This is similar to the idea of using a double knock and it is not necessary that all the ‘knocks’ are from a drum. Teaming an effective tank mix with crop competition and clean seed, for example, would reduce weed seed set in that season and lessen the pressure in following years, potentially allowing a less competitive crop to be grown.
This concept was proven through research in the United States following an observation that some fields were unaffected by waterhemp while a neighbouring field had this weed overshadowing the crop. Waterhemp is a big, competitive weed that sets a lot of seed and readily evolves resistance to herbicides. It is common for individual plants to be resistant to multiple herbicide modes of action.
It is now well known that over-reliance on a single herbicide group will inevitably result in resistance and US growers have created the world’s biggest herbicide resistance problem by abandoning all other forms of weed control in favour of glyphosate alone.
Researchers Pat Tranel, Jeff Evans, Aaron Hager, Adam Davis, Brian Schutte, Chenxi Wu and Laura Chatham from the University of Illinois, the USDA-ARS Global Change and Photosynthesis Research Unit, and the State University, New Mexico used spray records from a local spray contractor to compare 50 fields with glyphosate resistant waterhemp and 50 fields without. They looked at a total of 61 management and environmental variables and found that mixing herbicides was the single management strategy that made the most difference to whether or not glyphosate resistant waterhemp became a problem in any field.
In a review of herbicide application records from 2004 to 2006 and glyphosate resistance tests in 2010 the researchers found that adding more products to the tank at full rates for a single application causes the probability of resistance in these fields to decline sharply. (See graph below)
In a review of herbicide application records from 2004 to 2006 and glyphosate resistance tests in 2010 the researchers found that adding more products to the tank at full rates for a single application causes the probability of resistance in these fields to decline sharply. (MOA = mode of action = herbicide group).
In Australia, herbicide mixes are often used but in many cases the products included in the mix are not added at full label rates. While there are constraints such as crop safety or label restrictions for some product mixes, there are many product combinations that are safe and effective when combined at full label rates.
Look for product combinations of two or more herbicides to safely apply to the crop at full label rates for the target weed. The products must be physically compatible with no antagonism between them and no existing resistance to any of the herbicides in the mix.
This strategy needs to be used wherever possible and in addition to herbicide rotation, not instead of it. Mixing is most useful when managing weeds that use very specific resistance mechanisms to survive a herbicide spray. Where the target weed uses resistance mechanisms that can give cross-resistance to a number of herbicides, mixing may not be useful as the weed may be resistant to several modes of action and will survive the spray, going on to set seed and proliferate. In these situations, it is even more important that non-herbicide tactics are included in the weed management plan.
Mixing herbicides does not halt the evolution of herbicide resistance in a weed population, it can only delay the process.
You can read the scientific paper about this research here and check out the AHRI insight on this research.