From ONvegetables in The Grower, February 2014
Kristen Obeid, Weed Management Program Lead (Horticulture), OMAF/MRA
Herbicide resistant weed populations are now found throughout Ontario. The number of resistant species and areas affected by resistance continues to increase. Herbicides are the most heavily relied upon weed control method for many growers because they are both cost effective and simple, which has resulted in high selection pressure for herbicide resistance in populations of weed species.
It is often thought that weeds change or mutate to become resistant. However, weed scientists, believe that weeds do not change at all. Instead, populations change. The resistant weeds have always been present in low populations. When a particular herbicide is used, it controls the normal-susceptible types. This makes room for the population of the resistant weeds to increase. Consequently, when growers say that their “weeds have become resistant,” they really mean that the population of their resistant weeds has increased greatly and the population of their susceptible weeds has decreased.
As we learn about herbicide resistance, an unfortunate side effect is that some herbicide failures from bad weather, weeds that are too large or improper applications are considered herbicide-resistance problems. Do not suspect herbicide resistance unless a herbicide failure fits the following traits:
- The same herbicide was used year after year.
- One weed, which normally should be controlled, is not controlled although other weeds are controlled.
- A patch of an uncontrolled weed is spreading.
- Healthy weeds are mixed with controlled weeds (of the same species).
Even if a control failure exhibits these traits, it is not an absolute diagnosis of herbicide resistance. Get your weeds tested for resistance to confirm.
As we get familiar with herbicide resistant weeds we are finding that many species are developing cross resistance to herbicides. Cross resistance is defined as the ability of a weed population to be resistant to more than one herbicide. This may arise without the weed population ever being exposed to one of the herbicides. Why does this happen? Today, there are more than 100 different herbicides on the market, but many of these work in exactly the same way or, in other words, have the same mode of action. Fewer than 20 plant-growth mechanisms are affected by current herbicides.
If a field is infested with herbicide-cross-resistant weeds, the grower may lose yield because a competitive weed isn’t controlled. Growers also may have higher costs if they lose the use of several economical herbicides.
If you suspect a resistance problem:
- Use herbicides with a different mechanism of action to control the escaped weed.
- Do not let weeds go to seed.
- Use cultural practices such as cultivation.
- Contact OMAF, your dealer and your sales representative.
- Get your weeds tested for resistance.
Herbicide resistance is a complicated subject. Many weed scientists warn of hidden dangers in rotating modern herbicides. Because so many modern herbicides have the same mode of action, a grower could rotate crops and herbicides but still wind up with a resistance problem. The mode of action may not change even when crops and chemicals are rotated.
Look for the mode of action (also known as the Herbicide Group Number) on the top right corner of your herbicide label. OMAF Publication 75: Guide to Weed Control also lists all the herbicide group numbers in Table 4-1.