When Brian Jenks talks about weed control, one gets the impression that he also plays chess.
Jenks views weed control as a matter of strategy, where farmers have to consider the consequences of each decision they make. Keeping the weeds in check requires proper crop rotations and careful management of herbicides.
“A lot of guys get a product or a system that works for them and they just stay with it,” said Jenks, a weed scientist at North Dakota State University. “They need to change their system because the weeds will adapt.”
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Jenks spoke last week at the Manitoba-North Dakota Zero Tillage Workshop in Brandon, Man. He emphasized the dangers of continually using herbicides with the same mode of action, that is the way a herbicide kills weeds, such as inhibiting amino acid synthesis.
Herbicides are grouped in Canada and the United States according to their mode of action. Continuous use of herbicides from only one group increases the risk of creating weeds that are resistant to that group.
Jenks used kochia as an example. In a field with kochia, less than one percent of the weeds may be resistant to a herbicide. But if the same herbicide group is applied to the field for several years, the resistant kochia will spread.
“It just keeps getting bigger and bigger and all of a sudden you go out and spray and you don’t get any control.”
Jenks offered a list of documented resistant species in North Dakota. Those included: kochia resistant to ALS herbicides; kochia resistant to 2,4-D and Banvel/SGF/Clarity; green foxtail resistant to DNA herbicides such as trifluralin; wild oats resistant to ACCase inhibitor herbicides; and wild oat resistant to ALS herbicides such as Assert.
Resistance is also well-documented in Canada. Jenks said resistant weed species include green foxtail, wild oats, chickweed, hemp-nettle, kochia and wild mustard. Wild oat and green foxtail resistance were most common in the ACCase inhibitor herbicides.
Besides watching the herbicides, Jenks recommends careful crop rotations and timing of herbicide applications. While a species of weed may flourish in one type of crop, it may not compete as well in another.
Pigweed, for example, likes warm conditions. To better contend with that weed, producers may choose a crop that can be seeded early, giving it a head start over pigweed.
“If you can get the crop up and above the weeds, the crop will help give you some control over the competition.”
For wild oats, one option is to grow sunflowers, which tend to get seeded later in the spring. Wild oats can then be handled with tillage or a burn-down herbicide once they have emerged.
