Scientists call weeds the “bullies of the plant world.”

Weeds aren’t just bullies. They’re thieves, too. They cost farmers money by reducing crop yields. They even steal water from crops.

Worse yet, weeds can build resistance to the herbicides that are the main weapons farmers use in self-defense.

If a few individual weeds are resistant to a herbicide and allowed to reproduce, that herbicide will be less effective year after year. Increasing the amount of herbicide will only add to the problem, and the cost, experts say.

Resistance can increase herbicide costs by 30-40%, according to Brit Ausman, an Asotin, Wash., farmer and board member of the Washington Grain Commission.

When farmers encounter weeds that appear to be resistant to a particular herbicide — or herbicides that share the same mode of action — they turn to people such as Ian Burke and Drew Lyon. As weed scientists at Washington State University, Burke and Lyon and their colleagues at land-grant universities and extension services around the nation help farmers determine whether a particular weed population has developed resistance to a herbicide’s mode of action, or if there is another problem.

The main thing farmers need to do, though, is remember that more isn’t better when when it comes to herbicides. Using too little herbicide is a problem, too. It allows weeds to survive and develop seeds that will grow next year.

Crop advisers, extension agents and others can help farmers determine how best to break the hold that a weed has on a field.

It could be a matter of using another herbicide — or a combination of herbicides — to take down the weed. Or it could take a change in crop rotations or some other means of knocking out the weed such as plowing and making sure it doesn’t reappear next year.

Herbicide resistant weeds are a problem around the world. In the West, resistance is most prevalent in downy brome, Italian ryegrass, mayweed chamomile, prickly lettuce, wild oat, Russian thistle and mustard weed species.

If a herbicide is ineffective against those or any other weeds, a farmer or rancher needs to seek help.

If the farmer doesn’t come up with a game plan for that weed, it could become resistant to several herbicides. Because most weed seeds are wind-blown or can be transported on vehicles, this could cause a problem for neighbors as well. Horseweed, a native of North America, can travel 300 miles by air.

Whatever the weed, the worst thing a farmer can do is ignore the problem.

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