Purdue expert gives advice to help farmers whip weeds in their fields

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Trying to gain control of giant ragweed, common water hemp and marestail, also known as horseweed, in Indiana fields can be a challenge, but a Purdue University expert has some advice for farmers trying to control the weeds.

Weather, a big challenge so far this spring, kept farmers out of fields prior to planting and allowed weeds to get a jump start, said Bill Johnson, Purdue Extension weed management specialist.

“By the time farmers got their crops planted in late May and on into June, the weeds had started to grow rapidly in the middle part of June during warm weather conditions,” Johnson said. “This made it very easy to miss the timing for spraying by just a one- or two-day delay. When we miss those timings for spraying, it results in poor weed control and, ultimately, translates into yield loss.”

Most farmers have more recently applied a post-emergence treatment, but the effectiveness of the herbicides was lost because the plants were already stressed from a 10-day hot and dry spell; the weeds were relatively large and probably outside of the label limits for the rates being used, Johnson explained.

Spraying a second post-emergence treatment about three weeks after the first should help farmers get control, he said.

“At this time the weeds are starting to regrow, but they are still in a weakened state and should be more sensitive to herbicides,” Johnson said. “Farmers need to look at their fields, look at the weed size and apply the correct rate of herbicide to control the weeds, based on size.”

Also because it’s later in the year, growers should consider if a second herbicide application will restrict rotation plans for wheat to be planted in the fall or another crop to be planted next year.

Johnson also said he has noticed some crop injury such as a burning and cupping of the leaves.

“There is some crop injury, but I would not be concerned about it in terms of yield loss,” he said. “It’s weed control that’s important. It helps protect crop yield.”

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