Botanists: Farmers should not hesitate killing tall ragweed

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Purdue University botanists, in the latest Pest and Crop newsletter, urged farmers who find tall ragweed in their crops to apply herbicide before an infestation could become unmanageable.

In the article, authors Bill Johnson, a professor of botany and plant pathology, and Travis Legleiter, a weed science program specialist, said post-emergent pesticides should be applied when ragweed grows to be approximately four to six inches in height. If these plants are allowed to grow at will, they will block sunlight from getting to crop seedlings, including corn and soybean plants, they said.

"All too often, we see where producers were reluctant to make post-emergent application to a 4- to 6-inch giant ragweed because of a lack of other weeds that size and the potential need for a second post-emergence application," Johnson and Legleiter said. "Waiting for other weeds to emerge or for the soybean canopy to further develop will only allow the giant ragweed to continue to grow well past a manageable height and likely affect yields."

They also warn that fields that have had issues with ragweed in the past are especially vulnerable to infestation unless proper measures and anti-emergent herbicides are applied to fields.

"As farmers and consultants continue or begin to scout their fields, keep in mind that if giant ragweed is present among smaller weeds, that it should drive the decision for making a post-emergence herbicide application," they said.

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