Farmers have used Dicamba for more than 50 years to clear fields in the spring before planting, said Jason Norsworthy, an agricultural weed scientist at the University of Arkansas in Fayetteville.

But things changed in recent years as big biotech companies developed genetically modified, dicamba-resistant varieties of crops like soybeans and cotton.

Those crops are designed to help farmers deal with weeds that have evolved to tolerate glyphosate, the active ingredient in herbicides like Roundup, said Scott Partridge, Monsanto’s vice president for global strategy.

As mainland farmers have begun spraying dicamba on growing plants, the herbicide seems to have drifted to nearby fields of crops not engineered to withstand the chemical.

Monsanto’s Partridge said it appears many farmers did not follow label instructions for using Xtendimax, which include such details as the type of nozzle to use, the speed and height of the spraying rig, pressure for the tank and size of the buffer zones around the fields.