Dicamba-Resistant Crops – Stumbling Over the Starting Block

The discovery phenoxy herbicides (i.e. 2,4-dichlorophenoxyacetic acid or 2,4-D) in the 1940s ushered in a new the era of weed management. Since then, the agrochemical industry has developed and commercialised a wide panoply of herbicides with at least 25 recognised mechanisms of action. Today, farmers rely heavily on the use of chemicals for controlling weeds. The process of discovering, developing and commercialising herbicides has been successful because of the mostly positive interactions between researchers from industry, academia and governmental institutions. This process has been self-correcting, where new compounds initially discovered by companies were made available to independent researchers outside industry to evaluate in their specific geographic location to identify potential weaknesses or problems that needed to be addressed before global commercialisation of a product. This system has worked well for industry, researchers, farmers and consumers, except for where dicamba use with dicamba-resistant crops is concerned. Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a group 4 (auxin mimic or growth regulator) herbicide of the methoxybenzoic acid group. Group 4 herbicides mimic the action of auxin (indoleacetic acid). Their actual mechanism of action is fairly complicated, but in short, they bind to the receptor for auxin and disrupt normal hormonal regulation of cell growth, leading to abnormal growth and death of plants. Because auxin mimics tend to provide selective control of broadleaf weeds in grass crops, these herbicides were initially very popular for broadleaf weed management in corn and related cereals. Dicamba was first introduced in the US in the mid-1960s and was used for that purpose successfully for many years.