The herbicides 2,4-D and dicamba have been around for a long time. Both have been go-to herbicides in the fight against weeds, both in production agriculture and landscape maintenance. But their growing use by production agriculture is causing concern due to the tendency of both 2,4-D and dicamba to volatilize and cause collateral damage to nearby crops in nontargeted fields. Dicamba and 2,4-D are chemically related and behave in much the same way.

Dicamba and 2,4-D are also used extensively for weed control by professionals on turfgrass and are the active weed control component in many “weed and feed” consumer lawn care products. Concerns over the use of 2,4-D and dicamba in production ag could spill over into the landscape industry.

Background

The herbicide 2,4-D was developed in the 1940s and was one of the first modern herbicides. Since it had the characteristic of affecting broadleaf weeds but not harming turfgrass species, it quickly became widely used in the lawn care industry. Developed in the 1960s, dicamba had similar weed-killing characteristics to 2,4-D and also became a favorite for controlling turfgrass weeds.

The popularity of glyphosate

Dicamba and 2,4-D were also popular herbicides in production agriculture, but not as popular as glyphosate. In the 1990s, Monsanto introduced genetically engineered glyphosate-resistant row crops, such as corn and soybeans. This development meant that glyphosate could be used directly on growing fields of corn and soybeans, killing the weeds, while not harming the field crop. Subsequently, sales of glyphosate soared.

The extensive use of glyphosate in agriculture led to weeds developing resistance to the herbicide. After two decades of broad glyphosate use, agricultural pest weeds such as pigweed and Palmer amaranth came roaring back, gaining such a tenacious foothold in some fields as to make them unfarmable.

Dicamba to the rescue?

Several years ago, hearing the cry of farmers, Monsanto scientists went to work to develop genetically-engineered varieties of soybeans that were resistant to the old weed-killing stalwart dicamba. Problem solved!

Not so fast. Soon after the release of the new soybean varieties in 2016, complaints began to come in about crop damage in fields adjacent to the dicamba-resistant crops that had been treated with the herbicide.

One of the unfortunate characteristics of both 2,4-D and dicamba is that they easily volatilize, especially in warmer weather. This is not spray drift, but rather a cloud of gaseous dicamba that rises from treated fields and drifts on the gentlest of breezes.

Some crops, such as grapes, are very sensitive to dicamba and can be harmed from a dicamba application hundreds of feet away.

Monsanto and several other producers of dicamba believe they have solved the problem by developing a low-volatility form of the herbicide. There is some disagreement between researchers at several agricultural research universities and the dicamba manufacturers as to whether this low-volatility form of the herbicide has solved the problem or not.

EPA steps in

In late 2017, The U.S. Environmental Protection Agency (EPA) issued new labeling requirements for the application of dicamba over growing genetically-engineered soybeans and cotton. They also made dicamba a regulated use pesticide (RUP), one that can only be purchased and applied by a certified pesticide applicator. These new labeling requirements do not affect the use of dicamba for turfgrass applications, at least not at this point.

Impact on landscape management

If you manage a landscape in an agricultural area where soybeans or cotton are grown, dicamba drift could affect your landscape. You may want to contact adjacent farmers to ascertain their plans for using dicamba and controlling potential drift problems this season.

Although the new regulatory restrictions on the use of dicamba don’t directly affect the turfgrass use of the herbicide, this story is a cautionary tale. Many turfgrass weed management products incorporate dicamba and 2,4-D, so care should be taken when using them in proximity to sensitive plantings and gardens. Flowering annuals and many typical garden vegetables are sensitive, such as peppers, tomatoes, watermelon, cucumbers, cantaloupes and beans. Grapes are extremely sensitive to dicamba drift.

We’ll be keeping and eye on this issue and will let you know if EPA’s regulatory initiatives start moving in the direction of dicamba use for turfgrass weed management.

Photo: Jahoo Clouseau/Pexels