Specialty crops growers, especially those who grow grapes, will have new cause for concern in 2015 when new genetically modified crops are expected to come to fields near their orchards and vineyards.
The new ones have been engineered to tolerate two additional herbicides, 2,4-D and dicamba, as well as the herbicides glyphosate (Roundup) or glufosinate (Liberty).
Across the Midwest, where specialty crops grow like islands in a sea of corn and soybeans, more growers are expected to sign on and use the free Web site service called DriftWatch Specialty Crop Site Registry to identify areas growing crops that can’t tolerate drift from spraying or movement of volatile herbicides. Hopefully, their neighbors and commercial applicators will visit the DriftWatch Web site, see their posting, and take special care near their sensitive crops.
Diane Brown, a Michigan State University extension fruit educator in Berrien County, where about 8,000 acres—nearly half—of the state’s grapes are grown, is quite concerned. The herbicide 2,4-D has long been public enemy number one to grape growers, and Michigan has a public act, passed in 1963, forbidding use of 2,4-D esters in certain high-grape-acreage townships between May and October.
The 2,4-D threat came from several sources. It was used sparingly in corn, since there was risk of injury, but it was widely used by homeowners, businesses, and golf courses to keep the lawns and turf free of broad-leaved weeds, especially dandelion.
Brown is concerned about the possibility of vastly increased use of 2,4-D and dicamba, for two different reasons.
First, the new herbicides will come in complicated “packages” so growers won’t be able to choose to use 2,4-D alone to attack glyphosate-resistant weeds; they will now attack weeds with a cocktail of herbicides in premixes. Some of the companies selling the new products may have them labeled in such a way that it would be an off-label, technically illegal use to spray them with any other herbicide but the premix.
Second, she questions the merit of adding more herbicide-resistance genes when Roundup has already demonstrated that weeds can become resistant to even the most lethal herbicide. A decade ago, it was considered highly unlikely that any weed would ever become resistant to Roundup. Now there is a growing list.
Growth regulator herbicides, especially 2,4-D and dicamba, can cause severe leaf distortion and stunting in grapevines, according to Bruce Bordelon, the grape specialist at Purdue University.
“Since the introduction of glyphosate-resistant crops, the number of weedy plant species that have evolved resistance to glyphosate has increased dramatically.”
—Dr. David Mortensen
Late-season exposure to 2,4-D or dicamba may distort young leaves, but probably does not cause economic loss, Bordelon said. Grapes repeatedly exposed to these compounds become less productive, ripen later, and may eventually die from other causes due to their weakened condition.
In Indiana, 2,4-D is primarily a burn-down herbicide used in no-till crop production. It’s most frequently applied in April and May, which coincides exactly with the most sensitive grape growth period, Bordelon said. Dicamba is primarily applied postemergence to corn when plants are four inches or taller.
With the new herbicide-resistant crops, use patterns of the herbicides will change.
Ultimately, the resistant weeds that arise in corn and soybean fields will have to be fought in orchards, -vineyards, and specialty crops fields as well, where genetic crop modification is not a tool. Glyphosate, which is widely used in tree fruits, will become less effective as resistant weeds move in, warns Dr. David Mortensen, weed ecologist with Pennsylvania State University.
“During the period since the introduction of glyphosate-resistant crops, the number of weedy plant species that have evolved resistance to glyphosate has increased dramatically,” he said. This list includes many of the most problematic weed species, such as common ragweed, horseweed, Johnson grass, and several of the most common pigweeds. According to the Penn State research team, despite company-sponsored research that indicated resistance would not occur, 21 different weed species have evolved resistance to glyphosate, 75 percent of which have been documented since 2005. •