Chemicals produced in the herbicide dicamba float up from crops in warm weather and drift to nearby crops. The active ingredient in dicamba to kill plants is 3,6-dichloro-2-methoxybenzoic acid.
GMO soy, cotton and corn have been produced that are resistant to the active ingredient dicamba in herbicides. Dicamba-resistant soybeans have inserted the "dmo gene", which originates from a bacterial strain of Stenotrophomonas maltophilia. The dmo gene codes for the enzyme dicamba mono-oxygenase. This DMO enzyme inserted into soy cells can inactivate dicamba, which allows the plants to survive contact with dicamba in herbicides.
The manufacturers' user instructions for reducing the extent of damage have proven to be ineffective. Despite their recommended precautions, it is clear that there must be some factors over which they still have no control. Dicamba control has until now been about the product only being used at relatively low temperatures and wind speeds, so that evaporation is prevented. These parameters can be read from a presence message, or at locations with instruments.
However, it turns out that hidden in the product formula is another parameter with a significant negative effect on stability. Dicamba has increased volatility with increased acidity. Increased acidity in the soil will therefore increase the likelihood that Dicamba will rise and drift away from the resistant crops, to other crops where it will cause damage. Dicamba also damages honey production by damaging weeds in the bees' environment. Beekeepers near soy farmers have lost approx. half the hives.
To reduce the amount of Dicamba in herbicides, glyphosate is often used. Dicamba can be mixed with glyphosate for farmers who have dicamba and glyphosate-resistant soybeans, such as Soya MON87708. However, products with glyphosate lead to increased acidity in the mixture. Glyphosate mixed with dicamba will lead to a reduction in measured pH by 1 to 2 pH units. A Dicamba product will have a final pH of over 5.0. If glyphosate is mixed into the product, this will reduce the soil's PH to something lower than 5.0. Pure precipitation has a pH of 5.6. A pH between 5 and 4 is like acid rain.
Soybean MON87708 has the trade name Genuity®Roundup Ready™ 2 Xtend™. Soy MON87708 is available as a soybean variety with an inserted dmo gene that will form DMO (dicamba mono-oxygenase) to give soybean (Gossypium hirsutum) resistance to Dicamba (Genuity®) and Glyphosate (RoundupReady). These products are composed of glyphosate and dicamba. The products are marketed as necessary where there are tough weeds with resistance to herbicides.
Dicamba-resistant soybeans have already been planted on approx. 24 million hectares in the USA as of 2019. This can be compared with 8 million hectares in 2017 and 16 million hectares in 2018. Approx. 60% of all soybean plants planted in 2019 have been genetically modified for dicamba resistance. This means that there is still a high risk of damage to other crops, such as fruit and vegetables, in the vicinity of dicamba-resistant soybeans. By 2020, US authorities want only certified applicants to apply excess dicamba, and prohibit excess application of dicamba to soybeans 45 days after planting.
Links:
Despite federal and state efforts, dicamba complaints continue
Bees face yet another lethal threat in dicamba, a drift-prone pesticide
Spray mixture pH as affected by dicamba, glyphosate, and spray additives
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