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It is almost impossible to go through a day without hearing about further developments on the ongoing dicamba saga here in the US.
Recent developments have further escalated tensions between state governments, agrochemical manufacturers and producers and there are little signs of any resolution in the near future.
What is dicamba and when is it used?
Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a broad-spectrum herbicide on the market since the 1960s. It is an organochloride and is a derivative of benzoic acid.
The herbicide is praised for being highly effective against troublesome weeds such as pigweed, mare’s tail and lamb’s quarter.
This long-serving herbicide has become popular again and is used by farmers growing dicamba-tolerant genetically modified (GM) corn/maize. Over the past couple of years growers have come to use this herbicide on soya bean and cotton fields planted with newly engineered dicamba-tolerant GM seeds.
Volatilisation and drift
What makes dicamba chemistry stand out from the crowd is its tendency to volatilise. This characteristic has been known by the industry for years.
The chemical has a high vapour pressure, meaning it’s more likely to evaporate and rise into the atmosphere post application.
When present in the atmosphere the chemical can drift to off-target plants in neighbouring fields which, in most cases, are not dicamba-tolerant. Recent research has found that dicamba damage was observed over 70 metres away from application sites.
This year was the first season that significant numbers of US farmers used the chemistry on soya beans. This led to huge incidences of dicamba drift damage.
In total, there were 2,708 official dicamba-related injury investigations in the US. Of these reported cases, 3.6m acres were soya beans, which is about 4% of the US crop this year.
The volatilisation of dicamba post-spraying has been a huge issue in the US during 2017.
Non-tolerant soya beans are among the most susceptible crops to dicamba damage, with just 1/20,000th of the recommended dose enough to cause significant symptoms.
Many non-tolerant crops, such as tomatoes, peaches, vineyards, vegetables, apple trees, and watermelons also suffered serious damage.
Uncertainty among researchers
A recent DTN dicamba webinar featured contributions from University of Missouri weed scientists, a soya bean agronomist and a trade association representative. However, one got a distinct impression that the factors which contributed to dicamba volatilisation and drift were not well understood.
For example, Dr Kevin Bradley, a researcher at University of Missouri, conducted a number of regional experiments to determine levels of dicamba volatilisation based on time frames post-spraying. Dicamba was detected in the air for up to several days post application.
Dr Bradley then brought non-tolerant soya bean plants, which had been growing in a greenhouse, into that field 72 hours post spraying. These plants subsequently showed signs of dicamba damage.
The message from the researcher community was clear, there are more questions than answers. The exact effect on soya bean yield is equally unclear, with some farmers even claiming that it increased their yields.
EPA and state response
In response to the dicamba problems experienced over the summer, the US EPA announced an agreement with three agrochemical manufacturers, Monsanto, BASF and DuPont, to put new requirements in place for dicamba use in 2018.
The use of dicamba for this purpose is now labelled as a "restricted use", and it can only be applied by certified applicators with special training. So all commercial spray applicators must undertake dicamba spray training in 2018.
Other requirements include limiting application when wind speeds exceed 10mph to reduce potential spray drift; restricting spray timing to daylight hours and requiring farmers to maintain specific records relating to the use of these products.
The state-level response has been varied. When the extent of the dicamba problems became evident both Arkansas and Missouri issued a temporary “stop sale and use order” during the summer on all dicamba products.
Missouri then voted to ban the use of BASF's Engenia herbicide in 10 counties which had high numbers of complaints this year from 1 June 2018. This in-season ban extends state-wide on 15 July and ends in October.
This ban is eventually expected to extend to dicamba herbicides sold by Monsanto and DuPont.
The Arkansas Plant Board also voted to ban the use of any herbicide containing dicamba between 16 April and 31 October 2018. Many other states took no action.
Dicamba manufacturers response
Monsanto believed it had addressed dicamba’s longstanding volatilisation problem when it introduced VaporGrip in its new XtendiMax herbicide. VaporGrip was designed to keep dicamba in its salt form, which has a better chance of being retained in plant leaves than the volatile acid form does.
Estimates of Dicamba-injured soya bean acreage in the US in 2017 as reported by state extension weed scientists.
Following the first temporary state ban imposed by Arkansas, Monsanto moved to sue the Plant Board on the grounds that they were acting outside their authority.
However, following the vote to ban its use in 2018 during the spraying season, Monsanto asked an Arkansas County judge to strike down the rule approved by the board earlier this month, stating: “The ban severely curtails Monsanto’s ability to sell its new dicamba-tolerant seed and low-volatility dicamba herbicide within the state, and every day the ban remains in place costs Monsanto sales and customers.”
BASF came out swinging also, publishing survey results of 400 soya bean and cotton growers across the country. Growers rated weed control from Engenia herbicide a solid 8.6 out of 10.
The question of liability
The question of liability remains an incredibly murky area. The emphasis thus far has been placed on the spray applicator to ensure label directions are followed correctly, ie correct rates, suitable spraying conditions, correct sprayer set-up and nozzle selection etc.
If these steps can be proved, then the liability does not lie with the applicator as the damage was caused by volatility, a factor beyond the applicator's control.
Furthermore, as chemical volatility is not a covered risk under the majority of crop insurance policies, insurance companies are not obliged to cover losses that occur from dicamba drift.
Two sides to every coin
“I have not seen such divisiveness among farmers in my 20-something years in agriculture,” Dr Bradley said. Despite the millions of acres damaged by dicamba volatility drift, many farmers are still adamant that the chemical is essential for their weed control programmes.
Following the introduction of the in-state ban on dicamba use in Arkansas, a group of farmers filed a petition against its introduction.
The group say they’ve “seen first-hand the successes of dicamba technology in controlling the presence of pigweed” and have “been impressed by significant improvements in yield".
The 2018 season
The extent of the dicamba issues in 2017 caught regulators, agrichemical manufacturers and farmers off-guard.
With dicamba technology still widely available to farmers for 2018, all parties involved have scrambled to ensure the same problems are not repeated. With the new label requirements, in-season bans and deepening divisions among farmers, developments in the US dicamba dilemma will be worth watching.
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