The US state of Tennessee has become the fourth to place restrictions on the use of Monsanto’s dicamba, due to damages the herbicide is said to cause to crops not genetically modified to resist it.

Traces of the crop protectant sprayed by farmers have drifted, damaging soybeans, cotton and other crops across the southern US and leading to lawsuits over lost crops, according to reports.

The states of Arkansas and Missouri have also restricted the use of dicamba, and Kansas is said to be investigating complaints.

Monsanto, which has taken steps to limit drift, is campaigning to overturn the bans. The agribusiness giant, in the process of being acquired by Germany’s Bayer, blames problems such as wind drift and cross-contamination, issues similar to those it faced two decades ago when it launched its Roundup Ready glyphosate-resistant crops.

The company maintains that many of the dicamba issues are caused by farmers not following application labels, using contaminated equipment or buying older formulations of the pesticide that are cheaper but more prone to drift.

At the same time, Monsanto, along with BASF and DuPont, which also produce dicamba-based weed-killers, have agreed to additional safeguards for product use, according to Missouri Director of Agriculture, Chris Chinn.

The discussion adds to Monsanto’s woes, following a decision by the state of California last month to list glyphosate as a probable carcinogen.

The herbicide was key to Monsanto's biggest-ever biotech seed launch last year. The company’s Xtend line of soybeans and cotton are designed to tolerate the active ingredient, which replaces earlier products containing only glyphosate. Some weeds have developed resistance to glyphosate, which was introduced in the 1970s.

According to reports quoted by the news agency Reuters, the use of dicamba has spiked this season across the US after regulators last year approved it for crops already growing. Monsanto also sells a new dicamba formulation under the name Xtendimax, which it claims has less drift compared to older versions. BASF and DuPont also sell less drift-prone formulations.