Two of the world's biggest seeds groups, and bitter competitors, returned to trading slurs when DuPont accused Monsanto of breaking patents, only to be accused of "frivolous claims".

DuPont, the Iowa-based chemicals conglomerate, sued Monsanto for lost profits and compensation for patent infringement - and asked judges to impose a penalty of a trebling of damages on top - over allegations surrounding a harvesting technique that boosts "the vigour" of corn seeds.

Monsanto, which poached two of the scientists who developed the process while working at DuPont's Pioneer seed division, is "engaging in wilful and deliberate infringement" of the patents, a filing said.

"The infringement… will continue to cause Pioneer irreparable injury and damage.

"Pioneer is entitled to recover from Monsanto the damages sustained by Pioneer as a result of Monsanto's wrongful acts."

'Frivolous claims'

However, Monsanto denied that it used DuPont's defoliation technique, in which corn plants are sprayed off with weedkillers before harvest to boost seed quality.

"This approach is not used in any Monsanto production fields," the Missouri-based group said,  adding that it did not believe the lawsuit would have "any practical effect on our current or future business".

Indeed, it dismissed the filing as the latest "in a series of frivolous claims initiated by DuPont against our business and aimed at distracting us from our mission of investing in and delivering new product offerings to farmers around the world".

"We will defend our business against this latest attack."

'Dirty tricks'

The spat represents an escalation of tensions which had appeared to ease since 2009, when the groups sued eachother, after disagreements centring on use of Monsanto's herbicide-resistant seed technology.

DuPont accused Monsanto of gaining "illegal monopoly power". Monsanto in turn accused DuPont of "dirty tricks".

DuPont's latest claims surround defoliation techniques on which it was given patents in 1996 and 2000, based on research by three scientists including Barry Martin and John Schoper, both of whom later joined Monsanto.

Mr Martin, who became Monsanto's director of seed technology, left last year to take a similar position as Swiss-based Syngenta.