Pioneer Hi-Bred International Inc. expanded its grievances against Monsanto Co. with a lawsuit alleging infringement of two patents for producing corn seed that can withstand environmental stress, Bloomberg reported. Monsanto is defoliating corn plants between pollination and harvest to produce more vigorous seeds, a method patented by Pioneer, Pioneer said yesterday in a complaint filed in federal court in Des Moines. Two former Pioneer inventors now work for St. Louis-based Monsanto, Pioneer said. The patents, one issued in 1996 and the other in 2000, cover a way to defoliate plants before harvesting to improve the quality of maize seed. The goal is to produce better plants, even if the weather or soil isn’t optimal, according to the patents. The companies have been trading lawsuits since E.I. DuPont de Nemours & Co. bought Pioneer in 1999.