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Agrochemicals maker Syngenta’s logo is seen in front of the company’s headquarters in Basel February 6, 2013.Credit: Reuters/Arnd Wiegmann

China’s barriers to imports of some U.S. genetically modified crops are disrupting seed companies’ plans for new product launches and keeping at least one variety out of the U.S. market altogether.

Two of the world’s biggest seed makers, Syngenta AG and Dow AgroSciences, are responding with tightly controlled U.S. launches of new GMO seeds, telling farmers where they can plant new corn and soybean varieties and how can the use them. Bayer CropScience told Reuters it has decided to keep a new soybean variety on hold until it receives Chinese import approval.

Beijing is taking longer than in the past to approve new GMO crops, and Chinese ports in November 2013 began rejecting U.S. imports saying they were tainted with a GMO Syngenta corn variety, called Agrisure Viptera, approved in the United States, but not in China.

The developments constrain launches of new GMO seeds by raising concerns that harvests of unapproved varieties could be accidentally shipped to the world’s fastest-growing corn market and denied entry there. It also casts doubt over the future of companies’ heavy investments in research of crop technology.

The stakes are high. Grain traders Cargill Inc [CARG.UL] and Archer Daniels Midland Co, along with dozens of farmers, sued Syngenta for damages after Beijing rejected Viptera shipments, saying the seed maker misrepresented how long it would take to win Chinese approval.

In the weeks since Cargill first sued on Sept. 12, Syngenta’s stock has touched a three-year low. ADM in its lawsuit last week alleged the company did not follow through on plans for a controlled launch of Viptera corn.

Syngenta says the complaints are unfounded.

Bayer, told by Beijing in September that the new soybean seed, LL55, had not been approved for imports, says it will keep on trying, seven years after the company first filed its request. In the meantime, it will withhold the new seed. China granted its last import approval for any GMO grain in June 2013.