Japan suspends some US wheat imports after discovery of genetically engineered grain

Citation: http://www.straitstimes.com/breaking-news/money/story/japan-suspends-some-us-wheat-imports-after-discovery-genetically-engineere

Though it is not good to criticize whole the genetically engineered crops, I wonder why such crops were mixed with ordinary crops.
I guess farmland producing crops commercially is separated from farmland for carrying out experiment of genetically engineered crops. There seems to be no chance to mix both of them.