Pigweed
Gene flow from genetically engineered crops to organic and conventional crops is a big agricultural concern.

Read the Implications of Gene Flow and Natural Selection in Genetically Engineered Crops white paper. 

Gene flow is the change in gene frequency in a population due to movement of genetic material, individuals, or groups of individuals from one place to another. Gene flow was one of the first issues raised about the introduction the genetically engineered (GE) crops.