Monsanto's patent on Rounup runs out in the year 2000, but any farmer who adopts Roundup Ready seeds must agree to buy only Monsanto's brand of Roundup herbicide. To make Roundup Ready Technology legal, the US Evironmental Protection Agency had to accommodate Monsanto by tripling the allowable residues of Roundup that can remain on the crop. This increase of chemicals used on crops can lead to the development of resistance in insect pests, and knock on effects to the wider ecosystem. It is possible that resistance will reach such a level that these chemicals which world farming rely on will become useless. There are other risks involved with genetic engineering.
Genes can travel to nearby, related plants on their own.
This is called gene flow. In 1996 gene flow was discovered to be much more common that previously thought. According to Science magazine, many ecologists say it is only a matter of time before an engineered gene makes the leap to a weedy species, thus creating a new weed or invigorating an old one. Most people are opposed to genetic engineering. Of course there is resistance to this because the GM companies believe correctly that the majority of people would not buy GM food if it were clearly labelled.