Genetically modified food crops are widely cultivated in the US. The primary reason for this is that they have characteristics that make them tolerant to herbicides and resistant to insects.
This technology has been in use for the last 15 years and a lot of research has been conducted to review the impact of the technology on the environment. There are many dangers that the use of genetically modified food crops pose. The genes that protect GM crops from herbicides can move to actual weeds making them immune to currently used herbicides and very difficult to control. For example, before the introduction of GM crops, only one weed, ryegrass, was known to be resistant to a potent herbicide called glyphosate. Now, cases of weeds resistant to this have been reported from at least 6 states in the US.
GM plants that are designed to produce toxic substances to protect them from insects can prove dangerous for useful insects, birds and animals like deer, etc. An example to illustrate this was a toxin produced by certain GM crops that was lethal to the larvae of the Monarch butterfly. The Monarch butterfly travels thousands of kilometers to breed, is very beautiful to look at, and totally harmless, could have been eliminated due to the GM crops. Fortunately, the crops that produced toxins against the butterfly larvae had not been adopted widely and it was possible to put a ban on their use with no harm caused to the butterfly.