Genetically-modified crops are claimed to be perfectly safe, both for the environment and the animals and humans who eat the crops. And the manufacturers of such GM seeds argue that no disclosure needs to be made regarding their use.
But check out what can happen if the GM crops mutate.
Update: From The Raw Story:
The grass suspected of killing the cattle is not a GMO in the sense that food activists typically use, though it is a scientifically modified hybrid of African bermudagrass and an earlier hybrid grass, Tifton 68. The USDA and the University of Georgia jointly developed Tifton 85 and released it for commercial use in 1992. While it was not developed through some of the more controversial gene splicing methods used in GMOs —such as, say injecting fish genes into tomatoes — it is technically considered a genetically modified plant.