Question: “Is there any evidence, or any studies done regarding effects of Genetically Modified forages and grains on horses?”
Answer: The easy answer to your question is no.
The term ‘genetically modified’ is not well understood by lay people. Do you mean genetically engineered? As in inserting genes, such as creating resistance to RoundUp? RoundUp Ready alfalfa and soybeans, canola oil and BT corn are probably the only ones that might end up in a horse.