The GMOs definitely, also the monocrop farming destroys the soil, pesticides, herbicides, fungicides, and lots of erosion. Where I'm from it's corn and soy as far as the eye can see, and you'll have to drive your car at least 15km just to buy a tomato or anything fresh.
It makes no sense to farm like this, but the tax incentives cause profit-driven farmers to play along with the government incentives. Big meat, dairy, corn, soy, and wheat subsidies means those are the cheapest ingredients available, so it leaves vegan who likes whole foods having to spend 40% more on groceries every month compared to a processed-food eater.