Patents serve the same purpose in farming as they do elsewhere: it makes it worthwhile for companies to invest the hundreds of millions of dollars it costs to drive crop yields ever higher.
This rest on the assumption that the patent system is the best way to structure a market and incentivize agricultural R&D.
More accurately, it rests on the assumption that the patent system actually available now is the best way to structure this market activity. But I don't see GMO opponents offering better incentive structures, I see them making up non-scientific arguments that make no sense.
This rest on the assumption that the patent system is the best way to structure a market and incentivize agricultural R&D.