There are a couple of tricks the seed companies use to get around this:
First off, a year or two before the patents on a variety expire, they simple patent some other feature of the variety. Problem solved (from their perspective).
Second, several years before the patent expires on a variety they'll have a "new, improved" variety (often bred from the first) that their sales-people push hard onto farmers so that by the time the patent expires the variety basically has very little/no commercial value - it's been superseded by the "improved" variety in commercial use.
Comments
There are a couple of tricks the seed companies use to get around this:
First off, a year or two before the patents on a variety expire, they simple patent some other feature of the variety. Problem solved (from their perspective).
Second, several years before the patent expires on a variety they'll have a "new, improved" variety (often bred from the first) that their sales-people push hard onto farmers so that by the time the patent expires the variety basically has very little/no commercial value - it's been superseded by the "improved" variety in commercial use.