I've figured it out so thought I'd present my findings to the TD collective brain cell.
1: They create genetically modified seeds of everything and flog them any way they can to farmers who grow that crop promising better yields and ease of farming.
2: The farmers have to agree not to keep seed and buy new one next season. This is designed so that after a few years the farmer has no non-genetically modified seeds left. Now he realises that he wants to go back to the regular seed - but oh dear, he doesn't have any. Now he HAS to pay Monsanto big money to grow the genetically engineered version which they have patented - or...
3: He tries to go to some type of seed bank and buy fresh seeds and start over. Thing is, Monsanto have bought up almost every seed bank on the planet now and guess what - they ain't selling.
Absolute fucking scum.