The supermarkets decide the prices and the farmers decide whether to get paid or not.
In general (ie. when we don't have the current shortages), the surplus in the market will drive prices down until sufficient farmers go out of business to limit the surplus. It's the farmers' perpetual misfortune that unlike most other sectors of the economy, people can't eat more food when productivity improvements increase the supply*, so unless productivity improvements cease, neither will the farming redundancies. 500 years ago it took 58% of the
population working in agriculture just to feed everyone, now it's just 1%, and that can't do anything but continue, because in a competitive market everyone is forced to seek productivity improvements.
The supermarkets could offer higher prices of course, but what then? So long as the surplus remains, by the time the supermarket buyer has finished placing all the orders there will be some farmers left with no customer. What are they going to do, throw in the towel or ring the buyer and offer a discount if he'll give them the business instead? And when those farmers have won the contract by offering a discount, what will the ones who have just lost out do, if not undercut the farmers who have just stolen their customer?
By the time the farmers have finished their Dutch auction, prices have ratcheted back down to where they were in the first place, and some still remain on the verge of going out of business. You can also repeat exactly the same argument with supermarkets undercutting other supermarkets as consumers look for the cheapest food.
That's why I said that the farmers' enemy is the other farmer, just as the supermarket's enemy is the other supermarket. A cooperative is an agreement among suppliers not to compete with each other in this way, which is fine as long as regulators tolerate anti-competitive practices.
(*People can't eat more, but they can 'consume' more by wasting it, which is why we also have a quarter of all food going to waste in the UK. With such high levels of food waste, higher prices would be a powerful incentive for consumers to reduce it, but farmers might ask themselves what would happen to their profits, and agricultural employment levels if Hugh Fearnley-Whittingstall's food waste campaign were successful at removing 25% of their trade. This issue with economic growth is at the root of the hole we've dug for ourselves with the environment, and if we're going to get out of it, we'll have to get used to working less and consuming less. Unemployment needn't be iniquitous if we find ways of sharing the diminishing work fairly instead of leaving some with no job at all.)