US milk prices aren't "controlled" since they discovered the euphemism "supported." Some time ago they found that producers had more than they could sell at a price that covered the costs of production, so the Fed buys milk at some minimum price if the open market price falls below the "support level."

The result has been described as something like a "stretch bra" since the quantities the Fed can buy are limited, so it's "incomplete support" and there's nothing preventing the market price from rising above the support level so that the Fed buys none, although the Fed can "release for sale" some of what they bought previously to exert some control over temporary "peaks."

An insufficiently recognized result of the "price supports" comes from the fact that "raw milk" is a more complex product than most people realize, since it's the source for separate products like milk, milk protein, milk fats, whey, glue*, (barn) paint and the like. Selective purchase/sale of components in the Fed "reserve" can be used to exert some "regulation" of a very broad variety of industries. As it is a Federal program, even those who administer it have little understanding ... ... ... of all its effects.

* The appearance of consistently reliable casein glue, basically a milk product, saved thousands of chickens previously slaughtered for the "chicken blood" that held wooden airplanes (and a few automobiles? at the time) together, and greatly improved the smell in lots of factories. Possibly due to the wartime economy, no price supports for the chickens was created, although the fishbait industry was credited with making a valiant effort to provide an alternative use.

John