A feedlot buys a calf, buys corn to feed it for roughly five months, and sells a finished animal. The cattle crush puts that on the board: buy feeder-cattle-futures and corn-futures in the near months, sell live-cattle-futures about five months out.
The standard CME ratio is 2 feeder contracts and 1 corn contract against 3 live cattle contracts, which approximates the weight gain and feed conversion of a real pen. Feedlots use it to decide whether to place cattle at all; a negative crush means empty pens.
Example: feeders $250/cwt (50,000 lb contract = $125,000), corn $4.50/bu (5,000 bu = $22,500), live cattle $185/cwt (40,000 lb = $74,000). Two feeders plus one corn costs $272,500 against three live cattle worth $222,000, so the crush is deeply negative and placements would be cut.
Related: live-cattle-futures, feeder-cattle-futures, corn-futures, crush-spread, intercommodity-spread