Is this antitrust behavior?

Iowa Corn Co., Mid-Iowa Grains, and Center Cut Feeds all produce and sell cattle feed to beef producers throughout the Midwest. They are concerned that one of their customers, Bob’s Big Beef, is driving down prices. By doing so, Bob’s is forcing other beef producers to lower their prices, and they, in turn, are demanding that the feed suppliers reduce prices as well. The three feed producers have decided to not sell any more feed to Bob’s Big Beef.
Is this antitrust behavior? If so, what type? What standard will a court use to review the feed producers’ actions?