Oregon cattleman Gary Bailey says business at his ranch has thrived in recent years thanks to growing consumer hunger for meat produced without antibiotics.

But the 36-year-old has struggled in another role: recruiting fellow ranchers to leave the conventional beef market and join his rancher cooperative, Country Natural Beef, a supplier to Whole Foods Market Inc. Many of those conventional ranchers are already earning some of their biggest-ever profits.

“With a real strong market like that, there’s just no advantage to going to a natural program,” says Tim Knuths, 56, a Madras, Ore., rancher who has rebuffed Mr. Bailey’s entreaties.

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