SHANDON, Calif.—Steve Sinton, a rancher, was baffled when a company he’d never heard of began buying large tracts of agricultural land near his pastures at above-market prices. The firm, Brodiaea Inc., over a few months in 2012 acquired more than three square miles of a flat-bottomed valley.

“It was surprising, the prices they were willing to pay,” says Mr. Sinton, a partner in a family-owned ranch that raises cattle and grows grapes. A conventional agricultural business’s returns couldn’t have justified those prices. “It...