BANDERA, Texas—After a soft rain on Sovereignty Ranch, the earthy smells of rich humus and pungent juniper hung in the air. The clay roads became a sticky gumbo, clinging to cowboy boots and pickup trucks as guests converged on the ranch. Bandera, dubbed the Cowboy Capital of the world, was a hub for driving millions of cattle and horses through the region during the late 1800s. But the scenic 200-acre Sovereignty Ranch, set amid rugged hills and ancient limestone bluffs, does not follow traditional land-and-cattle ranching practices. The farm was founded by Mollie Engelhart, a self-described “recovering vegan” and California chef who became a Texas rancher and regenerative farmer. The first weekend of May, the place was packed with vehicles from Texas, Colorado, and California for the two-day American Regeneration Conference, featuring a visit from Health Secretary Robert F. Kennedy Jr., a longtime advocate of regenerative farming....