Heritage breeds are older, often pre-industrial livestock varieties maintained by small farms as alternatives to the handful of breeds that dominate commercial production. Think Red Wattle or Mangalitsa pigs rather than commercial hybrid lines; Bourbon Red turkeys rather than Broad-Breasted Whites; Dexter or Belted Galloway cattle rather than Angus or Holstein.
Heritage breeds tend to grow slower, cost more, and offer distinctive flavor. They're economically viable mostly through direct-to-consumer sales — heritage-breed meat is almost always premium-priced relative to commodity-breed equivalents.