Although there is no universally agreed-upon definition of what makes a particular animal breed a ‘heritage breed,’ it’s generally agreed that the breeds were adapted to their local environment, bred for advantageous traits, and existed well before the rise of modern (factory) farming.
According to The Livestock Conservancy, “traditional, historic breeds retain essential attributes for survival and self-sufficiency – fertility, foraging ability, longevity, maternal instincts, ability to mate naturally, and resistance to diseases and parasites.”