A true white buffalo happens only once every ten million births, according to the National Bison Association, but many white buffalo in North America result from breading a buffalo with cattle (sometimes referred to as bison hybrid, beefalo or cattalo). Breeding the buffalo with cattle has been a serious setback to wild American bison conservation. Most current bison herds are genetically polluted or partly crossbred with cattle leaving only four genetically unmixed American bison herds left. One of these herds is kept at the Fort Worth Nature Center near Fort Worth, Texas. The crossbred herd vary in type and color, depending on the breed of cattle used [e.g. Herefords and Charolais (beef cattle), Holsteins (dairy) or Brahman (humped cattle)]. Buffalo bred with Charolais cattle produce a much lighter colored calf sometimes almost white.