In the UK 'beef' means the meat of heifers (8-20 months), cows, bullocks (24-40 months) and bulls. Heifers are cows which have never had a calf. Bullocks are young bulls under a year old. Steers are males castrated when very young. Beef is at its best when the animal is 18-24 months. In North America meat over 2½ years, when it darkens to a purplish-red, is classified as "well matured beef". In the United Kingdom, since the recognition of BSE (Bovine Spongiforme Encephalopathy), beef cattle older than 30 months may not be used for meat. The so-called "noble" cuts come from the hindquarters of the animal and can be cooked rapidly, while the front quarters provide cuts which need slow braising or boiling. The quality depends on breed, age, feeding patterns, manner of slaughter and period of hanging. The meat should be fresh red and the fat white rather than yellow.