Traditionally, domestic sheep, the most common farm animal in Iceland, was the primary source of meat. Sheep were also used for their milk and wool, and were worth more alive than dead. When a sheep was slaughtered (usually the young rams and infertile ewes), most or all of the carcass was used for making food, which was carefully preserved and consumed. Traditionally lambs are slaughtered in the autumn, when they are more than three months old and have reached a weight of almost 20 kg. After Christianisation, horses were eaten only as a last resort. After the middle of the 18th century, attitudes changed. Horse meat, usually salted and served boiled or in "bjúgu," a form of smoked sausage, has been common in Iceland since the 19th century.