Breeding involves selective mating of cows with matched sires, and a marketing policy that targets unproductive cows. Less than 2 percent of the males are used for reproduction. Close monitoring of the herd allows early detection of oestrus and ensures that more than 95 percent of births result from match- making with selected males. A different sire is used for almost every oestrus of a particular cow, with an overall ratio of about one sire every four births. Pedigree sires are borrowed across large networks of (often related) breeders. Sire borrowing remains frequent (affecting about half the births) even when a breeder owns pedigree sires of his own. Match- making with non-pedigree sires, owned or borrowed, affects about 12 percent of births. Both practices are maintained explicitly in order to preserve variability. Matrilineal genealogies and the sire of each animal in the herd are usually remembered, together with pedigrees of special sires, and the identity and owner of all borrowed sires.