Sheriffs, who had first been head of shires, became powerful figures as the primary agents for enforcing royal edicts. There was no longer supervision of them by earls nor influence on them by bishops. They were customarily prominent barons. They collected the royal taxes, executed royal justice, and presided over and controlled the hundred and county courts. They were responsible for remitting a certain sum annually. If a sheriff received more than necessary, he retained the difference as his lawful profit of office. If he received less than necessary, he had to make up the difference from his own pocket. Before rendering this account, he paid the royal benefactions to religious houses, provided for the maintenance of stock on crown lands, paid for the costs of provisions supplied to the court, and paid for traveling expenses of the king and his visitors. The payments were initially paid in kind: e.g. grain, cattle, horses, hounds, and hawks. Sheriffs also took part in the keeping of castles and often managed the estates of the King. Most royal writs were addressed to the sheriff and county courts. They also led the county militia in time of war or rebellion. At times, a sheriff usurped royal rights, used royal estates for his own purposes, encroached on private land and rights, extorted money, and collected revenues only for his own pockets. Over the centuries, there was much competition for the authority to select the sheriff, e.g. by the king, the county court, the barons, and the Exchequer. There was also much pressure to limit his term to one year. Over time, the powers of the sheriffs slowly declined. Royal income came from customary dues, profits of coinage and of justice, and revenues from the King's own estates. For war, there was no change in the custom that a man with five hides of land was required to furnish one heavy armed horseman for forty days service in a year. The fyrd was retained. A threat of a Viking invasion caused the Conquerer to reinstate the danegeld tax at 6s. per hide, which was three times its old rate. (The price of an ox was still about 30d.) To impose this tax uniformly, he sent commissioners to conduct surveys by sworn verdicts of appointed groups of local men. A detailed survey of land holdings and the productive worth of each was made in 1086. The English called it the "Doomsday Book" because there was no appeal from it. The survey revealed, for instance, that one estate had "on the home farm five plough teams: there are also 25 villeins and 6 cotters with 14 teams among them. There is a mill worth 2s. a year and one fishery, a church and four acres of meadow, wood for 150 pigs and two stone quarries, each worth 2s. a year, and two nests of hawks in the wood and 10 slaves." This estate was deemed to be worth 480s. a year.