The productivity of agriculture is enhanced by a well-developed use of fertilisers and the traditional abundance of water, boosted since the Middle Ages by the construction (partly designed by Leonardo da Vinci) of a wide net of irrigation systems. Lower plains are characterised by fodder crops, which are mowed up to eight times a year, cereals (rice, wheat and maize) and sugar beet. Productions of the higher plains include cereals, vegetables, fruit trees and mulberries. The higher areas, up to the Prealps and Alps sectors of the north, produce fruit and wine. Cattle (with the highest density in Italy), pigs, and sheep are also raised.