Grassland is the UK's most important crop by area covering just over half of the entire UK - nearly three times as much as all other crops combined. Rough grazing and permanent grassland are maintained without reseeding whereas rotational grass which is sown every few years as part of an arable crop rotation where it is often known as a grass ley. Although grass leys are expensive to establish, they are very productive in their early life and play an important role in a mixed farm by returning organic matter to the arable rotation. The relatively wet temperate climate of the UK which favours grass and its growth.