The War had a profound influence upon rural areas, as the U-boat blockade required the government to take full control of the food chain, as well as agricultural labour. Cereal production was a high priority, and the Corn Production Act 1917 guaranteed prices, regulated wage rates, and required farmers to meet efficiency standards. The government campaigned heavily for turning marginal land into cropland.[135][136][137] The Women's Land Army brought in 23,000 young women from the towns and cities to milk cows, pick fruit and otherwise replace the men who joined the services.[138] More extensive use of tractors and machinery also replaced farm labourers.