The shift to large-scale field and food production involved changes in labor and technology. Labor shortages during World War II led to the Bracero program, which, between 1942 and 1964, brought more than two million Mexican men to the United States to work on short-term contracts. Labor unions such as the United Farm Workers sought to organize farm workers and fight for better wages and working conditions. They also staged boycotts to bring attention to these issues. Many growers turned to technological innovations to save costs and increase efficiency.