The picture was taken by Marjory Collins, a well-known American photojournalist, in 1943, which was one year after the Bracero program began.
The Bracero program (1942-1964) was a bilateral labor-contract program that started in 1942 between the U.S. and Mexico. Under the program, millions of Mexican workers came to the U.S. for temporary short-term labor contracts. The majority of these workers were under agricultural labor contracts. The U.S. government fueled and quickly adopted the program because of its manpower shortage after World War II.
Collins, Marjory, photographer. Stockton vicinity, California. Mexican agricultural laborers arriving by train to help in the harvesting of beets. United States California Stockton San Joaquin County, 1943. May. Photograph. https://www.loc.gov/item/2017853225/.