Maize was believed to have been introduced by Spanish or Portuguese traders in the sixteenth century. Export interest and profitability led to increased maize cultivation after World War II and the introduction of the so-called Guatemala strain in 1951. Output rose rapidly thereafter to almost 600,000 tons in 1961, over 1 million tons in 1965, and 2.3 million tons in 1971. A record 5 million tons were produced in 1985. Fertilizer use was limited, however, and there was concern that yields would gradually decline. The grain was grown throughout Thailand, but the uplands around the central plain were especially suitable. Weather conditions usually permitted commercial growers to produce two crops a year.