The relative importance of cotton exports dropped from 16% of the total in 1922 to 5% in 1932, then rallied to 10% in 1941, falling to 7% in 1951. The quality of cotton produced improved from the 1950s with stricter controls on pests and, although 80% of the crop continued to be grown in the lower Shire valley, it also began to be grown in the northern shore of Lake Malawi. Production varied widely, and increasing amounts were used domestically, but at independence cotton was only the fourth most valuable export crop. Tea was first exported from Nyasaland in 1904 after tea plantations were established in the high rainfall areas of Mlanje District, later extended into Cholo District. Exports steadily increased at first, and the importance of tea increased dramatically after 1934, from only 6% of total exports in 1932 to over 20% in 1935. It never fell below that level, rising to over 40% from 1938 to 1942, and in the three years 1955, 1957 and 1960 the value of tea exports exceeded that of tobacco and until the mid-1960s, Nyasaland had the most extensive area of tea cultivation in Africa. Despite its value to the protectorate's economy, the main problem with its tea on the international market was its low quality. Groundnut exports were insignificant before 1951 when they amounted to 316 tons, but a government scheme to promote their cultivation and better prices led to a rapid increase in the mid-to-late 1950s. At independence, the annual exports totalled 25,000 tons and groundnuts became Nyasaland's third most valuable export. They are also widely grown for food. In the 1930s and 1940s, Nyasaland became a major producer of Tung oil and over on estates in the Shire Highlands were planted with Tung trees. However, after 1953, world prices declined and production dropped as Tung oil was replaced by cheaper petrochemical substitutes. Until the 1949 famine, maize was not exported but a government scheme then promoted it as a cash crop and 38,500 tons were exported in 1955. By independence, local demand had reduced exports to virtually nil. The basis of estate agriculture in Nyasaland for much of the colonial period was the system of thangata which, in the early colonial period, meant that African on estates had to perform agricultural labour in lieu of the rent for a plot of land on which they could grow food. At first,