Query: How many novel types of this crop did Burma develop as of 1997 ?
Context: Rice accounts for 97% of total food grain production by weight. Through collaboration with the International Rice Research Institute 52 modern rice varieties were released in the country between 1966 and 1997, helping increase national rice production to 14 million tons in 1987 and to 19 million tons in 1996. By 1988, modern varieties were planted on half of the country's ricelands, including 98 percent of the irrigated areas.