The best time to harvest yams? Science says when the lower leaves turn yellow
(PHYSorg.com Biology)

Nigeria accounts for 60% of the world`s yam output and 74% of the total production in West Africa. Grown as a staple food, the tuber of the yam plant is its economically important part. The yam tuber, as in other tuber crops, is essentially a starchy or carbohydrate food and its principal nutritional function is the supply of calories.