Yams are a crop grown exclusively by men. Growing yams is labor intensive, and the size of a man's fields and harvest says much about his work ethic. Yams are grown to gain wealth and also to feed one's family. They are a symbol of masculinity and ability as a provider.
According to Umuaro religion, the harvest can't take place until the Feast of the New Yam is called by Ulu's chief priest, Ezeulu. And in order to punish Umuaro, Ezeulu stubbornly maintains that he can't call the feast until he eats the three remaining sacred yams. Ezeulu claims that according to Ulu, he can only eat one yam a month. While Ezeulu keeps to this rigid schedule, the rest of Umuaro begins to fall into famine and death.
At the end of Arrow of God, the Christian church invites the people of Umuaro to sacrifice their yams to the Christian god. When this happens, it symbolizes the triumph of Christianity over traditional Umuaro religion.