In E. E. Evans-Pritchards the Nuer he mentions maize being one of four crops (along with millet, beans, and tobacco) that is cultivated by the Nuer in South Sudan.
It is my understanding that Maize originated in the Americas. So when was it introduced to the Sudan? And how did it become a staple of the Nuer?
The earliest known possible reference to maize in Africa occurred in 1502, but there's some speculation that they might have been referring to a different grain. By the 1550s there were unambiguous references to maize in Africa. That was along the coast of West Africa however, and I'm afraid I couldn't find anything specifically referring to when maize arrived in Sudan.