Sustainable intensification in crop and livestock production is the foundation for smallholder agriculture in sub-Saharan Africa to adapt to a changing world, to respond to new economic opportunities, and to address poverty reduction and food security. For crop farmers, substantial gains in potential productivity have been made through crop genetics, but these do not translate into production without complementary investments in soil, water, and pest management. Nitrogen is the key driver for cereal crop performance across most environments, both in terms of yield and stability of yield (Vanlauwe et al. 2013). Understanding nitrogen use efficiency (NUE) – here defined as the amount of additional grain harvested per kilogram of nitrogen applied to the grain crop – thus becomes an urgent project that underlies success in agricultural development in the region. Indeed, nitrogen has been identified as one of the grand challenges of the 21st Century given its pivotal role in food production, and nowhere is this more important than in sub-Saharan Africa where fertilizer manufacture infrastructure is non-existent and landlocked countries face fertilizer costs five to ten-fold higher than in the Global North.