Feeding a Growing Population in Sub Tropics and Tropics With Maize Planted At a High(er) Population Density.

Population growth and adverse environmental conditions curtailing crop yields are endangering food security in the tropics and subtropics. To maintain food security, agriculture in areas with a high production potential will have to be intensified. We aimed to assess if grain yield per area (GYPA) in (sub)tropical germplasm could be increased by increasing planting density. We evaluated 8 adapted tropical hybrids in Kenya (6 from CIMMYT) and Mexico (8) in a factorial combination of 2 fertilization levels (optimum, nitrogen deficient fertilization) and 4 (Mexico) and 3 (Kenya) planting densities ranging from 5-11 plants m^-2. Under optimum conditions GYPA on average increased by 48%  when planting density was increased from 5 to 11 plants m^-2 attaining GYPA as high as 10.9 and 7.6 t ha^-1 in Mexico and Kenya, respectively. Under nitrogen deficient conditions GYPA was increased in Kenya (average GYPA of 4.4 t ha^-1)  by 41% while no increases in GYPA were observed in Mexico (3.3t ha^-1). Increases in GYPA were caused by increases in biomass per area (BMA; HN: r = 0.95; LN: r =  0.84) and the number of ears per area (EPA; r = 0.90; r = 0.72) increasing source capacity and sink potential, respectively. BMA (+83%; +42%) and EPA (+96%; +86%) increased with increasing planting density. We validated results obtained in CIMMYT germplasm in a broad set of 66 commercial (sub)tropical  maize hybrids marketed in Mexico at a planting density of 6.6 and 11 plants m^-2. Most entries showed increased GYPA as a result of increased planting density. Cultivation of maize hybrids at higher densities than are currently used by farmers is a promising route to increased productivity in (sub)-tropical maize germplasm.