Effects of planting density and nitrogen fertilization level on grain yield and harvest index in seven modern tropical maize hybrids (Zea mays L.)

To support tropical maize (Zea mays L.) breeding efforts, the current work aimed to assess harvest index (HI) in modern hybrids and determine the effect of different planting densities on grain yield and HI under well-fertilized (HN) and nitrogen (N) deficient conditions. Harvest index and grain yield of 34 hybrids on average reached 0.42 and 7.06 t/ha (five environments), indicating a large potential for improvement in HI relative to temperate hybrids. Ear weight (r = 0.88), HI (r = 0.78) and shoot dry weight (r = 0.68) were strongly associated with grain yield. In the second experiment, seven hybrids were evaluated at planting densities of 5, 7, 9 and 11 plants/m(2) under HN (six environments) and N deficient (LN) conditions (four environments) to assess the effect of planting density on grain yield and HI. Grain yield increased by 40.4 and 21.8% under HN and LN conditions when planting density was increased relative to the lowest planting density. Harvest index increased from 0.42 at 5 plants/m(2) to 0.45 at 11 plants/m(2) under HN conditions and decreased from 0.44 at 5 plants/m(2) to 0.42 at 9 plants/m(2) under LN conditions. Harvest index was maximized at planting densities of 8.33 plants/m(2) and 5.30 plants/m(2) under HN and LN conditions, respectively, while grain yield was maximized at 9.93 plants/m(2) and 7.89/m(2). Optimal planting density maximizing both HI and grain yield were higher than planting densities currently used in tropical germplasm. It can be concluded that productivity in tropical maize could be increased both under intensive (+40.4%) and lower-input management (+21.8%) by increasing planting densities above those currently used in smallholder agriculture in Latin America and Sub-Saharan Africa, in environments targeted by the International Maize and Wheat Improvement Center.