Changes in Kernel Number and Number of Ears Per Plant Increased Grain Yield in 15 Years (1995-2010) of Hybrid Breeding At Cimmyt.

We aimed to quantify changes in grain yield per area (GYPA) and yield components accountable for changes in GYPA for three tropical hybrids released by CIMMYT in 1995 (CML247/CML254; G1), 2005 (CML494/CML495; G2) and 2010 (CML549/CML550; G3) and their adaptation to four different planting densities (D5, 5; D7, 7; D9, 9; D11, 11 plants m-2).  GYPA between G1 and G3 increased from 8.43 t ha-1 to 9.65t ha-1. The number of kernels (KNO) was the most important factor contributing to increases in GYPA among genotypes as well as between planting densities. Irrespective of the planting density G3 formed more KNO than G1 and G2 with differences ranging from 5.35 (D7) to 7.7 mio kernel ha-1 (D11). Accumulation of greater shoot dry weight (+33%) for G3 compared to G1 at an earlier flowering date (-5.7d) indicated greater crop growth rates for G3 potentially explaining large increases in KNO. The number of ears per plant (EPP) equally showed a strong increase from 0.86 for G1 to 1.03 for G3. We show that line selection under higher planting densities did not have a direct effect on tolerance to higher planting densities and discuss potential beneficial effects on overall nutrient capture, reductions in ear barrenness and increases in KNO. Grain yield increased from 7.2 t ha-1 to 10.5 t ha-1 when planting density was increased from 5 plants m-2 to 11 plants m-2 as a result of increased KNO per area, while EPP (-17%), HKW (-7%) and ear weight (EWT; -21%) were reduced.  We show that it will be possible to increase grain yield by up to 46% by planting CIMMYT tropical hybrids at higher planting densities.