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Maize (Zea mays L.) kernel weight (KW) and grain yield depend on plant growth during active grain filling and reserves use. The objective of our study was to analyze the phenotypic and genotypic variation in these traits in a family of recombinant inbred lines (RIL). In two field experiments we measured plant grain yield (PGY) and its components (KW and kernel number per plant, KNP), biomass production per plant and per kernel during active grain filling, and apparent reserves use (ARU) per plant (ARUP, difference between PGY and plant biomass production during active grain filling) and per kernel (ARUK, difference between KW and plant biomass production per kernel during active grain filling). Heritability (h2) and phenotypic plasticity were computed for all traits. Large differences were always evident among genotypes, but phenotypic plasticity was (i) low for KW and plant biomass at R2 and physiological maturity; (ii) intermediate for KNP and PGY; and (iii) high for plant growth, plant growth per kernel after R2, and ARUs. Traits with highest h2 were KW (0.70), KNP (0.61), and ARUP (0.59). Final KW was related to plant growth per kernel (r2 = 0.64; P < 0.001) but not to ARUK, and ARUP was driven (r2 ≥ 0.49; P < 0.001) by KNP. Because of its positive relationship with KNP (main determinant of PGY), high h2 and high phenotypic plasticity, breeding must consider the increase in ARUP for improving grain yield, an objective that needs to be coupled with large reserves accumulation before silking to avoid the risk of lodging.