Understanding the heterosis in multiple environments between different heterotic groups is of fundamental importance in successful maize breeding.  A total of 737 hybrids derived from 41 maize inbreds were evaluated over two years, with the aim of assessing the genetic diversity and their performance between heterotic groups under drought-stressed (DS) and well-watered (WW) treatments.  A total of 38 737 SNPs were employed to assess the genetic diversity.  The genetic distance (GD) between the parents ranged from 0.05 to 0.74, and the 41 inbreds were classified into five heterotic groups.  According to the hybrid performance (high yield and early maturity between heterotic groups), the heterosis and heterotic patterns of Iowa Stiff Stalk Synthetic (BSSS)×Non-Stiff Stalk (NSS), NSS×Sipingtou (SPT) and BSSS×SPT were identified to be useful options in China’s maize breeding.  The relative importance of general and specific combining abilities (GCA and SCA) suggests the importance of the additive genetic effects for grain yield traits under the WW treatment, but the non-additive effects under the DS treatment.  At least one of the parental lines with drought tolerance and a high GCA effect would be required to achieve the ideal hybrid performance under drought conditions.  GD showed a positive correlation with yield and yield heterosis in within-group hybrids over a certain range of GD.  The present investigation suggests that the heterosis is due to the combined accumulation of superior genes/alleles in parents and the optimal genetic distance between parents, and that yield heterosis under DS treatment was mainly determined by the non-additive effects.

Cell wall architecture plays a key role in stalk strength and forage digestibility.  Lignin, cellulose, and hemicellulose are the three main components of plant cell walls, and they can impact stalk quality by affecting the structure and strength of the cell wall.  To explore cell wall development during secondary cell wall lignification in maize stalks, conventional and conditional genetic mapping were used to identify the dynamic quantitative trait loci (QTLs) of the cell wall components and digestibility traits during five growth stages after silking.  Acid detergent lignin (ADL), cellulose (CEL), acid detergent fiber (ADF), neutral detergent fiber (NDF), and in vitro dry matter digestibility (IVDMD) were evaluated in a maize recombinant inbred line (RIL) population.  ADL, CEL, ADF, and NDF gradually increased from 10 to 40 days after silking (DAS), and then they decreased.  IVDMD initially decreased until 40 DAS, and then it increased slightly.  Seventy-two QTLs were identified for the five traits, and each accounted for 3.48–24.04% of the phenotypic variation.  Six QTL hotspots were found, and they were localized in the 1.08, 2.04, 2.07, 7.03, 8.05, and 9.03 bins of the maize genome.  Within the interval of the pleiotropic QTL identified in bin 1.08 of the maize genome, six genes associated with cell wall component biosynthesis were identified as potential candidate genes for stalk strength as well as cell wall-related traits.  In addition, 26 conditional QTLs were detected in the five stages for all of the investigated traits.  Twenty-two of the 26 conditional QTLs were found at 30 DAS conditioned using the values of 20 DAS, and at 50 DAS conditioned using the values of 40 DAS.  These results indicated that cell wall-related traits are regulated by many genes, which are specifically expressed at different stages after silking.  Simultaneous improvements in both forage digestibility and lodging resistance could be achieved by pyramiding multiple beneficial QTL alleles identified in this study.