Maize ('Zea mays' L.) grain yield (GY) and density tolerance (DT) have increased during the last decades; yet plant yield potential (pYP) remains unchanged. In a first study, DT and root to shoot ratio (R:S) were investigated in hybrids representing different eras of genetic improvement in Ontario. Additionally, the DT-pYP association was explored in a set of experimental hybrids exhibiting the different trait combinations. The DT in modern era hybrids was explained mostly by maintenance of harvest index (HI). Increased dry matter accumulation (DMA) from silking to maturity and reduced plant to plant variability also contributed to DT. The R:S increased in modern genotypes but this effect disappeared under shading. In the experimental hybrids the pYP was associated with both DMA and HI under low density stress. For both studies reduced barrenness (stable HI) rather than high pYP was the main contributor to DT.