Factors Underlying Grain Yield Response to Plant Density in Maize.

Genetic improvement for maize grain yield in North America has been associated with increased tolerance to high plant density. It has been suggested that increased tolerance to high plant density is attributable, in part, to increased stay green (i.e., delayed leaf senescence during grain filling period), to increased dry matter partitioning to the ear and florets during grain-sink establishment, and to a decrease in pant-to-plant variability. The following hypotheses are tested in this study: (i) tolerance to high plant density is not associated with functional stay green; (ii) threshold dry matter at maturity for harvest index and threshold plant growth rate during the period of kernel establishment are negatively associated with tolerance to high plant density; (iii) tolerance to high plant density is associated with low plant-to-plant variability for dry matter throughout the life cycle and harvest index at maturity. A study was carried out at the Elora Research Station in 2007-2009 using a split-plot design with four replications. Main treatments were two of plant densities of 8 and 16 plants m^-2 and the subplots consisted of six short-season maize hybrids from different eras. Dry matter accumulation was determined at silking and at maturity, and light interception was measured at silking, and 4, 6, and 10 wk after silking.  The same protocol was employed in an experimental location near Guelph, which included four experimental hybrids. A combination of destructive measurement and non-destructive measurement in tagged plants was used in 2009 to determine through allometric relationship the effect of plant density on plant dry matter accumulation and plant-to-plant variability throughout the life cycle, and plant dry matter and harvest index at maturity.