The effect of planting date on maize: Phenology, thermal time durations and growth rates in a cool temperate climate

Delayed planting either reduced (Waikato) or increased (Manawatu) the emergence-flowering duration and this was associated with changes in leaf number and phyllochron length. The phyllochron averaged 47 °Cd, but increased to 51 °Cd when soil temperature, radiation and precipitation between emergence and tassel initiation were respectively >22 °C, ≤17 MJ m−2 d−1 and ≤30 mm. Sub-optimal temperatures and radiation under late plantings triggered a source limitation, leading to assimilate remobilization, reduced grain filling duration and resulted in higher grain moistures at physiological maturity (36% vs. 30%). When rainfall between emergence and flowering was ≥234 mm, increases in average daily irradiance (19.5-21.4 MJ m−2 d−1) and mean temperature (15-18 °C) increased pre-flowering crop growth rate (CGR) by 1 g m−2 (°Cd)−1. With late planting, higher pre-flowering radiation (≥21 MJ m−2 d−1) and temperatures (≥17 °C) increased CGR while low post-flowering radiation (13 MJ m−2 d−1) and temperature (15.7 °C) reduced CGR. Kernel growth rate (KGR) was more stable across PDs and hybrids when TT was used (0.36-0.38 mg (°Cd)−1). Provided temperature was ≥19 °C, low daily irradiance (11 MJ m−2 d−1) did not significantly reduce KGR. More rapid leaf senescence occurred for early and late plantings, and this was attributed to source-sink imbalances caused by assimilate accumulation or shortage. Leaf senescence rates in the absence of water stress were 0.03% (°Cd)−1 between anthesis and mid grain-fill, and increased to 0.2% (°Cd)−1 towards the end of grain filling.