The effect of planting date on maize grain yields and yield components

- Late planting affected grain formation more than stover, decreasing harvest index.
- Late planting or water stress reduced grain yield more in late than early hybrids.
- Temperature and radiation during grain filling affected kernel weight the most.
- Low grain fill temperatures (<15 °C) or radiation (<17 MJ m−2 d−1) reduced yields.
- High temperature around tassel initiation reduced leaf number by 0.1 leaf °C−1.

Four experiments were established in the Waikato and Manawatu regions of New Zealand in 2006 and 2007 to determine planting date (PD) effect on maize (Zea mays L.) leaf growth, grain yield (GY) and yield components. Five or six hybrids of three maturity classes (early, mid and late) were sown on four or five PDs between 18 September and 15 December of each year. Increasing mean daily temperatures in the range 13-19 °C immediately prior to tassel initiation reduced leaf number by 0.1 leaf °C−1. Highest leaf area indices were observed at mean daily temperatures of 17-19 °C. In the lower latitude environment of Waikato, maximum GY was obtained with earlier plantings than Manawatu. Lower spring temperatures, and consequently smaller canopy sizes in Manawatu depressed yields of early plantings. When planted early, late hybrids generally outyielded early hybrids while a better balanced source-sink ratio meant that early hybrids yielded consistently across PDs, matching or outyielding late hybrids when both were planted late. Lower grain filling mean temperatures (15 vs. 18 °C) and average radiation (11 vs. 20 MJ m−2 d−1) reduced yields more for late than early plantings. Grain yield was highly correlated with kernel number (KN) (r = 0.90***) and weight (KW) (r = 0.76***). Lowest KN, KW and GY values were obtained under late plantings, low rainfall (<20 mm) and/or radiation (<18 MJ m−2 d−1) 10-20 d either side of flowering, or when mean temperatures ≤15 °C or irradiance <11 MJ m−2 d−1 occurred during grain filling. Kernel number, KW and GY responses to late planting or water stress were more apparent in late than early hybrids. Kernel weight was more stable than KN under late planting or water stress conditions. Water stress during grain filling affected late PDs more than early PDs. Total biomass and harvest index decreased with delayed planting.