Slow development of maize (Zea tnays L.) canopies in northern areas of the USA may limit light interception and potential productivity. Whether adiation-use efficiency (RUE) and grain yield could be increased by earlier canopy closure was examined with two hybrids contrasting in canopy architecture and potential phytomass production. Early canopy closure was achieved using a combination of row spacings narrower and plant population densities (PPD) greater than typically used by local producers. Maximum interception of incident PAR (0,,, ~ x) and total PAR intercepted from sowing to 0m~x(IPAR) increased with PPD. Thermal time to one-half 0n,~(TUo. 0 decreased with increasing PPD. Sowing in narrow (38 cm) rows did not affect 0,,,~. IPAR, or TUn. s in the tall hybrid, Pioneer 3790: nor did it affect grain yield, which increased with PPD up to 10 plants m 2. Grain yield of the dwarf hybrid. SX123, was always less than that of Pioneer 3790. due to its low efficiency in converting intercepted PAR into phytomass. Both hybrids exhibited an optimum rate of canopy development in terms of 0,,,,~. IPAR. and TUo. ~ for grain production. Optima for these parameters varied across years, but were similar for both hybrids and row spacings. These results indicate that hybrids adapted to the northern corn belt may yield more grain if sown at PPDs greater than commonly used to promote arly canopy closure. Sowing to rows less than 76 cm wide will have less impact on grain yield. Productivity of hybrids prone to barrenness or with a low efficiency in converting PAR into phytomass, such as SX123, will not improve with earlier canopy closure. 1
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