Impact of Delayed Planting on Heat Unit Requirements for Seed Maturation in Maize

Delayed planting shortens the effective growing season for corn (Zea mays L.), increasing the risk of exposure to late season lethal cold temperatures prior to grain maturation. Consequently, growers often must decide whether to switch to early maturity hybrids to minimize this risk. Heat unit or growing degree day (GDD) ratings of hybrids are potentially more accurate descriptors of hybrid maturity than the traditional 'days to maturity ' system. The objective of this study was to determine whether delayed planting influenced the GDD ratings of silking and kernel black layer development of corn. The effects of delayed planting on the phenological responses of three corn hybrid maturities common to the eastern USA Corn Belt were investigated at four locations in Indiana and Ohio over four years. Delayed planting decreased the number of calendar days after planting to grain maturation by 9 d for early June versus early May plantings. Thermal time from planting to silk emergence decreased an average of 34 GDDs for June versus early May plantings, while the grain fill period decreased an additional 110 GDDs with late plantings. The total decrease in GDDs from planting to kernel black layer was 144 GDDs for corn planted in early June compared to early May, equal to a linear response to delayed planting of 3.8 fewer GDDs per day of delayed planting. The three hybrids responded differently to delayed planting, with greater GDD decreases occurring with late maturity hybrid. Linear rates of GDD decrease with delayed planting ranged from 4.5 to 3.2 GDDs per day of delayed planting for late and early maturity hybrids, respectively. Delayed planting decreases the GDD requirements of corn hybrids, resulting in less risk to grain maturation for adapted hybrid maturities from late season killing freezes than previously thought