USE OF HYBRID GROWING DEGREE-DAY RATINGS FOR CORN IN THE NORTHEASTERNUSA

Accurate characterization of corn (Zea mays L.) hybrid maturity is imp ortant in the northeastern USA to maximize yield and avoid the risk of frost before maturity. Producers frequently grow hybrids that are rat ed by seed companies at more growing degree days (GDD) than are availa ble in their area. This study was initiated to examine the relationshi p between the ratings and actual GDD accumulations to silking and blac k layer for corn hybrids at different locations and planting dates. Ni ne corn hybrids varying in GDD ratings to black layer from 2320 to 290 0 were planted in replicated trials at three locations in Pennsylvania during 1992 and 1993. The locations consisted of short (2100 GDD), me dium (2400 GDD), and long (2800 GDD) season environments. At the mediu m season environment, a second planting date approximately 18 d later was included in the experiment. Maturity was monitored every other day for silking and black layer development. Growing degree day accumulat ions between planting and silking averaged 91 GDD greater than the hyb rid ratings, due primarily to drought stress, which delayed silking in two environments. Where drought stress was not severe most hybrids si lked within 60 GDD of their rating. Growing degree day accumulations f rom planting to 50% black layer for the hybrids averaged 187 GDD less than their rating. Despite the discrepancy between the ratings and act ual GDD accumulations, the ratings predicted the relative order of sil king and black layer among hybrids reasonably well. When planting was delayed, the interval between silking and black layer was reduced in b oth years of the study for three hybrids by 89 GDD. This study shows t hat some hybrids frequently require fewer GDD to reach black layer tha n their rating in the Northeast and that, in some seasons, hybrids req uire fewer GDD to reach black layer when planting is delayed.