Accurate characterization of corn ( Zea mays L.) hybrid maturity is important in the northeastern USA to maximize yield and avoid the risk of frost before maturity. Producers frequently grow hybrids that are rated by seed companies at more growing degree days (GDD) than are available in their area. This study was initiated to examine the relationship between the ratings and actual GDD accumulations to silking and black layer for corn hybrids at different locations and planting dates. Nine corn hybrids varying in GDD ratings to black layer from 2320 to 2900 were planted in replicated trials at three locations in Pennsylvania during 1992 and 1993. The locations consisted of short (2100 GDD), medium (2400 GDD), and long (2800 GDD) season environments. At the medium 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 accumulations between planting and silking averaged 91 GDD greater than the hybrid ratings, due primarily to drought stress, which delayed silking in two environments. Where drought stress was not severe most hybrids silked within 60 GDD of their rating. Growing degree day accumulations from planting to 50% black layer for the hybrids averaged 187 GDD less than their rating. Despite the discrepancy between the ratings and actual GDD accumulations, the ratings predicted the relative order of silking and black layer among hybrids reasonably well. When planting was delayed, the interval between silking and black layer was reduced in both years of the study for three hybrids by 89 GDD. This study shows that some hybrids frequently require fewer GDD to reach black layer than their rating in the Northeast and that, in some seasons, hybrids require fewer GDD to reach black layer when planting is delayed. Problem Description Growing degree day (GDD) accumulations for adapted hybrids to reach black layer in central Pennsylvania are often less than the rating supplied by commercial seed companies. In this study, we were interested in determining whether this phenomenon was influenced by hybrid, location, or planting date. Rationale for the study In several previous studies, GDD accumulations between planting and black layer for adapted hybrids were often less than the published GDD rating. In addition, many corn producers grow hybrids that appear to be too late when the GDD ratings were compared with actual GDD accumulations available. Extension literature from New York and Wisconsin that estimated GDD accumulations for hybrids also suggested that the GDD rating for some hybrids may be greater than the actual accumulation necessary to reach black layer. Inaccurate estimates of GDD requirements make accurate maturity recommendations for different regions and planting dates difficult. This is especially difficult in Pennsylvania, where considerable differences in season length can exist within a few miles. Study Description Nine Pioneer brand hybrids consisting of three early maturity hybrids (3861, 3751, and 3795) three medium maturity hybrids (3573, 3527, and 3479, and three late maturity hybrids (3394, 3241, and 3245) were evaluated for 2 yr in long (2800 GDD), medium (2400 GDD), and short season (2100 GDD) environments. At the medium season location, a second planting date, approximately 20 d later, was added to study the effect of delayed planting. At each site, the GDD accumulations between planting and 50% silk and planting and 50% black layer were determined. Applied Questions What was the relationship between the GDD ratings and the actual GDD required to reach silking and black layer? Where water stress was not a factor, all of the hybrids except 3861 and 3795 reached 50% silking within 60 GDD of their rating. At two of the sites, water stress delayed silking, so averaged across all sites, GDD accumulations to silking were higher than the ratings (Table ). In general, hybrids required fewer GDD to reach black layer than their ratings. Accumulations necessary to reach black layer ranged fiom 235 GDD more than the rating to over 500 GDD less than the rating, depending on the hybrid and the environment. At five of the six locations, all hybrids except one required fewer GDD to mature than their rating. Averaged over all hybrids, GDD accumulations to black layer were 187 GDD less than the ratings (Table ). Did the location (length of growing season) affect the relationship between the ratings of adapted hybrids and the actual number of GDD required to reach silking or black layer? No. We found that when adapted hybrids were grown in their maturity zone, they accumulated more than the rating to reach 50% silking in each region. The hybrids accumulated an average of 92 GDD more than the rating to reach 50% silking, but this was due mostly to drought stress that delayed silking in 1993. In 1992, ratings and actual GDD accumulated to silking were similar in all three regions. On the average we found that these corn hybrids required 235 GDD less than their rating to mature at the location where they were adapted. Even in the long season environment, the late hybrids matured in 222 GDD less than predicted by their ratings. This discrepancy is probably a conservative estimate since development was delayed in 1993 due to drought stress. These results indicate that in most areas in Pennsylvania we can expect that the actual GDD accumulation to black layer to be about 100 to 400 GDD less than the current ratings, or about 2100 GDD for the early (92–97 comparative relative maturity [CRM]) hybrids, 2400 for the medium (103–108 CRM) hybrids, and 2600 for the late (111–114 CRM) hybrids. How did planting date affect the relationship between the measured and rated GDD accumulations? Delaying the planting date by an average of 18.5 d increased the GDD accumulation between planting and black layer in 1992 and decreased the GDD accumulation in 1993. In 1992, the increased GDD accumulation to black layer with late planting was associated with a larger GDD accumulation to silking. In 1993, delaying planting reduced the GDD accumulation necessary to reach black layer by 8 GDD/d of planting delay. Averaged over both years, the interval between silking and black layer for three of the hybrids was reduced by an average of 89 GDD with delayed planting. This suggests that hybrids can reduce GDD requirements for the silking to black layer interval and under some conditions this will translate into a reduced GDD accumulation between planting and black layer. These results are consistent with those from previous studies conducted in Indiana and Ohio. Our results also demonstrate that factors other than GDD can affect corn development during both the vegetative and reproductive stages of growth. What are the practical applications of this study? The practical applications of this study are: (i) discrepancies may exist among seed company ratings and actual hybrid GDD accumulations to black layer in some areas, (ii) GDD accumulations to black layer are not fixed and can be affected by factors such as drought stress and planting date, (iii) much of the discrepancy between GDD ratings and actual accumulations to black layer occurs as a result of fewer GDD required between silking and black layer compared with the ratings, and (iv) when calibrated to local conditions, GDD can be used to predict hybrid physiological maturity reasonably well but expect a standard deviation in accumulated GDD across locations between planting and black layer for a given hybrid to be about 170 GDD. Accumulated growing degree days (GDD) between planting and 50% silking for nine hybrids observed at multiple locations in 1992 and 1993 compared with the seed company rating. Short Medium
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Roth et al. (1997) studied this question.