Growing Degree Days to Black Layer Compared to Minnesota Relative Maturity Rating of Corn Hybrids 1

We evaluated 28 commercial corn ( Zea mays L.) hybrids at 2 dates of planting and 24 check hybrids used as standards for establishing Minnesota Relative Maturity (Minnesota RM) at 1 date of planting to determine growing degree days (GDD) to black layer for each hybrid. The hybrids were grown at four locations in Minnesota in 1970 and 1971. Date of 50% black layer formation and percent grain moisture on this date were determined. Growing degree days from planting to black layer were calculated for each plot using the National Oceanic and Atmospheric Administration formula. Hybrids, locations, years, and dates of planting were highly significant sources of variation for GDD from planting to black layer. Hybrid ✕ location and hybrid ✕ year interactions were nonsignificant for growing degree days to black layer and the hybrid ✕ location ✕ year interaction was considered to be negligible in size even though it was statistically significant. The variance for GDD among hybrids within Minnesota RM groups of the check hybrids was highly significant but considerably less than the variance among the maturity groups. The correspondence between growing degree days to black layer and Minnesota RM rating was good on the average for both the commercial and check hybrids, but in a number of instances, hybrids that differed by 10 or more relative maturity units had similar growing degree day ratings. In our opinion, these data cast serious doubt on the usefulness of the GDD to black layer system of rating hybrids for maturity in Minnesota. At black layer, moisture percentage values ranged from 22.6 to 32.9%. The use of GDD from planting to 30% moisture content would tend to increase the range in GDD for the hybrids included in this study.