Accumulated Heat Units for Classifying Corn Hybrid Maturity

Previous studies comparing the accumulated heat unit (AHU) method with the calendar day method for classifying corn (Zea mays L.) hybrids with respect to maturity do not clearly show the superiority of the AHU method over the calendar day method. They appear to have two limitations: (1) the range of climatic conditions under which the studies were conducted was not broad enough to elicit wide variation in the rate of occurrence of plant development and maturity, and (2) the studies were truncated at silking because the time interval from silking to maturity was assumed to be constant. The objective of the study herein was to determine, under a wide range of climatic conditions, whether AHU methods are superior to the calendar day method for classifying the maturity of corn hybrids and predicting the occurrence of phenological events. For a particular corn hybrid grown under a range of climatic conditions, the number of calendar days between any two phenological events may vary widely but the AHU should be constant. Four plantings of three corn hybrids were made at weekly intervals at seven widely dispersed locations in Ohio in 1969 and 1970. Calendar days and AHU, calculated by six methods, were determined from planting to tasseling and silking, and from planting to 40, 35, 30, and 25% kernel moisture and black layer formation. The coefficients of variation of AHU and calendar days to a particular phenological event for each of the three hybrids were determined on the pooled data from the four planting dates and seven locations for each of.2 years. The differences in coefficients of variation among the six AHU methods were relatively small. However, as a group, the AHU methods were about half as variable as the calendar day method for classifying corn hybrid maturity. The data indicated that the AHU method of classifying corn hybrids was superior to calendar days.