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The characteristics, limitations, and accuracy of 3 thermal (heat) unit systems were examined. One was a modified polynomial regression system and 2 were base temperature : cutoff temperature systems. All gave about the same minimum coefficient of variation if the proper parameters were used. Distinct differences were noted for the temperature-rate relationships found in 3 periods: planting to the 4th leaf, 4th leaf to silk, and silk to harvest. No consistent advantage was found for maximums alone, minimums alone, or both maximums and minimums as independent variables over the use of the mean daily temperature. Planting dates used alone or in conjunction with thermal units were effective sometimes in reducing error of the predictive system. It is hypothesized that they reflect differences in climatic factors that show a trend during the growing season. The estimation of the 4th leaf stage failed to increase the accuracy of the prediction of silking or harvest dates enough to make the effort practical. The establishment of the 80% silking stage by field observation increased the accuracy of harvest date prediction. The use of the Barger base temperature : cutoff temperature system with appropriate parameters is recommended.

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