Knowledge of growing season thermal units is essential for effective selection of corn (Zea mays L.) hybrids that can reach maturity within the short growing season on the Canadian Prairies. It is important to have adequate information about occurrence of phenological stages of corn so as to make effective management decision on seeding dates, chemical and fertilizer applications and anticipating harvest date. Therefore, a consistent index would aid with effective selection of corn hybrids for a given area. This study showed that the variation in the accumulation of the general thermal index (GTI) was consistently lower than that for the corn heat unit (CHU) but was not significant. The study also showed no significant differences in thermal heat unit accumulation required to reach physiological maturity by five corn hybrids with CHU ratings ranging from 2200 to 2700. The relative maturity (RM) rating determined by seed industries all showed a close relationship (r=0.75-0.99) with CHU rating. Therefore, the use of either tool would have similar results and thus RM would not be effective for selecting corn hybrids on the Canadian Prairies. There was an inverse relationship between cold night hours accumulated during the growing season and accumulated CHU from planting to maturity but no relationship between cold night hours and accumulated GTI. The GTI was not affected by cold night temperatures since the index calculation continues to accumulate thermal units at temperatures near 0°C. The relationship observed suggests that the current climate change trend for increasingly warmer overnight temperatures will also increase the number of CHU accumulated by corn hybrids from planting to maturity. The GTI showed an indication for accurate use as an alternative tool to CHU for selecting grain corn hybrids on the Canadian Prairies.