Corn hybrids come with two standard ratings: CHU (corn heat units) and RM (relative maturity). Both will help you choose a variety best suited to your area but exactly what information are these ratings giving you? CHU is a measure of the cumulative heat that a geographic area typically experiences during a growing season. Calculations are made daily, based on daytime highs and nighttime lows, and CHU maps show you an average year. Grain corn hybrids are given CHUs ratings based on the temperatures they need to reach physiological maturity. So the idea is that you look up the CHU of your region and choose a corn hybrid that has a CHU value that matches and you can grow that corn on your farm.
RM is a different beast altogether. Essentially, RM is a “days to maturity” rating that lets you know how many days it should take a specific hybrid to reach physiological maturity relative to a group of similar hybrids. It’s good to keep in mind that CHU and RM ratings are not standardized and each seed company assigns ratings differently. That’s why corn hybrids from different seed companies that carry the same CHU rating might not all mature at the same time in the same region. As well, days to maturity ratings are not written in stone – earlier and later planting can impact how many actual days it takes for a crop to mature to harvestable condition.