By Andi Anderson
Corn hybrid relative maturity ratings are commonly misunderstood by growers. The term “days” is often used with these ratings, which can lead farmers to think they represent calendar days from planting to harvest. In reality, relative maturity, or RM, is not a direct measure of time.
Understanding how these ratings work can help growers make better hybrid choices and improve harvest planning.
Corn maturity can be defined in two main ways. The first is physiological maturity, which occurs when kernels reach their maximum weight and a black layer forms at the base of the kernel.
At this stage, grain moisture is usually between 25 and 35 percent, depending on weather conditions. The second definition is harvest maturity, which refers to when corn is dry enough to be harvested, typically at 25 percent moisture or lower.
Seed companies assign RM ratings based on harvest maturity, not physiological maturity. New hybrids are compared to a standard hybrid at harvest. If a hybrid has higher grain moisture at harvest, it is assigned a higher RM number.
This system assumes grain dries down at an average rate of about 0.5 percent moisture per day after physiological maturity.
Research trials conducted over several years show that corn hybrids take more actual days to reach maturity than their RM numbers suggest. For example, a hybrid rated near 100 RM may require around 130 calendar days to reach physiological maturity and about 150 days to reach harvest moisture when planted in early May. These findings show that RM ratings should not be treated as exact time estimates.
The research also found that planting date matters. Hybrids planted later in the spring reached maturity faster than those planted earlier.
Delayed planting shortened the time to maturity by several days across different RM groups. This is important for growers adjusting planting decisions due to weather or field conditions.
Harvest timing can also vary depending on weather, dry down rates, and final moisture targets.
In some cases, corn may dry faster than expected, while in others it may take longer. Because of this variability, RM ratings should be used as general guides rather than exact schedules.
Experts suggest that growers also consider growing degree days when selecting hybrids. Growing degree days better reflect temperature and growing conditions that influence crop development.
By understanding how RM ratings work and using additional tools, farmers can improve hybrid selection, manage risk, and better plan for harvest each season.
Photo Credit: gettyimages-awakr10
Categories: Michigan, Crops, Corn