Interpret corn hybrid maturity ratings correctly

The word “days” is often loosely added to corn hybrid relative maturity ratings, which can lead to the wrong interpretation as calendar days. Seed companies assign relative maturity ratings based on grain moisture recorded at harvest maturity for a new hybrid in comparison to that of a standard hybrid. Clearly defining the term maturity is important to improve understanding of corn relative maturity ratings. Additionally, data on the actual duration for corn hybrids to achieve maturity from the time of planting can guide growers to optimize hybrid relative maturity selection and effectively plan harvest timing.

Corn maturity can be defined as the stage in corn development at which maximum kernel weight has been achieved and a visible black layer has formed. That stage is commonly referred to as physiological maturity with grain moisture content typically between 25 percent to 35 percent dependent on weather conditions. Maturity can also be defined as the stage where corn can be harvested for grain. That definition is often termed as harvest maturity with moisture typically 25 percent or less.

Seed companies use the harvest maturity definition to assign relative maturity ratings to a new hybrid. That process involves comparing harvest moisture content of the new hybrid to the standard hybrid assuming that kernels lose around 0.5 percent moisture per day after physiological maturity. If the standard hybrid has a 100 relative maturity rating and the new hybrid had two percent greater moisture content at harvest, that hybrid will be assigned a rating of 104 relative maturity, which is four units greater than the standard hybrid.

Field trials were conducted by the Michigan State University Cropping Systems Agronomy lab to estimate the actual number of days it takes for a corn hybrid to reach physiological and harvest maturity. The trials were conducted for five corn hybrids -- with relative maturitities of 89, 94, 99, 104 and 109 -- and planted in early and late May. The trials were conducted from 2021 to 2023 in Lansing, Michigan.