Are We Interpreting Corn Hybrid Maturity Ratings Correctly?

By Benjamin Agyei and Maninder Singh

The word “days” is often loosely added to corn hybrid relative maturity (RM) ratings, which can lead to the wrong interpretation as calendar days. Seed companies assign RM 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 our understanding of corn RM ratings. Additionally, data on the actual duration for corn hybrids to achieve maturity from the time of planting can guide growers to optimize hybrid RM selection and effectively plan harvest timing.

What are the definitions of maturity?

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. This stage is commonly referred to as physiological maturity with grain moisture content typically between 25 to 35% dependent on weather conditions. Maturity can also be defined as the stage where corn can be harvested for grain. This definition is often termed as harvest maturity with moisture typically 25% or lower.

Seed companies use the harvest maturity definition to assign RM ratings to a new hybrid. This process involves comparing harvest moisture content of the new hybrid to the standard hybrid assuming that kernels lose around 0.5% moisture per day after physiological maturity. For example, if the standard hybrid has a 100 RM rating and the new hybrid had 2% greater moisture content at harvest, this hybrid will be assigned a rating of 104 RM, which is four units higher (2 ÷ 0.5 = 4) than the standard hybrid.

Actual number of days to achieve physiological and harvest maturity

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 RM of 89, 94, 99, 104 and 109) and planted in early and late May. The trials were conducted from 2021-2023 in Lansing, Michigan.