Selecting Corn Hybrids

Corn is physiologically mature when kernels reach maximum dry weight and are safe from frost. All milky fluid disappears from the kernel, and the black layer, a dark layer of cells, forms near the tip of mature kernels. Most hybrids adapted to Wisconsin reach this stage when kernel moisture decreases to 30–32%, but it may not occur until moisture content decreases to 28% for some early hybrids. Harvest maturity depends on the intended use of the corn and on the harvest and storage methods of each farm. Corn for silage or high-moisture grain is ready to harvest about 10 days before physiological maturity. Corn that is field-shelled and dried or picked and cribbed does not reach harvest maturity until kernel moisture content reaches 23–28%. Companies that produce hybrid seed use different systems to classify maturity, which makes it difficult to compare hybrids from different companies and to choose hybrids appropriate for specific farms. The two most common classification systems are Relative Maturity and Growing Degree Days. The Relative Maturity (RM) System ranks hybrids in relative order of maturity using grain moisture percentage at harvest to evaluate the hybrids against check hybrids. This system does not give the absolute days to maturity. A 95-day RM hybrid planted May 5 in central Wisconsin may take 130 days to reach physiological maturity in a warm year and 150 days in a cool year. Figure 1 is a general guide to maximum RM for corn planted early in Wisconsin.