Most growers realize the importance of planting corn hybrids of proper maturity. Corn maturity is measured by the number of days from germination to black layer. Though this measurement is quite accurate, it’s greatly influenced by location, planting date, planting depth, soil type, temperature and moisture availability.

A more reliable way to gauge relative maturity is by the number of leaves a corn plant develops on the main stalk. For instance, in the central Corn Belt for hybrids planted in May, a hybrid averaging 16 leaves will shed pollen six to seven days earlier than a hybrid with 18 leaves.

The relationship between leaf number and maturity was recognized by European biologists, but hasn’t been widely used in the U.S. In 1932, the Russian scientist N. N. Kuleshov published data on the behavior of American corn varieties in Russia. He classified varieties into four maturity groups, using leaf number as an index of maturity. The Italian scientist V. Nozzolini reported very high positive correlations between leaf number and the length of the vegetative period.

In 1964, Sherret Chase and I conducted studies near Esmond, Ill., with 13 inbred lines and 13 hybrids. We counted all leaves on the main stalk of each plant, marking the fourth, eighth and 12th leaves as they appeared.

In 1965 we conducted replicated tests at three locations with 22 hybrids selected to represent the full range of maturities, from earliest to latest, that were grown in the U.S. and Canada. The first planting was May 15 near Waterman, Ill. The second planting was made on Sept. 10 near Homestead, Fla., and the third test plot was planted on Nov. 22, also in Florida.

4 key findings
Here are the results and conclusions of those studies.

1. There is a strong correlation between leaf number and relative maturity. That’s true in both inbred lines and hybrids. For the two sets of plants studied, we obtained highly useful correlation values of 0.95 for inbred lines and 0.99 for hybrids. A correlation coefficient value of 1.00 would be a perfect correlation.

2. The time as well as heat units required to reach pollen shed are similar. This applied to the May planting in Illinois and the November planting in Florida. However, the September planting in Florida grew rapidly due to the favorable temperature regime after planting.

3. One leaf difference equals about four days’ difference to pollen shed. This is based on what we saw in the inbred lines. Actually, the difference was a little more than four days. In the set of hybrids we tested, however, one leaf difference represented an interval of somewhat less than four days’ difference.

4. Short day regimes reduce the number of leaves formed. The rate of growth is increased by a rise in effective temperature. Later-planted corn in the Corn Belt produces fewer leaves and tries to hurry up to reach black layer if the grain fill period is warmer. This phenomenon was documented in the recent past by Purdue University Extension corn specialist Bob Nielsen and Ohio State University Extension agronomist Peter Thomison.

Nanda is president of Agronomic Crops Consultants LLC. Email him at [email protected] or call 317-910-9876.