Corn maturity status

Corn tasseling and silking have begun in many fields across the state. Early season delays and slow growth during the first half of the growing season have many farmers wondering if their crop will mature on time. One can make a rough estimate of crop progress by comparing 1996 growing degree days (GDDs) from planting to present to GDDs for normal planting dates in an average year. An even rougher estimate can be made by counting the leaves developed or by measuring plant height. However, the first solid indicator of crop progress is the average silking date. Once silking dates are known, we will have a better feel for the maturity prospects for the 1996 crop. Because the number of days from silking to physiological maturity is fairly constant, one can estimate maturity by adding 60 (±5) days to the silking date. The obvious value of a silking date plus 60-day estimate is to compare the expected maturity date with the average frost date.

Begin looking for silks soon after the tassels appear. Under normal conditions, 20 to 25 percent of the plants in a field will silk each day. When approximately 75 percent of the plants show silks, record that date as the silking date. In a typical year, the average silking date for Iowa falls between July 20 - 25. For much of the state, silking dates in 1996 will be somewhat later that average. Moving silking dates earlier is a major advantage of earlier planted corn. Planting-date studies have supplied considerable evidence that corn silking before July 25 yields more and has a lower moisture content than corn silking after July 25. Unfortunately, late silking is associated with lower yields and wet grain even if a freeze does not come early. Trading a couple of weeks of grain fill during late July and early August for a couple of weeks in late September or early October is not a fair trade as far as the corn plant is concerned. The problems are magnified when a killing frost comes early. July weather has been favorable for crop maturity progress, but the crop still is somewhat behind schedule.

This article originally appeared on page 142 of the IC-476(20) -- July 29, 1996 issue.

Updated 07/28/1996 - 1:00pm