When will my corn be ready for silage and grain harvest?

Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension

        The past two years have offered little room for speculation about whether corn will be mature or not before the first killing frost.  Both years provided more than adequate Growing Degree Units (GDU's) coupled with relatively early planting dates.  Although many 2000 corn fields appear on schedule, our proximity to normal is much closer this year.  At this point in the growing season, the best method to estimate when corn might be ready for grain or silage harvest is to make a comparison of how many GDU's are needed for corn to move through reproductive maturity stages versus what we can expect given normal weather conditions through August and September. 

What's needed for corn to mature?

        Table 1 indicates the approximate number of growing degree units needed for 95 – 105 R.M. corn to progress through the various reproductive growth stages. Full dent corn (65 to 70% whole plant moisture) approximates the optimum silage harvest period.

        Two management factors that may influence the actual number of GDU’s needed for corn to mature are planting date, hybrid characteristics, and, to a lesser degree, hybrid maturity.  Late-planted corn will usually require about 60 to 80 fewer GDU’s than early planted corn progressing from silking to physiological maturity (black layer).  Today, there is more variation between hybrids in terms of husk and stalk characteristics.  Husks that open and dry fast usually result in kernels that lose moisture at a more rapid rate.  Past experience has also shown that precipitation may alter drydown and maturity progression.  Very dry soil conditions tend to lower whole plant moisture relative to the actual maturity stage.  For purposes of estimation, the numbers provided in Table 1 will suffice for example purposes.

How many GDU's can we expect?

        Table 2 lists the expected number of GDU's for Fond du Lac based on a 30-year average.

        Obviously there are some basic mathematical skills needed to determine various corn maturity scenarios combining the information in Tables 1 and 2.  The easiest example to figure is corn silking on August 5.  The 880 GDU's needed to reach full dent for silage harvest wouldn't occur until the end of September.  Perhaps some of the earliest planted corn or short maturity corn that is at or near the kernel blister stage by August 5 will be ready to chop by September 10.  This range of dates from September 10 to September 30 appears to be the time period when most of the corn will be harvested for silage in 2000 (given "average" conditions).

        Last year our first frost came on October 6, which is near the expected first frost date of October 4.  With essentially all the corn beyond physiological maturity and much of the corn silage already harvested some 40 to 50 days prior to the frost, there were no real ramifications.  Unless things really heat-up over the next two months, that would not be the case this year.  Again, with normal conditions, corn silking after the first of August probably won't reach black layer by our normal first frost date.  A September frost has the potential to cause some real problems this year.  In case you're wondering, the full moon in September and October falls on the 13^th day in each month.

        To summarize, this year's corn crop (at least most fields) are right on target for an average maturity progression but will likely be 1 ½  to 2 weeks behind the previous two years.  Monitor fields closely for changes in maturity and moisture.

For more information contact Mike Rankin