Determining the most appropriate crop use can sometimes mean the difference between profit and loss. The potential for a given field of corn to produce dry or wet grain will depend upon the present plant maturity, the number of heat units accumulated between now and the first frost, and the date and extent of the first frost.   When a frost does occur to immature corn, the concern will be with impact on final grain yield and "dry-down rate".  This discussion is offered now so that growers have the opportunity to plan for the worst and hope for the best.  It is important that we have realistic expectations for the crop based on maturity stage and normal temperatures. 

Grain Yield:

        The influence of frost injury on final grain yield depends on how much leaf tissue is damaged and the plant's stage of development when the frost occurs.  The closer the plant is to physiological maturity, the less effect there is on grain yield (see Table 1).  Even if a frost kills most of the leaf tissue on the plant, the translocation of sugars from stalks to ears will increase kernel dry weight (and grain yield) unless the freeze is severe enough to kill husks, stalks and kernels.  

        What is the minimum stage of development that a plant must reach to make grain harvest a worthwhile venture?  If complete plant death occurs (leaves, husks, stalks, and kernels frozen), plants should probably be well into dent stage before grain harvest is considered an option.  At this point, about 65-75% of the potential grain yield is made, however, test weights will be below normal.  Most likely, the first frost will only be a "nipper" and corn at this stage will obtain an even higher percent of maximum grain yield.  Prior to a "solid" dent stage, high yield reductions, field losses, low test weights, slow dry-down, and harvesting problems make the grain option highly questionable from an economic standpoint. It's not until corn becomes fully dented (1/2 to 3/4 milk) that we can feel confident the crop is "made" for grain.  It is also at this stage where 100% of the maximum whole plant yield is obtained. 

Moisture:

        Grain and whole plant moisture contents for various maturity stages are presented in Table 1.  Many corn growers perceive that kernels from early-frosted corn dry very slowly or not at all in the field.  Minnesota studies simulating frost injury at early dent stage (about 55% kernel moisture) indicated that immature corn killed by frost dried when environmental conditions favored kernel drying.  Moisture loss was temporarily delayed immediately following frost that resulted in an extra 4 to 9 days needed to reach an acceptable grain harvest moisture range.  Where husks were loosened (simulating a mild frost), dry-down time was significantly reduced.  Many research trials have demonstrated that early frost has little impact on whole plant dry-down time. 

        Don't expect significant dry down of grain past November 1st.  Grain moistures taken from standing corn in Wisconsin studies have indicated a 10% unit drop in October but only a 2% unit drop during the entire month of November.  Dry down rates during this time period were surprisingly similar for grain that ranged from 37% to 59% moisture on September 28. 

The Current Situation:

        It’s really become a race to the finish line at this point.  Much of the early planted corn has a good chance to make grain, even with the low number of heat units accumulated to date.  As for the late corn.....think silage. Calendar day, maturity, and moisture relationships are presented in Table 1.  These numbers provide only estimates and will be driven by a number of factors.  For example, the plant’s “maturity clock” will sense it’s getting late in the season and will push development a bit faster than when maturing under normal conditions.  Obviously, our heat unit accumulations will help drive (or not drive) maturity.  Note that from early dent to black layer an average of 475 GDDs are needed.  The table indicates this equates to 20 - 25 days (an average of 20 GDDs per day).  However, with an average high temperature of 70 degrees F and low temperature of 50 degrees F (not unreasonable for the month of September), only 10 GDDs per day accumulate.  In reality, corn will probably take somewhere between the 25 to 50 days to mature (more than what is indicated in the chart but less than what would figure simply by using GDD estimates).   

        Hybrid maturity differences in development time occur primarily from emergence to silking, not from silking to maturity.  However, fewer GDDs are needed for later plantings of the same maturity rated hybrid compared to early plantings.  This will be to our benefit this year as many fields will fall into that "late planted" category.