The May monsoon season has brought about a plethora of questions about corn replanting and late planting decisions.  Experience has taught us that the first rule of thumb in making these decisions is to be rational and realistic.  Perhaps some of the “old timers” can cite a year during the past 100 when they got dry grain and planted on July 1^st, but I would have plans B and C in place if it doesn’t occur this year. 

How many growing degree units are left?

        From June 1 through the first week in October we typically accumulate about 2150 Growing Degree Units (GDU’s).  Of course this number can vary.  In 1998, we accumulated 2471 GDU’s while last year we only accumulated 2033 GDU’s during this same time period.  Let’s put these numbers in perspective based on the relative maturity (RM) of a corn hybrid.  A 90 to 95 hybrid needs approximately 2100-2200 GDU’s to reach physiological maturity.  For each 5 day drop in RM, decrease the GDU’s by 100 (e.g. an 85-90 RM hybrid requires 2000-2100).  Initially, this may not sound so bad but we need to remember that grain moisture at black layer is still around 35 percent and dry down rates slow considerably as we push through October and November.   

The great migration

         In years like 2004, bags of corn seed move south faster than Canadian geese in November as shorter season RM hybrids are supplied to growers in different regions. For those wishing to harvest dry grain, planting anything greater than an 85 RM hybrid after June 1 is riskier than laying even odds on the Devil Rays to win the World Series this year.  After about June 10, planting with the intent of harvesting dry grain is highly discouraged.  At that point, planting either soybeans or possibly running with the crop insurance money is clearly the better decision.  Making late planting decisions is not easy.  It often comes down to an individual’s desire to accept risk.

        There is some good news along the lines of late planting and hybrid RM.  Researchers in some states have found that it takes fewer GDU’s to reach black layer when a hybrid is planted late.   For example, Purdue agronomists estimate that a typical corn hybrid's GDU requirements decrease about five GDU’s per day of delayed planting from late April - early May through at least the early part of June.  This means that a 30 day delay in planting may result in a hybrid maturing in 150 fewer GDU’s (30 days times 5 GDU’s per day).  

The dreaded yield penalty for late planting

        Harvesting grain and harvesting high grain yields are very different.  It’s been well documented that late planted corn will yield less than early planted corn.  How much less yield will depend factors such as GDU accumulation, hybrid planted, and fall frost date.  In Wisconsin, yield of corn planted in early June decreases at a rate of 3% per day delay.  Table 1 lists the anticipated yield loss for corn planted at different dates and at various stand densities.  This table can also be used to assess replant decisions by comparing a stand of known planting date and plant density with a replanted one at a later date and a likely higher plant density. 

What about corn for silage and high moisture grain?

        Planting corn for silage or high moisture grain offers at least some late planting risk reduction.  However, some of the same rules and principles apply.  On-going studies in Wisconsin have been conducted to assess the yield and quality penalty for late-planted corn harvested as silage.  In corn silage, a key feed quality factor is the ratio of grain to stover.  Less grain yield usually lowers silage quality.  Similar to grain, corn silage yields decrease with later planting dates.  Wisconsin studies show that corn silage yield from June 10 planting dates is about 30 percent lower than earlier planting dates in May and April.  By the end of June, yield levels are less than 50% of the maximum yields obtained in early May.  

        The grain:stover ratio decreases with later planting dates primarily due to lower grain yield.  Lower grain:stover ratio results in lower overall feed quality.  As with corn for grain, producers need to choose appropriate short season hybrids to increase the probability that the crop will reach harvest maturity before the first fall frost.