With a late start for some and corn fields not yet planted throughout the region, growers are asking, “Do I need to be switching to earlier maturing corn varieties?” and “Should I replant problem fields?”
It seems like each year I need to address these questions. We learned in the past that corn requires fewer Growing Degree Units (GDUs) to mature when planted after May 1 than listed in product brochures. From my own research trials and University trial data, we can expect corn planted in our region to require on average around 6.8 fewer GDUs to mature for every day of delay beyond May 1. Another way to interpret this is that it would require approximately 200 fewer GDUs from May 1 planting versus June 1 planting to reach black layer.
For example, a variety listed at needing 2,400 GDUs to reach black layer would only need 2200 GDUs to reach black layer when planted the end of May versus planted May 1. This happens based on how growing degree days are calculated and how corn develops at temperatures over 55 degrees.
It is agreed corn does nothing for development at temperatures below 55 degrees and above 86 degrees. However, when calculating GDUs using the current formula, one minute at 56 degrees counts the same as 12 hours at 56 degrees for a given day. The corn plant “disagrees with that thinking” and in fact 12 hours at 56 degrees contributes to development much more than one minute at 56 degrees. Typically, temperatures at the start of June are above 55 degrees more minutes a day than in April or the first of May, which validates the research above.
My advice to growers is to have patience with your variety selections until later in May. Planting earlier hybrids south of their adapted maturity zones could increase their inability to tolerate temperatures, as well as leaf or stalk diseases associated with southern zones.
Planting date is but one of many yield influencing factors:
- Historically, we know that early planting favors, but does not guarantee, higher yields.
- Do not assume that this year’s ideal planting date has already passed.
- 90% of corn yield and maturity date is typically determined by the environment (temperature and rainfall) after June 1.
- Late May or June planted corn typically emerges twice as fast as April planting due to warmer soils and longer hours of sunlight.
- Risk of replant is dramatically reduced for May plantings compared to April planting.
Replant decisions are also a likely hot topic for growers at the moment. With warmer temperatures, we should be to able to evaluate stand establishment. Remember to base replant decisions on expected yield and dollar returns, not on emotions. History tells us to finish planting other fields not yet planted prior to replanting.
Bob Nielsen, Agronomist Purdue University, writes replant guidelines to consider first:
- Productive plant population: Determine the productive plant population in several areas of the field if left as is.
- Stand uniformity: If the productive plant population is not uniformly distributed within the row, additional loss will likely occur.
- Likely replanting date and target plant population. This will help calculating the yield potential of the replanted field.
- Can the original yield goals be accomplished by spotting in thin areas or would it be better to replant the entire field?
- Likely replanting costs.
- Expected normal yields. Estimate the yield potential of the damaged field and replanting yield potential. Suggest using a five-year average.
- Expected market price for corn. The dollar gains or loss by replanting obviously depends greatly on what you expect to receive for the grain this fall.
It is important to recognize that there is no guarantee of success for later-planted replanting situations.