In most years, there is little advantage to planting corn before soil temperatures consistently rise to 50 degrees or higher.  Under certain conditions, planting too early can even have a yield limiting effect.  Corn emergence and early season development can be projected using the following information.

• Corn typically emerges when 100 to 120 GDDs accumulate after planting.  Dr. Bob Nielsen, Purdue University Agronomist, noted that this is best calculated using soil temperatures, since this phase of growth is under ground.  He also noted that soil temperatures continue to provide a better estimate of corn growth until about V6, when the growing point is lifted above the soil line.
  
• Prior to the V10 growth stage, corn adds one new leaf collar about every 82 GDDs, and after the V10 stage, corn will add one new leaf collar about every 50 GDDs.
• Growing Degree Days (GDDs) can be estimated by taking the daily low temperature (Do not enter a value lower than ’50.’), plus the daily high temperature (Do not enter a value higher than ’86.’), dividing by 2, then subtracting 50.  Dr. Nielsen pointed to a website that can make the GDD calculation simple - https://mygeohub.org/groups/u2u/gdd.  This site provides real-time GDD estimates, plus future GDD projections, down to a county level.
• Using the observed corn growth stage in the field, along with estimated or calculated GDD values, one can work backward to confirm key corn growth events, or can work forward to predict when key growth events are likely to take place.

Remember that these are only estimates, and corn hybrids vary somewhat.  Stress on the growing corn plant can slow normal development and extremely favorable growing conditions can result in slightly faster development.  Also remember that late-planted corn has a tendency to make up for lost time and leaf collar emergence may occur at a faster rate.

The image shows the early season difference in corn growth rate between no-till (left) and conventional tillage (right).