Have recent below average temperatures adversely affected corn growth and yield potential? No, corn actually yields best with moderate temperatures (and adequate soil moisture). Temperatures that occur in Ohio in July and August (especially at night) are often warmer than optimum for corn. The ideal daytime temperatures for corn are about 80 to 86 degrees F (and higher if moisture is plentiful at all times). Although some believe that corn grows best when nights are hot, past research shows that warm temperatures adversely affect yield potential. While temperatures in the 40’s may impair photosynthesis, high night temperatures (in the 70s or 80s) result in wasteful respiration and a lower amount of dry matter accumulation in plants. With high night temperatures, more of the sugars produced by photosynthesis during the day are lost; less is available to fill developing kernels, thereby lowering potential grain yield. Research conducted at the University of the Illinois indicated that corn grown at night temperatures in the mid - 60s outyielded corn grown at temperatures in the mid- 80s. High night time temperatures result in faster heat unit (GDD) accumulation that can lead to earlier corn maturation, whereas cool night temperatures result in slower GDD accumulation that can lengthen grain filling and promote greater dry matter accumulation and grain yields. Cool temperatures may also slow the development of foliar diseases and insect problems.

Average corn yields are generally much higher with irrigation in western states, which have low humidity and limited rainfall. While these areas are characterized by hot sunny days, night temperatures are often cooler than in the Eastern Corn Belt.  Low night temperatures during grain fill are associated with some of our highest corn yields in Ohio – 143, 158, and 174 bu/A in 1992, 2004, and 2009, respectively.

So what’s the “downside” to these lower than average temperatures? Cooler temperatures (if they continue) could delay grain harvest and result in higher grain moisture.  Growers may want to consider this possibility when they estimate fuel costs for drying grain. Moreover, the cool, wet growing season of 2009 was associated with ear rots and mycotoxins. The corn harvest in 2009 was delayed by frequent rains which allowed molds to grow resulting in major mycotoxin problems for many farmers.