If you have been paying attention to farm news in our region, you heard concerns about getting the corn crop in the ground as it was getting late for planting. You seldom hear those concerns in Kansas regarding soybeans. Have you ever wondered why? Well it has to do with the difference in the importance of heat and light for these crops.
Likely everyone knows our crop plants need light (solar radiation) and heat in order to germinate seed, grow, and reproduce. Some like winter wheat need a period of cold in order to flower and produce seed. Plants have a range of temperatures they function best in and above and below that range, plants are inactive, damaged or even die. Plants need a minimum amount of light in order to grow and photosynthesis can increase as does light intensity with the amount depending on the species. The amount and intensity necessary vary depending on the species and in many cases the variety or hybrid. Why are Kansas producers so focused on length of growing season and adequate heat for corn and not nearly as much for soybean? Keep in mind this discussion assumes other factors such as water and nutrients aren’t limiting plant growth and development.  
• While a bit simplistic, the easiest explanation is that corn is heat driven and soybeans aren’t. You hear farmers speaking about how many “days” it takes for their corn crop. Here 100 day corn is short season and 118 days or so is full season. The reality is that it is more complicated and these days are shorthand. What matters is how much heat the corn plant accumulates and the accumulation of heat determines everything from when it emerges and how often new leaves emerge to when it will tassel, silk and the grain is physiologically mature.  
• Corn accumulates heat units every day and the accumulation uses a simple formula: [(Tmax + Tmin)/2] – 50o F. If Tmax is above 86o then 86 is used. If Tmin is below 50o then 50 is used. The formula assumes no growth occurs above 86 or below 50 degrees. For example, if the high was 80 and the low 60, the crop accumulated 20 heat units. The typical “full season” variety will silk when approximately 1550 units accumulate and be physiologically mature around 2800 units. For a short season variety the numbers are approximately 1400 and 2600 respectively. Emergence from the soil takes approximately 125 units and a new leaf emerges about every 85 units.  
• The fuller season the corn, normally the greater the yield potential but also the more heat that must accumulate. Farmers, to maximize yield potential, will normally select a fuller season hybrid which means they want to take advantage of the growing season and hope to have the crop flower and develop seed before the worst part of the summer heat and drought stress. Having corn develop kernels during excessive temperatures compresses the time for grain fill and hurts yields. That, and unlike soybeans, our corn is a determinant crop and only has one chance to flower.
Next week how soybeans differ from corn and what it means.