Great Bend Tribune
Published June 7, 2015
Corn and Soybean Production - Heat and Light Part 1
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.
Part 2
Published June 14, 2015
The first part of this series described how the growth of crops like corn is driven by heat accumulation. For corn the accumulation of heat determines the growth stage of the plant. Naturally light, water, and nutrients are important but heat accumulation determines the rate of growth. For crops such as soybeans heat is certainly important and the proper temperature range enhances growth but day length, or more precisely night length determine the onset of flowering and therefore bean production. So how is soybean production in Kansas determined by light?
- If you were a summer annual row-crop like soybeans, your primary function is to make sure the species continues. You do this by producing seed before a hard fall freeze comes along and you die. How does the plant sense when it is necessary to start flowering in time so it can produce viable seed for next year’s soybean plants?
- First, air temperature won’t work since a soybean plants flower for an extended period of time, determined by the maturity group, and need approximately two months from the beginning of flowering until seed is maturing. Air temperatures can be summerlike well into September and even October. And soybean plants can essentially shut down when exposed to cool temperatures not that close to freezing.
- Light is the key to initiate flowering or more precisely dark. Day length increases until the summer solstice and then starts to gradually decrease until the winter solstice when day length increases again. Soybean plants have evolved taking this decreasing day length, increasing night length, into account.
- Soybeans are termed short-day plants. They start to flower in response to a critical night length. This critical night length varies by the maturity group and even within it. Simply put, once the night increases to a certain length, it keys the plant to begin flowering and reproduction. This is why when producers double-crop soybeans after wheat they seem to begin flowering almost immediately while those planted in early May take almost two months. How does this work?
- Soybean plants contain a compound termed phytochrome. This compound is found in two forms red and far red. In the presence of light the red form very rapidly turns into the far red form. The far red form turns back very slowly to the red form. So when night reaches a genetically determined length, floral initiation occurs based on the ratio in the plant of far red to red.
- Interestingly even with long enough nights, if you expose the soybean plant to a brief period of light the red turns far red rapidly enough it will prevent flowering. If you did this every night, the plant would never flower.
Finally, wheat is just the opposite. To protect itself from flowering when it is too cold, in addition to vernalization, wheat is a long-day plant and requires an increasing night length in addition to proper temperatures to flower.