Why does Corn Yield more than Soybean?
DR. DENNIS B. EGLI
LEXINGTON, KENTUCKY
Did you ever wonder why corn yields so much more than soybean? The two crops grow in the same environment in our production systems, often in adjacent fields. Photosynthesis is the main driver of yield production in both crops and yield is the weight of seeds harvested at maturity, but the yield of corn is, on the average, about three times higher than soybean. What are the characteristics of corn that allow it to produce so much more yield?
There are three primary reasons for the yield differential. First, photosynthesis of a corn community is higher than soybean. Soybean has C3-type photosynthesis (first stable product is a three-carbon compound) which includes a process called photorespiration. Photorespiration results in the loss of CO2 from the leaf in the light thereby reducing photosynthesis and plant growth. Photorespiration is inhibited by the CO2 in the air, so the rate of photorespiration decreases (photosynthesis increases) as the CO2 concentration in the air increases. The CO2 we are adding to the atmosphere that is causing climate change is also doing a little good by increasing soybean photosynthesis.
Corn has C4-type photosynthesis (the first stable product is a four-carbon compound) that does not exhibit photorespiration, so photosynthesis in corn is higher than it is in soybean. Higher photosynthesis contributes to higher yield. Since corn does not have photorespiration, it does not respond to increases in the CO2 concentration in the air. The more upright corn leaves also contribute to higher community photosynthesis by providing a more uniform distribution of sunlight over all the leaves. The upright leaves give corn an additional advantage, photosynthesis wise, over soybean which has more horizontal leaves that intercept most of the sunlight near the top of the canopy.
Seed composition is the second reason that corn has higher yield than soybean. The synthesis of the oil, protein, and carbohydrates (starch) in the seed requires energy and this energy comes from respiration of the simple sugars produced by photosynthesis. Biochemists have calculated the energy cost (expressed as the amount of glucose that must be respired) of each of the seed components. They found that oil requires the most energy per unit weight, protein is next, and carbohydrates are the lowest. The soybean seed with its high protein and oil concentrations requires 2.64 grams glucose per gram of seed while corn (low in oil and protein and high in starch) only requires 1.42 grams glucose per gram of seed. Consequently, corn will produce more seed weight from a given quantity of simple sugars from photosynthesis than soybean. Corn would have a higher yield because of its lower oil and protein concentrations in the seed even if the photosynthesis of the two crops was equal. These energy relationships explain why it is so difficult to develop soybean varieties with higher oil and protein seeds without losing yield.
A slightly longer seed-filling period in corn over soybean could be the third and final reason for the higher corn yields. Yield is produced during the seed-filling period – the time from when the seeds first start accumulating dry weight until they reach their maximum weight at physiological maturity. The length of the seed-filling period is under genetic control, and it is related to yield. There is some evidence that corn may have a longer seed filling period than soybean, but this evidence comes from separate experiments. No one has compared modern high-yielding corn hybrids and soybean varieties in the same environment to conclusively determine if corn does have a longer seed-filling period.
The roughly three-fold higher yield of corn is a direct result of higher photosynthesis, lower oil and protein concentrations in the seed (more seed weight per unit photosynthesis) and possibly a longer seed filing period. Soybean is penalized by its lower rate of photosynthesis and its high oil and protein seeds. These traits are fundamental characteristics of the two species that cannot be changed by management. ∆
DR. DENNIS B. EGLI: Department of Plant and Soil Science, University of Kentucky