You planted the best-yielding variety. You scouted and battled against weeds, insects and diseases in a timely fashion. You managed soil fertility and practiced crop rotation. You even had plenty of rainfall through the growing season. Still, no single practice, problem or preventive step seems to push your soybean fields past that 50 bu. per acre yield barrier.
What gives? That's a question many farmers, soybean specialists and our own Farm Journal Field Agronomist, Ken Ferrie, continue to ask as they attempt to unlock the mystery behind soybean yields. "I'm convinced part of the answer is environmental, rather than simply agronomic,” Ferrie says.
"In corn, it's easy to find 50 bu. per acre yield swings from one area to the next, but with soybeans, average farm yields of 45 bu. to 55 bu. per acre seem to catch the whole country.
"We devote half our Farm Journal plots to soybeans and it's sometimes a struggle to get a 2 bu. to 3 bu. per acre yield bump,” he says.
Ferrie says most of the agronomic practices currently in place—such as using soybean cyst nematode–resistant varieties and spraying for aphids—are part of the process of saving yield, rather than adding to it. Other things, such as length of growing season, light interception, biomass accumulation and rising carbon dioxide levels in the upper canopy, are environmental factors that are also part of the equation.
From 1924 to 2008, U.S. soybean yields have increased at an average rate of 0.34 bu. per acre per year. New genetic combinations and innovations have inched soybean yields along, but the national average in 2009 is still expected to be in the 40 bu. per acre range.
USDA–Agricultural Research Service (ARS) soybean geneticist Randall Nelson observes that reports of 100 bu. per acre soybean yields first came from state yield contests in 1968. "Over the years, the number and intensity of diseases and insects have increased and new genes have been added to combat these problems, but the genetic base of soybean production in the U.S. has not changed substantially in 40 years,” he says.
There's still potential to increase yields by including new germplasm in our breeding populations, Nelson adds. He sees important genetic resources being uncovered from exotic soybean germplasm that could provide valuable genetic resources for future variety development.
Yield champ. Missouri farmer Kip Cullers isn't waiting. His soybean yield contest test plot near Purdy, Mo., hauled in 154 bu. per acre in 2007 and his noncontest plots consistently yield near or above the 100 bu. per acre mark. In 2009, he says, one 14-variety test plot of late Group III to late Group IV varieties yielded more than 100 bu. per acre without the benefit of any seed treatment and while being grown in 30" rows. He estimates that at least 20 bu. was lost due to inefficient plot harvesters. "The point is, the genetic potential is there,” he notes.
"The problem is, many growers use the same practices year after year and hope for the miracle of a perfect year to result in more yield,” Cullers says. "Perfect years don't exist.
"High-yield soybean production is a matter of constantly searching for new answers and trying new practices and inputs to help maximize the genetic potential of the variety,” Cullers says. "It is finding a high-yield formula that treats soybeans like the specialty crop that it is.”
Complex crop. University of Missouri soybean specialist Bill Wiebold says the path to increased soybean yields is a season-long journey that is complicated by the plant itself.
Although flowers are produced on every stem and branch node of a soybean plant, the majority of yield is produced in the upper portion of the middle one-third of the soybean canopy. "Approximately 70% of soybean yield is determined by the number of nodes on the stem,” Wiebold says.
This localization of yield is important to remember as you consider what is critical to the plant. "This is the portion of the plant where sunlight is absorbed,” Wiebold explains. "Light absorption drives yield. Leaves are like solar collectors that use solar energy to produce the sugars required for all the other processes needed to produce yield.
"Once reproductive growth begins, these sugars do not move very far within the plant,” he adds. "They are used in producing seeds and very closely associated with the leaf that captured the sun and made the sugars.”
Another unique characteristic of the soybean plant is that under normal circumstances, 65% or more of the soybean flowers produced are aborted and never result in a pod that contains seeds. This abscission increases as you go deeper into the soybean canopy. How many of the flowers are lost depends on many things—including stresses on the plant that increase competition among pods for sugars and other nutrients.
With that in mind, Wiebold offers the following suggestions to put you on the path to higher yields:
1Choose the right variety. Review data from many locations, including those that have low to average yield potential. Yield stability is important because you can't predict weather conditions.
"If you've made the decision to enter the 100 Bushel Club, you've made the commitment to provide a high-yield environment,” Wiebold says. "You need a variety that will take advantage of that environment. Disease resistance is important, but yield potential remains most important.”
2 Capture sunlight. Sunlight powers soybean yield. Capture more sunlight by planting early. Day length and the angle of the sun decrease after the first day of summer. Planting early moves the critical seed-filling period earlier into summer, when sun power is greatest. Cloudy days will decrease yields.
Narrow rows can increase light capture and quick canopy closure. Closing the rows earlier and rapid crop growth at flowering is important to keep photosynthesis going at a high rate and maximize pod and seed set.
Seeding rate and even stands are also related to light capture. Protecting leaves from foliar diseases and chewing insects that remove soybean leaf area is also important. Scout to properly time fungicides and insecticides.
3 Worry about water. You want short rain events with few clouds and rain events evenly spaced, with rainfall amounts related to crop needs. "Crop need is not necessarily the same as water use,” Wiebold notes. "Water use by most plants decreases during grain-filling, but that doesn't mean the need for water decreases.” If irrigation is used to increase pod number, you need to continue to supply water to completely fill that extra pod load.”
Some growers who have irrigation available like to mist plants to reduce leaf temperature. Heat builds up in the leaves as light is captured for photosynthesis. To reduce temperatures, leaves either increase water evaporation or turn away from the sun. In both cases, sugar production decreases, setting up a scenario for increased competition among pods.
4Get down and dirty. Treat the soil as if it is alive. Remember that nearly one-half of the soybean plant is growing below ground. An actively thriving community of microbes and other living organisms are needed for healthy roots.
Soils act as banks for water and nutrient storage for plants to draw upon as the season progresses. Compaction squeezes the life out of soil and has season-long (or longer) impact.
5 Pamper plants. Cullers' favorite saying is "Keep your plants happy.” A commitment to producing 100-bu. yields means the soybean plants will exploit their environment to a greater extent than in normal fields, Wiebold adds.
Cullers swears by the use of inoculants. He foliar feeds a variety of micronutrients and makes multiple fungicide treatments. While most trials do not show a yield advantage from the addition of nitrogen, Cullers includes it in his soybean program.
Wiebold notes that few nitrogen experiments have been conducted in high-yield environments. "As developing seeds demand more sugars, roots may not receive all they need to function at a high level. Foliar fertilization may be helpful, but be careful not to harm salt-sensitive tissues,” he says.
6 Try new things and keep track. Do your own experiments. Raising the yield bar is likely to take more than one season. Scout fields and record everything you see. If you don't like to write, buy a small video
recorder and make a visual history.
You can e-mail Pam Smith at email@example.com.
- February 2010