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Many soybeans got off to a seemingly slow start this past season. I remember soybeans in some no-till fields seeming to take forever to get taller than last year’s corn residue. Cool temperatures in May and unusually dry conditions that persisted until late June in many areas slowed vegetative growth. Once the growing environment went from cool and dry to warm and wet, soybeans made up ground in a BIG way. By mid-August it was obvious that soybeans across much of the state were excessively tall, or at least taller than normal. Walking plots in late August and early September, it was not uncommon to find soybean plants that were 60” or taller when you pulled them upright. Usually when I hear people talking about “5-foot tall” soybeans it’s usually just an exaggeration for effect, but this year it was an accurate statement.

80:40 Rule: A colleague of mine in southern Illinois has an 80:40 rule I am fond of repeating that applies to “big” soybeans … “80-bushel fodder and 40-bushel beans.” This saying reflects that big impressive looking plants don’t necessarily result in big impressive yields. In fact, more often it is the short, unimpressive looking fields that tend to produce the surprisingly good yields. Some of this is due only to the psychology of failing to meet vs. exceeding expectations. If the field you thought would make 80 makes 60 you are disappointed. If the field you feared would not make 50 makes 60 you are thrilled. The same yield produces totally opposite reactions because of our expectations.

Big Plants Doesn’t Mean Big Yield: So … why don’t bigger soybean plants tend to produce big yields and why this year did enormous soybean plants produce record yields? The purpose of the soybean plant is to convert sunlight and nutrients into yield. In theory, a bigger plant should capture more sunlight and take up more nutrients and thus produce more yield. Where this theory falls apart is that on an acre basis a dense soybean canopy is already capturing all the available sunlight. Making the plant taller or the canopy denser won’t capture any more sunlight. Unfortunately that bigger plant—that will take up more nutrients or remobilize more nutrients from its biomass to seed later in the season— also requires more nutrition and energy just to maintain itself, so there may not be enough resources available to devote to grain fill. Diverting resources from grain fill to leaf and stem biomass production will likely be detrimental to yield.

Lodging Liability: Another downside of excessive plant growth is lodging, which will reduce harvestable yield. Even varieties with excellent standability lodged in some cases. The relative differences between varieties with good, average and poor standability remained but almost everything lodged more than normal in 2016. Lodging can interfere with sunlight interception, air movement through the canopy and interrupt pod fill. Even if yield isn’t negatively impacted harvestability certainly is.

On my own farm, I noticed my worst lodged field was yielding 10bu/acre more when traveling north vs. south … so we cut most of that field one way. Without the glaring, visual evidence on the yield monitor it would have been easy to overlook the plants I was missing and lose 5 bu/acre off my farm average.

Restricting Height: Many soybean yield contest winners have their favorite methods to limit excessive plant height and encourage more branching. Burning or stunting soybeans with foliar applications of fertilizer or herbicides like Cobra® or using rollers to smash plants are examples. These practices are designed to damage the apical meristem, thus diminishing apical dominance and allowing more lateral branches to form. This results in a plant that tends to be more compact and branched. Mowing or clipping the terminal bud in early vegetative growth stages has also been tried. A well-timed hail storm can produce this effect as well.

Typically, taller plants have a similar number of main stem nodes as shorter plants. The nodes of taller plants just tend to be spaced further apart. Looking at plants this August I noticed that, considering how excessively tall the plants were, the nodes were spaced close together and there were a LOT of main stem nodes. This was the first indication I had that this year might produce 80-bushel fodder and would also make 80-bushel beans. I am not sure exactly what mysterious thing happened this year that enabled soybeans to set as many main stem nodes as they did. But I believe that was the secret to the high yields we achieved.

No two years are the same and some combination of environmental factors in 2016 enabled soybeans to achieve excessive vegetative growth and set the potential for big yields. Generally good late season growing conditions and good plant health due to low disease pressure (in most fields) enabled soybean plants to live up to their high potential.

Soybean yields in Illinois have been tremendous the past three years. A record state average yield was set in 2014, equaled in 2015 and surpassed in 2016. This trend can’t continue indefinitely year after year but in these high yield years we are learning and identifying practices that do increase yields … information that will help us in good years and bad.

Lance Tarochione is a technical agronomist with Asgrow/DEKALB in west central Illinois. His work has focused on crop production, research and product development, and through his role at Monsanto^® he currently supports the Asgrow^® and DEKALB^® brands in seven counties in western Illinois.