The Race to Soybean Maturity
Raise your hand if you planted some soybeans later than you would have liked this year. That it seemingly the reality for northeast Kansas and southeast Nebraska, and really, large parts of the Midwest. When we first approach these situations we are bombarded with a lot of information on changing or keeping the same maturity, plant population adjustments and more. But we don’t often talk about how this will impact harvest, other than trying to avoid a frost. So how does this late planting affect plant maturity and our harvest timeline?
How Soybean Plants Mature
Soybean plants are what we refer to as a short day plant. This can be a little bit misleading, as it’s the length of nighttime that impacts soybeans. While many plants will respond to day length differences, this is an important factor in initiating reproductive stages for soybeans. Essentially, when the length of night is longer than a critical point for soybeans, flowering will start. We usually think of those longer nights happening after the summer solstice—but remember, if soybeans are planted early enough, plants can also experience those longer night/shorter day combos before then and initiate flowering. The magnitude of this night length change will differ based on your latitude. Higher latitudes will have shorter nights during the summer than lower latitudes. So how do soybeans know to flower? Well, these varieties have been bred for these locations and usually have a reduced sensitivity to variation in photoperiod (night length). This is why we have different maturities adapted to different latitudes.
How long does it take to get from vegetative to reproductive stages in soybeans? That has a host of factors. The threshold for each variety is a little different, and just simply the fact its dependent on changing light conditions means we can’t very accurately estimate this timeline. However, after a soybean plant starts to flower, photoperiod is not the main driver of its development; temperature is. At this point, we can estimate the time until maturity.
How Late Planting Impacts Maturity
Various things happen in the plant with late planting, and these are generally the ones we focus on most.
1. The duration of time for late planted soybeans to reach R1 is less.
2. The duration of time between R1 and R6 in late planted soybeans is reduced.
3. The number of nodes per plant is reduced.
So, what are the impacts of these three points. Let’s start with number 1. A study by Pioneer showed that 3.5-4.5 maturity soybeans planted the 15th of April will have 53 days from planting to R1. During this time the plant grows vegetatively, increasing canopy cover and adding nodes. A similar maturity soybean planted July 7th will only have 36 days to maturity. How much does this matter? The amount of time a soybean plant spends in the vegetative stage is surprisingly significant. When given more time in these early stages plants are able to grow larger, canopy, and add additional nodes all before reproduction. Simply by having this larger leaf surface area available by reproductive stages, and consequently by the time of year with max solar radiation, we are able to achieve higher pod set and pod fill. Imagine looking straight down on two different rows of soybeans on a bright sunny day. One set of rows was planted early and just reaching canopy. How much light is intercepted by leaves? How much is reaching the ground? Now look over at the late planted beans. They are less than 30% canopied. And look how much light is reaching the ground. Light hitting the ground=lost light energy.
On to number 2. With smaller plants during the reproductive stages from R1 to R6, less surface area is present and available to do what needs to be done for pod fill: collect as much sunlight as possible. This sunlight is desperately needed as energy for filling and retaining soybean pods. In the same Pioneer study, 51 days were available for podfill with the April planted soybeans. Only 26 were available for the July planting date. Half the number of days to complete the same task.
Finally, number of nodes. The longer plants can spend in the vegetative stages before beginning flowering, the more nodes available on the plant. And what are each of these nodes? Nodes are the sight where soybeans will produce flowers, pods, seeds, and YIELD. The more nodes=the more possible seeds. The plant can compensate for less nodes but producing larger seeds. And it does happen. But it is far easier for the plant to put on yield by having additional nodes. This is really one of the main benefits of early planting. “Earlier soybean planting can increase crop yield potential by allowing plants to generate more stem nodes earlier.” (UNL)
Planting Date x Maturity
A lot of research has been done with planting different maturity groups at different planting dates. And I am not going to go into the full details. Those are largely pertinent when making planting decisions, not harvest decisions. I will however say this: with late planting, earlier maturity groups don’t show as big of yield decline as later maturity groups. (Keep in mind when I say late and early, this is based on a study out of Wisconsin, so late maturity groups are 2.5’s and early maturity groups are 0.5’s.) This is largely because the later maturity groups have much higher yield potential to begin with, so as we get farther away from optimum planting, the decrease in yield is much more significant.
Largely, with high yielding varieties selected from the appropriate soybean maturity range for our location, you aren’t going to see a major difference in in yield by maturity. Now if we bring some group 7’s and group 0.5’s in to NE Kansas we might start seeing some differences. (Interestingly, a study from Wisconsin did look at beans ranging from 0.2-7.5. The group 7 beans didn’t start reproductive stages until the beginning of September!)
Summary: If you are planting in our typical maturity range, it shouldn’t significantly impact yield, maturity date, etc.
Looking towards harvest
What should we be on the lookout for as we move towards harvest? Let’s look at a quick scenario. Using the figures from the Pioneer study, lets say you got some beans in “early” on May 4th. In their study, those beans started flowering on June 24th. This is right around the summer solstice and with plenty of time for vegetative growth to reach canopy by then. Excellent timing for maximum solar interception. Next, lets say your late planted beans were planted on June 11th. These beans will be reaching flowering around July 20th. Total daylength has already begun to decrease and less sunlight is available. When will both of these plantings reach maturity? (In this case we are looking at R6 or max pod fill). For the early planted soybeans we are looking at August 22nd. For the later planted beans, this date would be around August 20th.
WAIT. Can that be right?
Yes, it is. And it all goes back to those same issues with photoperiod affect as well as temperature dependency. In the end, it all evens out. And that is what makes planting early so significant.
Of course, all this doesn’t seem to matter much with all the rain we are getting and that is in the forecast. By the time we get back to the field, I imagine we will have a selection of both late and early planted beans to choose from.