Soybean harvest has pretty much wrapped up across most of the territory and growers are nosing into corn as the weather allows. Two key observations and questions have repeatedly popped up so I looked into finding an explanation for you.
1) Soybeans with green stems but mature pods.
2) Delayed black layer in corn and high harvest moistures.
I'll tackle the soybean question first.
Why are there so many green stems in the soybean crop?
Green stem syndrome is a disorder in soybean that results in mature pods and seeds hanging onto green stems. This phenomenon occurs when there are more leaves and stems producing and storing photosynthate than needed to fill the available seeds. In other words, the source-sink relationship is unbalanced and the plant has stored photosynthate with no where to go.
Typical plant on the left has mature pods and stem, while the plant on the right is showing symptoms of green stem syndrome. Photo courtesy Shaun Casteel, Purdue University.
A 2009 study at the University of Kentucky investigated the impact of depodding on green-stem syndrome in soybean. 25-50% of pods were removed from a soybean plant at the R6 growth stage, effectively mimicking reduced pod set due to early season stress and limiting ‘storage-space'. Pod maturation was not delayed when pods were removed, BUT stem maturation was delayed by 4 days to a month. The more pods that were removed, the more pronounced the green stem symptomology was (Egli and Bruening, 2006). The exact stresses and conditions that cause green stem syndrome are still being investigated.
Green stem syndrome makes a lot of sense considering the dry conditions through the vegetative growth and early flowering stages of the soybean crop followed by average rainfall in August.
We are seeing delayed black layer across large areas of the province and it was only last week that I started to see hybrids well into their maturity zones and with more then enough CHUs actually black layer. In addition to this, there's a lot of reports from growers that have been into their corn fields that moistures are unusually high for the relative maturity of the crop their harvesting and the time of year. So.. what on earth is going on?
Now, this question was tricky. I usually try to look up as many resources as I can, cross-reference or fact-check them to confirm the findings and then present them. With this one I was coming up empty- no one seemed to have a great answer for what is causing this delayed maturity. In academic distress I reached out to a few of the kind folks at U of G for some help and guidance (i.e. Dr. Liz Lee and Dr. Dave Hooker). The reply started with a sentence I'm not unused to hearing “Hello Lauren- I do not think anyone has a definitive answer…”
Fortunately, they did point me in a better direction and linked an article that may not have all the answers but offered some great insight.
I'm going to include some of the comments from Dr. Lee (along side a few of my own comments) and link the article that Dr. Dave sent along (I don't want to risk butchering it trying to put it in my own words and it feels futile to even try).
Without further ado-
First things first: what is black layer?
Black layer occurs when kernels reach physiological maturity. The placental cells in the kernel die and darken, creating a barrier that blocks movement of photosynthate into the kernel, essentially sealing off the kernel and signaling physiological maturity of the crop.
A corn kernel at physiological maturity. You can see the thin black layer at the base of the kernel. Photo courtesy Bob Neilson, Purdue University.
What causes black layer?
Black layer occurs when movement of photosynthate into the kernel falls below a certain threshold- either due to the plant reaching maturity under favourable growing conditions or crop stresses (such as frost) which limit the supply of photosynthate. The time between full dent (R5) and black layer (R6) can vary widely depending on environmental conditions and hybrid charactersitics. A 2300 CHU corn product can take between 18-30 days for the milk line to drop from full dent (~55% moisture) to black layer (~30% moisture) (Bayer CropScience Internal).
Progression of black layer formation.
Photo courtesy: Iowa State University Extension.
So, why is black layer delayed this year?
Annotated thoughts from Dr. Liz Lee.
Modern corn hybrids are well equipped to achieve complete or near-complete kernel set even under drought stressed conditions- think of how little tip back we are seeing in the corn crop this year compared to the dry conditions through the summer. This creates a very strong sink for the plants to fill relative to the amount of biomass (oddly, opposite to the soybean crop).
Under favourable conditions the plant would continue active photosynthesis through out grain fill and we would see a fair amount of stay green in the crop- this year we are seeing significant amounts of top dieback (Hey! I wrote about that! HERE). Remobilizing nutrients causes senescence and that tissue is no longer able to actively photosynthesize- further limiting a crop that had limited biomass to begin with.
The midwest is experiencing the same delay in maturity but with a different explaination.
Emerson Nafziger at the University of Illinois wrote an article on the same topic. He mentions cooler than normal temperatures through September and October and dry weather through out the summer as possible contributing factors. He doesn't mention any signs of top dieback in the midwest crop and it's important to note that their crop was planted later than normal (compared to the Ontario crop that was largely planted on time). It's a very interesting article but one that's important to read with the understanding that it was written about the midwestern corn crop and not the Ontario one.
For me a key take away from this article was that although black layer may be delayed the extended grain fill period may be setting us up for a very high test weight crop.
I'll let you read this article to gather your own take-aways.
Actually, yeah! It has, here are some thoughts from Dr. Bob Neilson at Purdue in 2019 when they experienced a very similar phenomenon with some of their later planted corn.
Oh the joys of science and agronomy. I presented a lot of data and opinions- some of which were conflicting. To conclude this email I would just like to say- there's a lot of things we still don't know, and sometimes the best we've got is just a really good guess. At the end of the day the only thing you can do is get out there and scout your field to understand where your crop is at and when it will be ready to harvest.
If anyone has any other thought's or observations to add I'd be very interested in hearing them.
