**This post was originally posted in September 2018 but has been updated to include 2021 data and weather conditions
As we’ve gotten closer to Labor Day we’ve spent more time checking corn moisture to assess which hybrids and fields should be ready to harvest first. We wanted to spend some time on a little refresher of how corn matures, dry down rates of corn, and if our conditions have affected our dry down at all.
How Corn Matures
Corn physiological maturity is considered the point at which transfer of moisture and nutrients from the plant to the grain is no longer occurring. We can see a visual indication of this stage called the black layer, shown in Figure 2. The black layer is an abscission layer that cuts off the flow of nutrients and water in and out of the plant. Before the plant reaches maturity, kernel moisture is very high. At milk stage, kernels are around 85% moisture, around 70% at dough stage, around 55% at dent stage, and then around 30% at physiological maturity. As the plant matures, water is lost from the kernels from evaporation and as the grain accumulates more starch. After the plant reaches maturity, all moisture that is lost is through evaporation from the kernels themselves. The speed at which each hybrid matures is determined by accumulation of heat units or growing degree days throughout the season. Plants described with a days to maturity value can be misleading as maturity and developmental stage of a corn plant is dictated by the GDD. Depending on the growing season, this may take more or less time than the “days to maturity” value. Additionally, the days to maturity value is not standardized across the industry. While it is easy to compare relative maturity within a company, it is very difficult to compare across companies.
Across Kansas, 63% is dented, and 12% at physiological maturity as of August 30th. That is compared to 69% dented and 17% and physiological maturity last year.
When I first wrote this blog in 2018 71% of corn was dented, and 23% was at physiological maturity as of August 26th, 2018. This is in comparison to 55% dented, and 13% at maturity on the same date the previous year, August 26th, 2017.) This was likely due to an above average accumulation of GDU’s early in the growing season driven by above average May temperatures. Since that point, Brown County had been between 10-14 days ahead of normal in terms of GDU’s. Figure 1 illustrates the difference in 2018 GDU’s versus the 30 year average.
However, that was not the case this year. If anything we lagged behind our average GDD accumulation. All season long we stayed about 70 GDD below the 5 year average, as seen in Figure 2.
Figure 2: 2021 GDD accumulation compared to 30 year average GDD accumulation
How Grain Dries
After reaching physiological maturity, moisture will continue to evaporate out of kernels until it reaches somewhere around 15%. At this point it will remain fairly stable around 15%. The rate of dry down is very dependent on environmental and plant physiological factors. If we are experiencing hot and dry conditions, dry down can be as high as 1 percentage point a day. If conditions are cooler and wetter, perhaps even just high humidity, dry down can be as low as 0.3% a day, or even stagnate. On average, dry down is around 0.5-0.6%. In general, to dry grain from blacklayer to 25%, about 30 GDU’s are needed per point. A study from Purdue University shows a decrease in 0.5% moisture for every 12 GDU’s and 0.75% when 22 GUD’s are accumulated. Using these values, it could take anywhere from just over 2 weeks to 4 weeks to dry grain in the field from 30 to 15%.
Physiological characteristics also play a role in how grain dries down. Plants with less layers of husks, thinner husks, or looser husks will dry down faster. Ears that extend beyond the top of the husks will dry down quicker. Ears that drop sooner will also dry sooner. Finally, grain with a thinner pericarp, the protective layer on the outside of each kernel, will also dry faster.
Each hybrid will differ in dry down rates. This is affected by both physiological characteristics of the grain and plant, but also by relative maturity and planting date. Plants that have a difference in relative maturity of one day but were planted the same day will have about a 0.5% difference in moisture.
Drought Effect on Corn Dry Down
In 2018 we were talking about how drought impacted dry down. It’s a common question among researchers, agronomists and farmers. How do season long drought conditions affect how corn dries down? According to Purdue University, dry down will be “normal”. While our drought conditions may have contributed to smaller, shallower kernels, each kernel will still reach black layer. This will happen very prematurely as no water and nutrients are available for transfer. Generally, since black layer was reached faster, dry down will be occurring in conditions that promote faster drying, such as hotter August temperatures. Because of this, drying rates may be faster than normal. Conversely, the rapid color change of our crop may be deceptive in assessing crop moisture. The crop color will change faster than the grain can lose moisture. Because of this, a plant may look like it has been dead for weeks and would have grain moisture readings around 15-20%, however, the plant may have abruptly changed in a short time period leaving less time for the process of moisture loss in the grain.
Stalk Strength
In any year, we need to keep an eye on plant health and stalk strength. While drying should be relatively normal, we do need to take stalk strength and ear retention into consideration. In 2018 we were at increased risk for weak stalks and ears from fungal diseases as well as weakened stalks from the plant cannibalizing resources during the growing season to use towards grain fill. However, in any year, it's important to take special care in creating a harvest plan to prioritize fields with the weakest stalks or ear shanks. If you do find a field with some stalk issues, some management adjustments might be needed to maintain as many ears on the stalks or upright stalks as possible, we will have to harvest at wetter conditions than desired. Take time to test a representative sample of grain for moisture, and consider stalk strength at each field to prepare yourself for harvest. If you have any questions about how corn matures or dry down rates of corn, contact any member of the Pederson Seed Team.
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