corn stalks

Corn Maturity and Drydown

Can I capture an early market premium?

It’s that time of year again when growers look at their corn crop and go through the exercise of deciding when to harvest. They also wonder if they will be able to harvest a bit early to capture an early market premium for their corn. With the overall later planted crop and cool summer temperatures, corn maturity and drydown will likely occur later for most and harvesting to capture this market premium will be more difficult.

corn milk lineHow many days is my corn away from harvest?

Most corn in Ontario is well into the R5 stage (Denting) although quite a few are still behind. At this stage in the crop you can begin to notice a distinct line near the top (dented end) of the kernel. This line is referred to as the “Milk Line.” There will be a distinctive difference in color above and below this line. In Ontario, it can take approximately 20-23 days to reach R6 stage from R5. The R6 stage is referred to as the “Physiologically Maturity or Black Layer” stage. This is the stage when the crop has completed its life cycle. It just needs to dry to proper harvest moisture.

Generally if the weather is good it will take approximately 10 days from 50% milk line to reach black layer. By splitting your cob in half and looking at the kernel you can start to make estimations as to where the milk line is and how long (if the weather cooperates) that it will be until black layer.

corn maturity

What is the black layer?BlackLayerCorn

Black layer is when the cells at the bottom of the kernel discolor and eventually collapse which block photosynthate movement into the kernel creating what looks like a dead black tip at the bottom of a kernel. At this stage kernels are then considered mature and safe from frost and the moisture in the kernel is typically around 30% to 35% (can range from 25% to 40%). Any stresses following this stage generally does not have much of an impact on yield. The kernels have achieved maximum dry weight but moisture loss will still be needed for suitable threshing. OMAFRA states that on a normal year, R6 should occur around September 26.

Frost potential

As discussed above, yield is not affected significantly by frost once black layer is reached. With the current crop situation and long range forecast, it is unlikely that frost will be a issue for majority of this season’s corn crop.

Stresses that affect final maturity date

There are many factors that affect the final maturity date of a variety. Cool temperatures, especially nighttime temperatures, slows the transfer of photosynthates to the kernel, delaying maturity. Insect and disease stress that cause damage to the plant will lower the overall energy reserves. In this case maturity will occur earlier as there is less energy for the plant to move into the grain. Keeping the plant healthy through the use of fungicides, nutritional products, insecticides, etc. will maintain the plant’s energy reserves that can be transferred into the grain allowing the plant to reach its full maturity potential.

Predicting black layer date

The maturity date of a corn hybrid is determined by the accumulated crop heat units (CHU). CHUs are based on the daily maximum and minimum temperature throughout the growing season. You can calculate the rough maturity date based on the current accumulated CHUs from your planting date until now. is great local Canadian site allowing you to view your field and look at the total CHU accumulated near your field and help predict the actual maturity date.

Dry down following maturity

It is possible for corn to lose 1% moisture per day when weather is favorable. Generally 0.4% – 0.6% per day is more realistic in Ontario. The temperature and hybrid traits will determine the speed of dry down. Warmer temperature contribute to faster drying. Hybrid traits such as: husk leaf number, husk thickness, husk tightness, coverage of the ear, and how the cob folds over when it is mature will all

influence the dry down time. Corn can always be harvested wet and dried to proper storage temperatures but this can be expensive. Never harvest above 28% as damage to the kernel can occur.

If your corn has reached R6 around the general target date of September 26th (which is very unlikely this season) and the moisture at this time is 30% (best case) it will take roughly 24 days to reach 18% moisture (given average drying weather). This means harvest could beginning around October 20th, unless it is harvested at higher moisture and dried.

This season much of the corn would be expected to reach black layer from September 26th to October 10th. Moistures are expected to be slightly higher. If you figure black layer on October 3rd at 35% moisture, expect harvest timing at 18% moisture would occur around November 4th, leaving no time to capture the early corn market.

Before You Book Early Corn Premiums

  • CornTransportWhat is your cost to dry corn down to 15.5%? It takes more dollars to dry from 26% moisture than it does 23% or 22%
  • What is your cost for freight since wet corn is heavier which means more trips back and forth from the field?
  • What types of drying systems do you have and how long will it take to dry?
  • What is the penalty if I book early premium corn and cannot get it harvested due to later than expected maturity or weather?


This year will be challenging for many growers to hit the early corn market for premiums. As we watch the corn prices drop, many growers will be anxious to get harvesting and capture whatever premium they can. Get out into your fields and start checking what stage your corn is at and where the milk line is located. Contact your local Thompsons Sales Staff to look at your fields with you to determine a rough harvest estimate.

Contact us

For more information contact one of our Agronomy Solution Specialists or any local Thompsons branch.

Chris Hunt, CCA
Agronomy Solutions Specialist
Phone: 519-676-5411, Ext. 20478 | Cell: 519-365-3593

Jevin Vyn
Agronomy Solutions Specialist
Phone: 519-676-5411, Ext. 20303 | Cell: 519-401-2120

Mike Wilson
Advanced Agronomy Solutions Manager
Phone: 519-676-5411, Ext. 20480 | Cell: 519-809-0284

Sources:,, Hyland Corn and Soybean Field Guide, OMAFRA,,,,