How can I estimate the rate of drydown for my mature grain corn?

Morgan Cott on October 7, 2026

By Morgan Cott, Agronomy Extension Specialist – Special Crops, Manitoba Crop Alliance

I was hearing rumours of some grain corn being harvested last week with ranges of 25-33% moisture, which is pretty darn accurate for the last week of September. Considering a few days of fog have passed and some beautiful, mild temperatures are ahead, let’s talk about drydown rates to expect in October.

Prior to physiological maturity (black or brown layer), grain fill occurs during the reproductive stages of corn development. As grain fill progresses, dry matter replaces water content in each kernel. Water is removed from the kernel via the pedicel (point where the kernel is attached to its cob) and via evaporation during the grain fill period. As physiological maturity is reached and those cells that connect each kernel to the plant die, water is no longer lost at that point of contact and can only occur via evaporation, which is much slower.

So, black layer will occur consistently when the grain reaches about 30% moisture and from that point, we rely mainly on weather for evaporation and, to a lesser degree, kernel characteristics of different hybrids. Unfortunately, September 2026 was quite humid and damp, overall, which is the opposite of what farmers require for grain drydown. Aside from environmental factors, hybrid traits also play a major role in the plant and grain’s ability to dry down in the field. Here are some noted characteristics:

  • Husk leaf number – the fewer, the better for drydown;
  • Husk leaf thickness – the thinner, the better for drydown;
  • Husk leaf senescence – the quicker, the better for drydown;
  • Husk coverage of the ear – the poorer the coverage, the better for drydown;
  • Husk tightness – the looser, the better for drydown;
  • Ear declination – the earlier the ears “drop”, the better for drydown;
  • Cob diameter – the narrower, the better for drydown; and finally,
  • Kernel type – flint-dent kernels dry down more slowly than dent types because they have a harder kernel.

Moving on to the actual question, how to estimate drydown in the field. October is starting out beautiful with sunshine and warm temperatures from mid-teens to 20oC. The only things that would make this better drying conditions would be even warmer temperatures and stronger breezes, that are also warm. The general estimation is that during these warm temperatures, you can assume up to 1% moisture loss per day, but I wouldn’t bet the farm on that information!

According to Ohio State University, when the grain is between 30 – 25% moisture, it takes approximately 30 GDD’s to lower moisture by one point. So, 150 GDD’s to lower moisture from 30 – 25%. After that, 45 GDD’s are required to lower the moisture from 25 – 20% (225 GDD total). This is the easy answer and calculation, but there are other factors at play, of course. These drydown rates won’t be the same for every hybrid and will very much depend on weather conditions. I took a quick look at Carman’s Growing Season Report and see that from Oct 4 – 5th (24 hours), we only accumulated 3 GDD in that period. However, on October 6th, as I write this, the air temperature is 21oC and the sun is warm with a gentle breeze, so I assume we are accumulating more GDD today than yesterday. I will also assume that we could be expecting drydown on days like this to be about 0.5 – 0.75% daily, if we are being conservative. As the temperatures cool a bit next week, calculate roughly 0.25 – 0.5% per day, as long as the sun is shining and there is a breeze. Using that information and the Weather Network’s 14 day forecast, my very rough estimate would be that we could lose about 6 – 10% moisture over the next two weeks, depending on potential rains and cloud cover, plus overnight low temperatures.

The only thing we can do is make a best-educated guess on how long it will take the crop to drop “x” points in moisture given the information above and what you know about the hybrids you grow. Maybe you have experience with a certain hybrid and that it took seemingly forever to drop those last 3% in the past, or maybe you have experienced the opposite. Keep detailed notes so you have a record for the future, so when you’re planting early (or late) in spring 2027, you have more decision-making tools on what to plant, and where. 

Wishing you all a very safe and productive corn (and sunflower) harvest!

Other resources

 

What is the difference between GDD, CHU and RM? Why and when are they each important?

Morgan Cott on June 1, 2026

By Morgan Cott, Agronomy Extension Specialist – Special Crops, Manitoba Crop Alliance

This is such a great question and I understand the reasons for asking. First of all, I think it is understood that GDD & CHU both basically measure how efficiently a day’s heat will benefit a growing or developing crop. One main difference between the two measurements is that CHU does account for extremely high temperatures (>30oC) that negatively affect crop development. CHU also looks at both the nighttime low temperatures (4.4oC as the base) and daytime temperatures between only 10oC (base) and 30oC (optimum ceiling).

GDD has historically been used to help estimate certain agronomic events like insect emergence, weed emergence, frost-free days and specific crop staging. My understanding is that CHU is used more for maturity than for the ability to accurately predict these different timelines throughout the season. This might be why certain seed companies use CHU and RM for maturity ranking and GDD for staging references.

Pride Seeds is an example of this, because they break down maturity with both CHU and RM, in addition to two key reproductive stages in GDD accumulation.

When calculating GDD or CHU, you will start from the day after corn was planted. It takes from 100 – 120 GDD for corn to emerge following planting, which is in ideal conditions, including soil moisture and soil temperature, which were variable this spring (and every spring). Start your calculations from day 1 until the day of emergence and see if that fits the above. Now that corn has emerged and is actively growing in your current conditions, continue to monitor accumulating GDD with the following formula. This will give you a head start on expectations during the growing season. When to expect pollination, for example. Note that this GDD accumulation in relation to corn staging is all in relation to each individual hybrid. A shorter season hybrid will need fewer GDD or CHU to reach black layer than a longer season hybrid, of course.

 

Helpful Resources:

How much 10-34-0 can be applied with my corn seed?

Morgan Cott on May 5, 2026

Oddly, I have had this conversation more this winter/spring than ever before. On paper, there is a finite answer. Anecdotally, there are a few different options and it is all dependent on soil type and soil conditions, moisture, etc.

First of all, side-banding any type of fertilizer is much safer than placing it with the seed. Some fertilizers are safe in certain quantities with the seed, but very few. Side-banding is much safer and provides quick access to the roots. Midrow banding is the safest method, but roots take that much longer to access the fertilizer row, which negates the “starter” effect. The other factor that indicates the level of safety is soil moisture; the drier the soil, the more risky it is to place any fertilizer with or near the seed.

I’m guilty of thinking that fertilizer toxicity to the seed is mainly due to the nitrogen content and a result of ammonia burn. Salt injury is actually more common and affects germination and early season growth, so applying fertilizers that have a low salt index in closer proximity to the seed is best, if any has to be close to the seed at all. Bonus points if there is good soil moisture at the time of fertilization.

As I mentioned, on paper there are defined rates of 10-34-0 that can safely be applied in-furrow with corn on 30” rows. This table shows those rates and placement that will help to avoid salt injury to the corn crop.

Table: Amount of 10-34-0 (gal/acre) to help avoid salt injury to corn grown in 30” rows.

After having some discussions with Manitoba corn farmers on various soil types, I did get some reasonable responses explaining increasing rates in heavier, wetter soils. But more importantly, decreasing the above rates in dry and/or lighter soils. These were purely anecdotal and not research-based.

I recommend being very conservative in 10-34-0 rates if you are new to trying this method of application, and especially if you are planting in dry conditions and/or coarse soil textures. Start small and have many conversations with your peers on their experiences with various rates of 10-34-0 in-furrow. Use that information to make a conservative decision of your own.

Remember, start your season with success and make smart choices. Do not make decisions that could get your crop off to a bad start. We live in Manitoba – Mother Nature is hard enough on us in spring.

Preventing and Monitoring for Insects in Stored Grain

Morgan Cott on August 25, 2023

By John Gavloski, Extension Entomologist at Manitoba Agriculture

Grain beetles can be an issue in high infestation years, so it is important to review management strategies. The first step is knowing what species of beetle is present and whether it is a species that feeds on the grain itself, or more on fungus growing on the grain or in the bin. Without magnification, it can be difficult to determine the difference between rusty grain beetles, one of the more common beetles feeding directly on many stored grains, and foreign grain beetle, which is primarily a fungus feeder.

Note the club-shaped antennae on the foreign grain beetle in the photo. If placed in a glass jar, foreign grain beetles will climb up the sides, while rusty grain beetles cannot.

Preventing insects being introduced into or establishing in grain bins.

A few steps to reduce stored grain insects establishing in grain bins include:

  • Clean in and around storage facilities.
  • Inspect grain storage facilities for signs of deterioration, leaks or holes.
  • Clean equipment used to move grain. Combines, grain wagons, truck beds,
    augers and other equipment used to move grain should be cleaned of grain
    residue before being used.
  • Dry and cool grain (ideally below 15°C) as quickly as possible after harvest.

Controlling insects found in stored grain.

Knowing the species you are dealing with is important when determining the most effective and economical means of control. If it is a fungus feeder such as foreign grain beetle, practices that result in grain drying may be all that is needed.

For insects that feed directly on the grain, if the grain is being kept over the winter, lowering the grain temperature through aeration or moving the grain can kill stored gain insects.

Grain vacs, if used at a slow enough speed (such as 200 bushels per hour), can kill stored grain insects. Removing too large a volume of grain at a time results in the grain protecting the insects and reduces kill. Insect killing efficiency is greatly increased when a 90° angle is made in the grain vac pipe. The grain should be dry for this method to work well.

Depending on the crop being stored and insect pest, chemical control options can include fumigants (such as phostoxin or fumitoxin), diatomaceous earth (Protect-It, Insecto), or malathion. There are things that should be considered before using any of these chemical control options, however.

Fumigants such as phostoxin and fumitoxin are restricted to applicators with a valid stored agricultural products license. These products can not be used when the grain temperature is below 5°C, as the tablets release the gas too slowly.

Diatomaceous earth damages the cuticle of the insect, reducing the insect’s ability to retain moisture. The insect eventually dies from dehydration. It can take weeks for diatomaceous earth to control insects in grain bins, assuming the grain is dry.

Important note: Some commodities, such as canola, flax and sunflowers, should not be stored in facilities recently treated with malathion.

For more information:

Prevention and Management of Insects and Mites in Farm-Stored Grain, Manitoba Agriculture

 

Top