I’ve tried to write pertinent topics that can give you food for thought.  Many of you are finished or nearing completion of your corn harvest.  Others are working diligently to complete it prior to any appreciable losses.  Given the current conditions, I’ve decided to stab at field dry down.

Let’s look at how grain dries in the field after maturity. Grain drying in the field after physiological maturity (black layer) is an evaporation process.  Moisture must travel through the pericarp (outer layer of kernel cells or skin) of the kernel at a rate determined by the relative humidity surrounding the grain.  Pericarp thickness is a factor as well as plant structures such as cob volume or thickness and husk leaves length, thickness and adherence (tightness) to the grain.

Moisture in corn goes from a high concentration to a low one. Grain evaporation rate is very much dependent on relative humidity immediately surrounding the kernels. Transpiration from the senescing, but green, leaves in plants for a time after kernel black layer contribute to higher humidity in the field. As the leaf tissue dries, the remaining ear structure is the only barrier to respond to atmospheric factors such as relative humidity and wind affecting the dry-down of the grain.

This drying process will vary between hybrid characteristics and the environment from black layer onward to harvest.  Grain moisture loss in the field occurs at a fairly linear rate within a range of moisture content from ~ 35-40 percent down to 15 to 20 percent, and then tapers off to much smaller daily rates. The exact rate of field drying varies among hybrids and years.

Because grain dry-down rates are greater when the dry-down period is warmer, it stands to reason that a corn crop that matures in late July will dry-down faster than one that matures in late August or early September.  When corn is maturing in late July, it is not uncommon for grain moisture to decline more than one percentage point per day over a period of days when conditions are warm, sunny, windy and dry (low humidity). In contrast, there will be little grain dry-down on cool, cloudy or rainy days.

Depending on location, studies have shown that corn dry-down from black layer varies.  Early on after maturity, corn will average .7 to 1.0 percentage points per day and slowly taper down.  As temperatures drop, dry-down will be closer to .3 to .5 percentage points per day depending on planting date and hybrid factors.

What are the hybrid characteristics that affect dry-down?

• Hybrid relative maturity, thickness of pericarp, and ear angle after maturity can all affect dry-down rate. Hybrids with thinner cobs will lose moisture faster. As corn matures, moisture is lost through cob and ear shank, exposed ear tips and husks. Grain with thicker skin and higher test weight dries slower; chaffy or light weight grain dries faster.
• Upright ears tend to capture moisture in the husks and slow down the drying process. Droopy ears lose moisture faster than upright ears as the husk protects the grain from rewetting and will also shed moisture.
• Husk number, and kernel depth also influence how fast the grain dries. The greater the number of husks, the slower moisture is to evaporate. The deeper and heavier the kernel, the slower grain loses moisture.

Weather has a major effect on grain moisture at harvest. Temperature, rain fall, relative humidity and amount of sunshine influence grain drying. Weather conditions after the grain-fill period is over have a major effect on how fast the grain will dry in the field. So, there are lots of factors that influence moisture at harvest.

Delaying harvest until corn dries down to 16% to 18% moisture content can save on artificial drying costs. However, as corn dries down in the field there is greater potential for excess harvest losses from stalk lodging and ear-drop. Most harvest losses are mechanical, caused by kernel shattering or corn never getting into the combine. Allowing corn to dry-down in the field could lead to excess harvest losses, as much as 2 to 8% above the normal level with a timely and efficient harvest at 21% +.

Why is this information important?  We have entered into the season when we see more and more tropical depressions occur with late afternoon heavy rains. See the table below.  This takes a toll on good drying days and will certainly cause yield and quality losses.

Summary of the number of rainy days as of Aug. 20, selected sites

We have a good crop and it is important to capture as much of it as possible.  I encourage any corn grower that has corn in the field to finish harvesting as quickly as the current weather allows.  It may require drying and if your buyer has drying facilities, I encourage you to take the discounts and remove the results of your hard work from the field and avoid greater losses that will come with the delays associated with frequent rainfall.