December 18th, 2020

posted by Andrew Blomme on 12/15/2020 in Weekly Newsletter

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December 18th, 2020

Last week's newsletter covered the the physiology of a corn crop during the early part of its life cycle. This week we will look what goes on in the second half of the growing season. If you would like a refresher on the topics covered last week you can find that newsletter at the following link: December 11th, 2020.



Tasseling is one of the most important stages in a corn crop's development. It marks the halfway point of a corn plant's life. At this point a corn plant is done with vegetative growth and starts to focus on reproductive growth and grain production. This begins with pollination. 

Pollination, when you get down to the basics, is a fairly simple process. Pollen is created at the tassel and gets dropped down onto the silks of an ear. The silks we see are actually pollen tubes that transport the pollen to individual embryos which later develop into corn kernels. In order for a plant to develop the maximum number of kernels possible, pollen has to reach every silk on the ear. 

That doesn't always happen. There are several things that can cause some concerns during pollination. One of the most common is insect damage. Corn silks are like sugar to Japanese beetle, adult corn rootworm, and corn earworm. Silk feeding is also called "silk clipping" and causes abnormal ears like the ones pictured below. If silk clipping is noticed early enough an insecticide application is very effective at preserving yield this stage. 

Corn Silk Clipping

Aside from insect damage, heat and water stress can also effect pollination. Iowa State Extension has a good article about the effect of hot and dry conditions on corn at tassel time. You can follow this link to that article: Iowa State Extension-Corn Pollination


Grain Fill

After pollination the last major task required of a corn plant is to pack sugar and starch into it's kernels. There are several factors that can influence the grain fill period. Water availability and daily temperatures are two of the biggest environmental influences on grain fill. 

From VT through R5 a corn crop uses 1/4" to 1/3" of water every day. That means a crop needs about 2" of moisture each week. As we saw this year, water stress can happen quickly during grain fill when rainfall is lacking. This stress can have a negative impact on grain fill and test weights. 

Daily temperatures can also effect grain fill. Ideal grain fill days have cool nights and are full of sunshine. Corn develops based on GDDs/GDUs, but produces sugar/starch based off of sunlight. The cooler temps slow down corn development and can prolong the grain fill period by several days. These extra days of grain fill allow the plant to convert a couple more days worth of sunlight into starch that get packed away in the kernels.  

While cool nights help grain fill, hot nighttime temperatures hurt. Hot nights increase plant respiration rates. Respiration is simply the process of converting sugar into energy. Humans are constantly respiring to create energy to live. Plants are no different. During the day, a plant is able to create more sugar through photosynthesis than what it consumes in respiration. At night, there is no photosynthesis and so the plant consumes its sugar reserves for energy. The warmer the night gets, the more energy the plant needs and respiration rates increase. 

Iowa State had a really interesting article from 2019 about a model that tracks how many bushels are gained each week during grain fill. It is specific to 2019 but has some good information for every year. You can find it at the following link: Iowa State Extension-2019 Corn Grain Filling Process


Black Layer

At the end of the grain fill period, corn reaches black layer or R6. At this point growth and development of the plant ceases and the grain begins to dry down. Corn grain is typically around 35% moisture at this point. As a rule of thumb, grain loses 0.75% of moisture everyday until it reaches 25%. At the point it lose 0.5% of moisture everyday. Temperature and humidity have effects on drying but this is a good rule of thumb. 

There is an awful lot that goes into producing a corn crop. The material covered in the last two weeks is just a couple pieces of the puzzle. There is always something more to learn. During the next summer, we will cover crop development as it is happening in the field and what to scout for. 

About The Author

Andrew Blomme
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