Grain Fill Stages in Corn

The reproductive stages of corn are often over looked and not fully understood, but as discussions of fungicide applications, maximizing yield and monitoring grain development continue, having a basic understanding will help make better production decisions:

Corn reproductive stages generally consume 60 days starting with silk emergence and ending at black layer.  There are six recognized growth stages during the reproductive stages and are designated with an “R,” followed by a number designating subsequent stages. 

R1: Silking

R2: Blister

R3: Milk

R4: Dough

R5: Dent

R6: Physiological Maturity

Silking (R1) begins when silks emerge from the ear and are ready to receive pollen.

Blister (R2) stage begins 10 to 14 days after silking begins and the kernels are whitish blisters and contain a clear liquid if busted.  By this point the silks are brown and drying rapidly.  R2 is also often referred to as the “Brown Silk Stage.”  Starch begins to accumulate in the kernel while the radicle root, coleoptile and first leaf have formed in the embryo.

Milk (R3) stage begins 18 – 22 days after silking.  R3 is commonly referred to as the roasting ear stage.  The kernels are mostly yellow or dull yellow and filled with milky white fluid.  By the R3 stage the ear kernel number is generally determined and additional kernel loss will be minimal.

Dough (R4) stage begins 24 – 28 days after silking.  The kernel’s milky fluid is changing to a “dough” consistency as starch accumulation.  The kernels are gaining consistency and size and the color is becoming more yellow and could develop a small dent in the kernel.  Stress during this period will not lead to kernel loss, but will primarily affect the kernel weight.  The cob is beginning to show some color from light red to pink.

Dent (R5) occurs 35 – 42 days after silking and all kernels have fully dented and the hard starch begins forming at the top of the kernel.  The best way to recognize and identify this stage is to look for the top of the kernel turning bright, shinny, dark yellow color and the inside of the kernel is of a firm consistency.  Kernels will continue to fill and add weight for another 20 days and hot dry weather can still reduce yields by 15-20% during this stage.

Dent stage also is when the “milk line” can be seen.  The milk line is the separation between the liquid and solid starch areas of the kernel.  As the kernel continues toward maturation the milk line moves from the tip to the top of the kernel.

Physiological Maturity (R6) starts 55 to 65 days after silking and is often referred to as “black layer.”  Physiological maturity occurs shortly after the milk line disappears and just before the formation of the black layer at the tip of the kernel.




2017 Rootworm Revival?

Corn rootworm (CRW) damage has been held at bay the past few years.  Poor winter egg survival, excessively wet springs, and plentiful moisture during grain fill have limited populations and the effect of CRW feeding in our fields.

In 2016, we observed a dry June for the first time in 3 years, and late season CRW adult beetle counts were higher than 2015 and 2014.  Late season rainfall masked most root damage that did occur from the larvae pressure last year; however, the higher beetle counts created more capacity to reproduce and set our fields up for potential issues in 2017.  I have received more CRW reports in the past two weeks than I did in all of 2016; adult beetle pressure appears to be much higher than previous years.

Peak hatch occurred during or shortly after the week beginning June 11, 2017 according to Iowa State University.  Approximately 50% CRW hatch is expected when soil degree days, base 52F, reach the range of 684-767 DD. (See Figure 1).

Adult beetle emergence timing is right on top of tassel and silk emergence in many corn fields this year.   Silk clipping and poor kernel set were not much concern in 2015 and 2016 as silk emergence was well ahead of adult beetle emergence.  Begin scouting fields ASAP to limit damage to silks and kernel set if populations are high.  Silk clipping can devastate corn yields if not managed properly.  If silks are still green and clipped to less than 1.5”, adult beetles should be controlled with an insecticide application.

The plan for 2018 must also be considered when scouting for CRW adults.  If continuous corn is in the plan for 2018, adult CRW populations should be controlled with an insecticide application when female populations reach ¾-1 beetle/plant and are fully gravid.  Extremely high pressure fields should be considered for rotation to soybean.

The risk of CRW damage can be attributable to many factors.  The following is a list of risk factors that can increase the risk of CRW infestation:

1.       Continuous corn, especially if 3rd year or more in corn

2.       Conventional corn, and/or corn with no below ground Bt traits (ie. VT2Pro)

3.       Fields with a history of hog or cattle manure

4.       Single mode of action Bt trait planted

5.       Fields with a history of higher than expected feeding on Bt rootworm corn

6.       No in-furrow insecticide applied at planting

7.       Same mode of action Bt trait planted for 3 or more years in a row

8.       Late planted/maturing corn last year attracting late hatch CRW beetles from neighboring fields

9.       High CRW beetle counts in the field the previous year that were not controlled with insecticide

10.   Soybean the previous year with unsprayed volunteer corn and/or weeds

11.   Fields with a history of CRW populations expressing extended diapause to survive corn/soybean rotation

I personally observed rootworm larvae damage on long-term corn-soybean rotation in Audubon County this year.  Extended diapause is the likely cause of the CRW population in this rotated field.  The situation highlights the need to scout every acre during pollination and avoid harvest surprises.  Late summer and fall wind events this year may highlight increased root feeding this year.  Be prepared to make decisions for next year and actively scout now for 2018 success.

Wind Analysis Tool for Predicting Pollen Drift

Growers of non-gmo corn, white corn, and waxy corn have struggled for years to predict where unwanted pollen might come from.  Now growers of IP yellow and white corn near fields of Enogen® corn are doing this as well.  Most pollen falls in the field it came from.  Yet, with only a 15-mph wind, pollen can travel as far as ½ mile within a couple of minutes while it is still viable (Nielsen, 2016). After discussing this challenge with Dr. Mark Westgate, Iowa State Professor of Agronomy, he directed me to a website developed by a fellow ISU professor, Daryl Herzmann.  The website shows detailed information of wind speed, direction, and duration in a diagram called a Wind Rose.

This article will help guide you through using the website so you can develop a wind rose for your fields.  The wind data comes from airports around the Midwest.  You will be able to choose the location you use.    

Gather Information

Before going to the website, it is helpful to gather some information.  First map out where the fields you are concerned about, I call them the “target” fields, are located.  You will have hybrid and planting date information on those fields.  You will need information on what corn is planted near your target fields.  You will want to know what hybrids and when they were planted.  You need to go at least ½ mile away on all sides of the target field.  Don’t assume the wind only comes from the south or west in the summer.

Calculating Pollination Window

Once you have the hybrid and planting information you will need to calculate silk emergence in your target field.  This can be estimated by using Growing Degree Unit (GDU) data and the data from your hybrid profile for GDUs to Mid-Pollen.  GDU data can be calculated from several websites/apps or gotten from Advantage Acre® if you are a subscriber.   For example, A6499 is 1362 GDUs to Mid-Pollen from the AgriGold Hybrid Profile Guide.  Using the Advantage Acre timeline, I can find what date that occurred on.  If you use a GDU calculator you can estimate that date as well.  Pollen shed is normally a two-week process, so go one week to both sides of the mid-pollen date to determine your silk exposure window.  If your corn field had uneven emergence and/or replant you may want to widen that window.

You have now determined the time period you are concerned about the wind.  However, if you want to refine your time period you can add the information about the surrounding fields.  By going through the same process, you can determine the pollen shed windows for those fields.  Then you can overlap the pollination windows to possibly narrow the time period you are concerned about.  Offsetting planting dates is a good away to lessen the risk of cross pollination.

Now you are ready to go to the website.  The directions will walk you through step-by-step on how to use it.

IEM Site

This example is for the AgriGold office near Lawrenceville, IL for a period last summer. (Click on image 4).

Select By Network – From drop down select “state” ASOS.  If in Iowa Select “Iowa AWOS”  Click “Switch Network” button.

Select By Station – Select nearest location to your target field from map or drop down list.  Click “Select Station” button. (Click on image 5).

Click on “*Custom Wind Roses” tab. (Click on image 6).

Select Start/End Time:

Start:     Select Year, Month, Day, Hour (Beginning date of your calculated time period)
End:       Select Year, Month, Day, Hour (Ending date of you calculated time period)

Check box by #3. Limit to Range of hours given by start and end time.  Pollen usually sheds in the morning to early afternoon.  Try using 8:00 AM to 2:00 PM.

Direction Bins: Change to 24.  You can experiment with this.  Fewer bins consolidates wind the readings.

Image Format: Change to Portable Document Format (>PDF).  This will make Wind Rose easier to print.

Click “Submit” button.  Wind Rose will generate after a brief moment. (Click on image 7).

Interpreting your Wind Rose

  • Imagine your target field as the center point of the wind rose.  The wind is blowing in to the center.
  • Each triangle represents a wind direction during the designated time period.
  • Wind speed is designated by color key at the bottom.
  • Wind direction is designated on the outside perimeter.
  • Frequency (%) of speed and direction is designated by the concentric circles.  Each wind rose may have different frequency scale.

Example: The red circled triangle tells us there was a period of wind from the NNE approximately 9% of the time period.  Approximately 5% of the time period the wind was NNE at 20+ mph.  Approximately 1.5% of the time period the wind was NNE at 15-20 mph.  The remaining 2.5% of the time period the wind was NNE at 10-15 mph.  This adds up to the 9%.

How can you use this information to help you lessen the risk from contamination causing the corn to be rejected and not qualifying for a premium?  Hopefully now you have a clearer picture of what parts of your target fields have possible issues.  Standard harvest procedure for IP corn fields is to harvest border rows all the way around the field and store separate from your IP corn.  If your isolation distance is less than 165 feet, the recommendation is to harvest 16 border rows.  If the isolation distance is between 165 feet and 660 feet, the recommendation is to harvest 8 rows (Thomison and Geyer, 2016).  The wind rose could help you modify border row harvest to reflect possible risk areas.  I recommend harvesting a minimum of 8 rows whether there is any perceived risk or not.

For those who have their own weather stations with recordable wind information there may be an option to use that with a different website or software.  I am currently working on that with a wind logger at a field near me.  When I get that figured out I will put out another article.

I hope this information can help lessen the risk of mixing contaminated corn with your IP corn.  If you have questions about this article, please contact me.  If you have questions about the website you can contact Daryl Herzmann at ISU.  Our contact information is below.

Chuck Hill - AgriGold - Specialty Products Manager, Chuck.hill@agrigold.com, 217.473.6063, Twitter @agold_chuck

Daryl Herzmann - Iowa State University - Analyst, akrherz@iastate.edu, 515.294.5978

Enogen® is a registered trademark of Syngenta Seeds, Inc.

Advantage Acre® is a registered trademark of AgReliant Genetics LLC
Nielsen, Bob. 2016. Tassel Emergence & Pollen Shed.  Purdue University
Peter Thomison and Allen Geyer. 2016. Managing “Pollen Drift” to Minimize Contamination of Non-GMO Corn.  The Ohio State University

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