Stage Your Corn: Critical for Post Herbicide Application Decisions

The spring of 2017 has been a roller coaster of temperature extremes.  Early April started warm with a small window for planting in some areas; only to be followed by an extremely cold week to end the month.  Picture 1 from WeatherTrends360 displays the year-over-year comparison of Weekly Average Temperatures 2017 vs 2016; the gray line displays the average temperature recorded for the week.  The average temperature for the seven day period ending May 5 was only 48F near Fort Dodge. 

Picture 2.  Soil temperatures were hovering at or below 40F for 5 consecutive days, followed by 1-2” of snow on May 1 for much of western, Iowa.

However, most of the corn planted in April avoided significant stand loss due to the lack of crusting from the cold rains in late April, and/or timely crust-busting rains in early May.  The stand loss issues we observed in 2016 due to a similar cold and rainy stretch of weather in late April did not materialize on a large scale in 2017.  Stand establishment this year throughout western, IA is much stronger this year vs last.

Even though corn stand establishment has been solid, April planted corn growth has been limited due to the cold and stressful conditions it has endured.  Nodes are stacked tight at the base of the plant.  I am consistently finding V4-V5 corn planted April 9-12th that is very similar in plant height to V2 corn planted in early May (~6” tall).  As a producer, it is important to distinguish between corn growth and corn development, and make proper herbicide application decisions based on corn development stage.

Corn growth is measured by plant height and size.  Consistent warm temperatures, with plenty of moisture can extend internodes on a young corn plant and create larger plants with only a few collars.  New leaf collars still develop approximately every 90 GDU’s on a young corn plant, but the leaf itself, and the height of the plant can be larger if those 90 GDU’s accumulate in a short time span with rapid growing conditions.

Corn development is heat unit driven based on GDU accumulation described above; however, development is strictly measured by counting leaf collars.  Cool weather slows corn growth, shortens internode length, and increases the time needed to add collars to the corn plant.  As a result, cool springs often lead to squatty, slow growing corn plants that may be well developed, with very little plant growth by late May.

Picture 3 Growth vs Development.  Corn growth is 6” tall.  Corn development is V3 = 3 collars present on the stalk.

Herbicide labels are restrictive based on corn growth stage and/or plant height.  In either case, I always error on the side of caution as the corn plant nears the V6 development stage (6 collars developed).  A corn plant begins determining kernel rows around the ear shoots at this growth stage, and stinging the corn plant with certain herbicides can limit yield potential.  Be extra cautious with Sulfonylurea (ALS) herbicides when applying to corn at later growth stages.  The label on many of the ALS pre-mix herbicides restricts application based on plant height (11”); however, if your corn was planted in April, a plant height of 11” or less could encompass V6 corn when you drive the sprayer to the field.  Be sure to count your collars, and avoid ear pinching due to ALS herbicide applications when your corn plant is determining kernel rows around the ear.

Picture 4.  Ear pinching due to ALS herbicide application past the V6 growth stage.

Other herbicide groups can have detrimental effects to corn when application timing restrictions are not followed.  This website is a great resource for updated herbicide application timing and restrictions: https://www.roundupreadyplus.com/resourcecenter/corn-growth-stage-and-herbicides-applied-postemergence

Always read and follow all herbicide labels and restrictions.


Cold and wet weather impacts on corn and soybean herbicides

Many parts of Wisconsin, Minnesota, Northern Iowa and Northern Illinois have experienced abnormally cool and wet conditions throughout most of the planting and growing season thus far. Some areas have been fortunate enough to get most of the crop planted, but what has been planted has been slower to emerge and is likely stressed from saturated soil conditions. For both corn and soybeans, cold and wet conditions can interfere with the plant’s ability to metabolize herbicides that are normally safe and labeled for these crops. This can be observed as twisting and buggy-whipping, stunting, bleaching, and/or browning on the young leaves as these plants struggle to grow in a stressed environment.


The most common case of early-season herbicide damage comes from PPO-inhibiting herbicides that are becoming more common again as farmers try to stay ahead of herbicide-resistant weeds. Sulfentrazone and flumioxazin, the base ingredients in Authority, Valor, Fierce, and many other products, are popular options because they provide very good weed control. These products often cause some brown lesions on soybean hypocotyls, stems, and young cotyledons and leaves if a heavy rain event occurs around the time of emergence or water splashes up on the young leaves after emergence. A few plants may die, but soybeans typically “grow out” of these conditions with the return of warm weather with no long-lasting effects. In severe situations of prolonged cool conditions, a large percentage of the soybeans may be killed and significant reductions in the stand can occur. This is not common, and oftentimes does not outweigh the benefits of an herbicide program which also does a good job of providing acceptable weed control.  


Bleached corn is often associated with the HPPD-inhibiting herbicides tembotrione,  mesotrione and isoxaflutole, which are ingredients in Lumax/Lexar, Acuron, Balance Flexx, Corvus, Halex, Capreno and many others. These herbicides are typically metabolized by corn, but in stressed situations where little growth is occurring, the plant can end up taking in more than it can effectively break down. The symptoms usually show up as partially or completely bleach white plants and in severe cases, the plant will die. In most situations the plant outgrows the symptoms with the return of warm and drier weather with no long-lasting effects. 

Another common early-season injury symptom that is sometimes seen in corn growing in stressed conditions is “buggy-whipping”. Chloroacetamide herbicides can be one of the causes of this. These include s-metolachlor, acetochlor, and dimethenamid-P, which are found in Degree, Harness, Warrant, Dual II Magnum, Outlook and are included in many other corn premixes as well. This may end up stunting or killing a few plants when this occurs, but typically has no long-lasting effect as the plant is usually able to eventually unfurl and grow normally.  







Seedling Blight

The recent extended cool wet periods have caused many corn fields to need to be replanted due to less than ideal stands.  The saturated soils, crust and cold temperatures greatly decreased the viability of the corn seed that was plant 3-4 weeks ago.  Fortunately not all the corn stands failed and many fields were left, although the stands were not ideal, they were still acceptable for maintaining high yield potential.  Those fields survived the first round of stand establishment concerns, but, recent phone calls and evaluations from the field have led many corn fields into the next challenge in stand establishment: seedling blights. 

Seedling blights is the term used to describe corn plants that have been attacked by soil borne pathogens during the critical stage of emergence to V3.  The soil pathogens are present in every soil and are designed to decompose any plant material that is present in the soil.  A healthy growing corn plant’s natural defense against these seedling blight pathogens is rapid growth.  If the corn plant is able to grow faster than it can be attacked, the corn plant wins.  But, if the corn plant cannot grow faster than the attack, the pathogens win.  The seed treatment that comes on every bag is designed to help the corn plant have a 14-21 day window of protection, to germinate and outgrow these pests.  But after that window of opportunity, the corn plant is on its own.

Seedling blight pathogens thrive in cool, wet soils, the same environment that causes corn growth to be slow to non-existent.  The last 3-4 week window has been a perfect environment for seedling blight attacks and the corn has been sluggish long enough to be in the cross hairs for infection.

Scouting for seedling blights is relatively simple.  The first signs of seedling blights will show on the mesocotyl.  The mesocotyl is the part of the corn plant that attaches the seed to the above ground portion of the corn plant.  The mesocotyl is the umbilical cord that connects the new plant to its food source, the seed.  An infected mesocotyl will have brown lesions or be completely brown.  Once the mesocotyl turns brown and shrivels up, the young corn plant can no longer receive water and nutrients to survive.  Often the plants look healthy above ground one day and the next they are wilted and struggling, this is the above ground symptoms of the below ground problem with seedling blights.

Until the corn plant has reached the V3-V4 growth stage, it is solely dependent on the seed for its survival, after V3 it begins to grow its nodal roots that will provide the water and nutrients for further growth.  Therefore once the corn plant is living off its own roots, the seedling blight pathogens are no longer a concern for the corn plants survival. 

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