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What’s Going On In Those Corn Fields

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This is the time of year, it seems like a million things are going on, especially in the corn field.  Corn in some parts of the county has been planted for 6-7 weeks while other fields are recently planted.  No matter what stage it’s in, this weekend’s forecast of 80-90 degrees will encourage rapid corn growth.  Corn is a grass and responds to heat in a way that we can measure and predict leaf development.   Typically, this done using a system called modified growing degree days.  The math is simple:  Find the average daily temperature (high temperature plus low temperature and divide by two) and subtract 50.  If temperatures go over 86, then use 86 and if temperatures fall below 50, use 50.  Yesterday’s high and low were 75 and 63 so (75+63)/2= 69-50= 19 MGGDs.  From emergence to about V10, a new leaf emerges every 85 MGDD’s.   If temperatures continue on the same, a new leaf should emerge every 4-5 days.  From V10 to the final leaf, a new leaf emerges every 50 MGGD’s.  A free MGDDs calculator can be found here:  http://www.weather.com/outdoors/agriculture/growing-degree-days/19943   Like 2011 at this time, we are running about a 100 MGGDs higher than normal. This is an important concept to understand as many of our field activities revolve around growth stages in corn (i.e. postemergence herbicide applications, PSNT, sidedressing nitrogen).  
Postemergence Corn Herbicide Height Restrictions:  
Recommended Soil Nitrate Tests (Recommended Soil Testing Procedures for the Northeastern United States):   http://ag.udel.edu/extension/agnr/pdf/soiltesting%202011/CHAP4.pdf
Kent Conservation District Contact (The conservation will take and process PSNT’s):  http://kentcd.org/
On another note, I have received phone calls from growers with purple corn wanting to know what’s going on.  Like typical extension answers, it could be a number of things.  If it is widespread across the field and the field is planted to the same variety, then is probably “genetic.”  It’s a response to the weather conditions we had last week and not all corn contains the genes for the purple corn so this is why some hybrids stay green.  A good explanation of why purple corn is around this year can be found here:  http://www.agry.purdue.edu/Ext/corn/news/timeless/PurpleCorn.html
Don’t be too quick to write it off as genetic though (remember, it could be a number of things).  Purple corn could also be caused by insect feeding injury to the roots, low phosphorus, soil compactions, dry conditions, and herbicide carryover.   Be sure to take a shovel and dig up a few plants and take a look at the roots.  Compare to taller plants in the same field and see if the roots look healthy.  The next 2 weeks will be a busy time as herbicide applications are made, PSNT’s are taken, and nitrogen side-dressed.
Phillip Sylvester, Extension Agriculture Agent, UD, Kent County
References:
Nielson, Bob.  2012.  Prevalent Purple Plants Perennially Puzzle Producers.  Corny News Network, Purdue University.  Online.
Nielson, Bob.  2002.  Corn Growth & Development, What Goes On From Planting to Harvest.  AGRY-97-07.  Purdue University.  Online.  http://www.agry.purdue.edu/Ext/pubs/AGRY-97-07_v1-1.pdf
VanGessel, Mark and Brad Majek.  2012.  2012 Corn Weed Management Guide for Delaware and New Jersey.  University of Delaware and Rutgers University.  Online.  http://www.rec.udel.edu/weedscience/CornGuideWeb/CornWeedguide.pdf

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