Introduction
What a change a few days can make!  Corn planting has begun across much of Iowa, with about 25% of the corn ground planted in west-central Iowa.  Most planting activities have stopped due to the nice rain (1/2 to 3/4 inch) the area received over the weekend.   I noticed a lot of dust behind tillage and planting equipment last week, hopefully the rain was enough to thoroughly wet the upper soil profile.  The rapid cool-down in soil temperatures will slow germination and emergence; the earliest planted corn should be emerging in the next 5 to 7 days (120 GDD).  Is there potential for ammonia burn on corn in recently applied fields; the answer is yes if it was not applied 6 to 8 inches deep.  Alfalfa weevils will be emerging soon, look on south facing slopes first.

Weather update
We’ve experienced a roller-coaster in temperature this last week.  This type of temperature swing can be very stressful on plants.  The area forecast is for a short warming trend by mid-week, but then returning to cooler temperatures by week’s end.  The warming trend will not return until around May 1.

 Planting and Crop Development
Average 4-inch soil temperatures peaked at 65 degrees Fahrenheit last week (Figure 1) but have dropped now to the mid-40s.  With the rapid soil temperature cool-down, frequent field inspections should occur on a regular basis to monitor stand establishment and crop health.  If a stand problem is found, spend the time to determine what caused the reduced stand, whether it may be from seedling disease, uneven seed placement by the planter, rodents, or insect pests.  Make it a habit to pull plants and inspect roots.  The root system will tell a lot about plant health and general soil conditions (compaction, herbicide injury, disease, and insect feeding).  Early inspection of roots may help answer questions later in the season.  The following two tables may be useful when inspecting plant populations and stand development. 

Growing Degree Day Accumulation
With the very warm days last week, the area accumulated several growing degree days which can be used to predict corn growth and development.  Some corn was placed in the ground and started the emergence process, but is most likely at a stand-still today.  For those that started planting corn last Monday (April 15), we’ve accumulated about 70 GDD and forecasted to collect an additional 40 GDD over the next 7 days.  Growing degree days are calculated by the following equation:

 (Daily high temp + Daily low temp)/2 – base temperature.  The base temperature for corn is 50 degrees F and the maximum temperature is 86 degrees F.  Corn typically requires 110 GDD50’s for emergence.  This number will vary depending on soil temperature, soil type, planting depth, and soil moisture.  Using the information, we can predict that corn planted on April 15 – 18 may be emerging by April 28.

In Figure 2, the line with squares equates to Monona, Crawford, Carroll and southern Ida, Sac and Calhoun counties.  The line with Diamonds equates to Pocahontas, Buena Vista and northern Ida, Sac, and Calhoun counties.  The dotted line is predicted GDD and crop development in 7 days.

Soil Fertility - Ammonia Injury to Corn
If you just applied anhydrous ammonia to your cornfield, you may be wondering how long to wait to plant corn after the ammonia application.  If there is good soil separation between the ammonia zone and the seed, planting can be done the same day the ammonia is applied. If the seed is to be placed in the ammonia zone, the longer the waiting period the less potential for root injury. There is no magic number of days to wait.

 Ammonia injury causes desiccation of roots and die-back from the root tips. Injury is first noted in the field by uneven emergence, slow growth of some plants, and wilting of plants in dry weather. Root injury is noted by browning of the roots and in severe cases roots will be completely killed and turned black back to the seed. Ammonia injury is detected more frequently in dry weather because roots are slow to develop and destruction of a portion of the root systems limits water uptake.

(Information from Integrated Crop Mngt 4-16-93)

Pest Management
Alfalfa weevils will be hatching soon across northwest IA, this is a bit sooner than normal.  Scouting should begin at 250 GDD (base 48).  Currently we are at 236 GDD with a projected target of 250 GDD between April 24 and May 1.   Alfalfa weevil can be very destructive to first cutting alfalfa. They remove leaf tissue, beginning with the new leaves in the top of the plant, then work down the stem to other leaves. Their feeding reduces forage quality and quantity. Begin scouting on south-facing hillsides. Larvae will hatch here first because these areas warm up quicker than northern hillsides.  For more information on the alfalfa weevil, refer to the April 23 2001 issue of the ICM newsletter or go the ICM newsletter website http://www.ipm.iastate.edu/ipm/icm/ and search on “alfalfa weevil”.

Newsletters and Websites

·         ISU Integrated Crop Management Newsletter
http://www.ipm.iastate.edu/ipm/icm/default.html

·         Crop Update information from Joel DeJong, ISUE Crop Specialist NW Iowa http://www.extension.iastate.edu/nwaeo/Crop%20Update%20Newsletters/CropNews_JD.html

·         Crop information from Virgil Schmitt, ISUE Crop Specialist east-central Iowa http://www.extension.iastate.edu/Pages/eccrops/cu.html
 

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366
Email: vagts@iastate.edu  

For questions or comments please respond to vagts@iastate.edu

This page last updated on 05/03/02