University of Vermont

Assessing Field Corn Soon After Emergence Assessing Field Corn Soon After Emergence

Sid Bosworth and Heather Darby, Extension Associate Professor, Department of Plant and Soil Science, University of Vermont and Extension Assistant Professor, Northwest Region, University of Vermont Extension

The most critical time to scout field corn is that three to four week period after planting.  During this period problems such as poor emergence, insects, and weed pressure can have a big impact on yield and quality.  Many of these problems can still be economically rectified if caught and treated quickly. The following is a rundown of potential items to look for in that early part of the corn’s life.

Poor Germination and Emergence
Corn should emerge anywhere from one to three weeks after planting depending on soil temperature.  According to Greg Roth, Extension Corn Specialist at Penn State, corn should begin emerging after approximately 100 to 125 growing degree days have accumulated following planting.  He recommends looking for the following potential emergence problems:

  • No seed present. May be due to planter malfunction or bird or rodent damage. The latter often will leave evidence of digging or seed and plant parts scattered on the ground.
  • Coleoptile (shoot) unfurled underground. Could be due to premature exposure to light in cloddy soil, planting too deep, compaction or soil crusting, extended exposure to acetanilide herbicides under cool wet conditions, or may be due to extended cool wet conditions alone.
  • Seed with poorly developed radicle (root) or coleptile.
  • Coleoptile tip brown or yellow. Could be seed rots or seed with low vigor.
  • Seed swelled but not sprouted. Often poor seed-to-soil contact or shallow planting- seed swelled then dried out. Check seed furrow closure in no-till. Seed may also not be viable.
  • Skips associated with discolored and malformed seedlings. May be herbicide damage. Note depth of planting and herbicides applied compared with injury symptoms such as twisted roots, club roots, or purple plants.
  • Seeds hollowed out. Seed corn maggot or wireworm. Look for evidence of the pest to confirm.
Note the patterns of emergence as these might indicate the cause of the problem. At times poor emergence has been associated with a particular row, spray width, hybrid, field or residue. Often a combination of two or more stress factors can interact to reduce emergence where as the crop would have emerged well with just one present. Also, note the population and the variability of the seed spacing. This information could be valuable in the future when repairing and adjusting your corn planter.

Early Season Corn Insect Damage
There are several insects that can cause problems during early season.  Seed corn maggot is the most common early season corn insect found in Vermont.  Other early season insects that cause losses in isolated cases include the black cutworm, armyworms, wireworms, white grubs and potato stem borer.

Seed corn maggot – Adult flies lay eggs close to decaying plant material or germinating seed to provide a food source for the newly hatched larvae.  Large seed, like corn, is very attractive.  Fields with regular manure applications also provide exceptional habitat for egg laying.  So, there is a very high likelihood that most cornfields in Vermont will have some level of this insect.  At high maggot populations, corn losses can typically range from 3,000 to 8,000 plants per acre.  There is no treatment once the problem occurs, so the best treatment for this insect is prevention.  It is usually quite easy (and relatively inexpensive) to control this insect with a planter box insecticide treatment.  Captan-Diazinon Seed Treater or Kernel Guard Supreme will both control seed corn maggot.
(Iowa State Univeristy)
Wireworms and white grubs – Wireworms are the larvae of click beetles whereas white grubs are larvae of scarab beetles such as June beetles.  They feed on the roots of grasses, germinating crops seeds and the base of young plants.  Both can cause occasional problems and are most commonly found when corn is following a grass pasture or hay field; however, there is no way to predict a problem in order to justify a preventative treatment of insecticide.    In fact, there are no insecticides labeled for white grub.  If damage from wireworm is severe enough, the field may have to be replanted.  At that time, a preventative insecticide could be justified.
Black cutworm – an occasional problem in corn.  Conditions most likely to see this problem include late plantings, weedy fields, low, wet areas and fields previously in pasture or sod. It is critical to start scouting early.  The damage from black cutworm is most visible when the cutworm larvae reach about 1 ½ inch in length.  But by then it is too late to get effective control with a foliar insecticide.  The larvae need to be smaller for the insecticides to work.  Symptoms of damage from this insect depend somewhat on soil moisture conditions.  When moist, the larvae will emerge from the soil and cut the corn plants off above the ground.  When soil is dry, the larvae stay below ground and the damage looks more like wilting of corn seedlings.
True Armyworms – an occasional problem in corn especially in weedy fields.  They can start early feeding on corn and other grass crops.  Check fields regularly for ragged holes chewed from the leaf margins.  Look for pellet droppings in the whorls and larvae can often be found in the whorls or at the soil surface. Larvae usually feed late in the day or at night.  During the day hours, they typcially stay at ground level under the shade of crop residue or grasses.

Early Weeds
Scouting early for weeds is critical.  Many of the post emergence herbicides that can be used as an emergency treatment need to be applied when the weeds are small in order to be effective.   Each herbicide has it’s own set of restrictions based on corn size and maturity and weed size which also varies according to the particular weed specie.  Of course, it is important to be able to identify the young weed seedlings so that you can apply the most appropriate products.   For example, the newer product Callisto can be applied post emergence to corn up to 30 inches in height.  It is effective on many summer annual broadleaf weeds when applied up to 5 inches in height, but it will control crabgrass (large only, not smooth crabgrass) but it has to applied when the grass is less than 2 inches tall.  An effective and economical way to control weeds is by cultivation.  Cultivation should be implemented before corn canopy closure.

Scouting Report
Well kept field records can be very helpful when making management decisions.  Click here (pdf format) for a two page scouting report for field corn.

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Last modified July 02 2009 02:46 PM

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