By Delbert Voight, Mena Hautau, William Curran, Dwight Lingenfelter

Some fields with poor corn stands are being evaluated for replanting. The Penn State Agronomy Guide has a discussion about the different factors when considering replanting on pages 68-69.

Population Assessment

Once corn germinates, the population in the field can be determined. First take the width at planting (15 or 30 inches for corn) and convert the row width from inches to feet by dividing by 12. Divide feet by the square feet per acre (43,560). This gives you linear feet. Take the number of plants you find per linear foot in the field and multiply by 1,000 to get the plants per acre (ppa).

Here is an example: Thirty-inch rows divided by 12 is 2.5 feet. 43,560 square feet in an acre divided by 2.5 gives us 17,424 linear feet. One plant in a foot will yield 17,424 plants per acre. If there are two plants in a foot, then the plant population will be 34,848 plants per acre.

To simplify this you could now take 17,424 and divide by 1,000 to get the number of feet you need to represent 1/1,000th of an acre. In this case it would be 17.4 feet. In the field, measure 17.4 feet, count the plants, multiply by 1,000 and you will have your plants per acre. Do this in several places and take the average of the field.

Stand Evenness

Often, the greatest challenge to assessing whether to replant is the unevenness of the stand. Penn State research suggests that within-row unevenness robs 12% of yield, while unevenness between rows only robs 5% of yield. For each inch of deviation, university research suggests a 2.5-5 bushels-per-inch loss. This becomes more critical as populations are increased.

Planting Date and Relation to Yield

Planting corn by April 25 is 100%, May 19 is 90% and the May 29 is 80% of yield potential. In the Agronomy Guide, Tables 1.4-8, 1.4-8A and 1.4-8B on page 69 can offer help in making a determination whether one should keep or replant when comparing the plants per acre at harvest by planting date.

Pest Management

Weeds will invade anywhere light is allowed through to the soil, competing with poor stands, so evaluate potential herbicide costs. Important: Review any pesticides that were previously applied and re-crop restrictions.

Consult Your Dealer

Check with your dealer to determine the policy for replant support. There could be online tools offered by your seed dealer to help assess a stand.

Terminating Roundup Ready Corn for Replanting

If you do decide to replant, there are times when corn has to be removed from a field. Tillage is one effective method, but it is not appropriate in no-tillage situations. Use of glyphosate is highly effective for non–Roundup Ready corn. But the challenge is in removing Roundup Ready hybrids. There are limited herbicides to consistently kill small corn plants. Gramoxone SL, Liberty and Select Max are three products that have shown the most activity.

A few years ago we conducted a trial that simulated volunteer Roundup Ready corn control in a replant LibertyLink corn scenario. Roundup Ready corn was planted in mid-May. On May 31, pre-plant herbicides were applied and LibertyLink corn was replanted about 7 days later. The 6 fluid ounces of Select Max provided 96% volunteer corn control. 

Control with Gramoxone ranged from 76-99%. The addition of a photosystem II inhibitor (group 5 or 7) herbicide to Gramoxone, as well as increasing the rate, generally improved performance. Gramoxone plus 2 ounces of metribuzin had the highest numerical value (99% control). Liberty applied post-emergence on June 20 provided at least 91% control.

Below are data from the study:

  • Select Max (6 fluid ounces): 96% control
  • Gramoxone (24 fluid ounces): 76%
  • Gramoxone (32 fluid ounces): 84%
  • Gramoxone (48 fluid ounces): 90%
  • Gramoxone (24 fluid ounces) + metribuzin 75WG (2 oz): 99%
  • Gramoxone (24 fluid ounces) + Lorox (8 oz): 90%
  • Gramoxone (24 fluid ounces) + atrazine (32 fl oz): 95%
  • Liberty 280 (in-crop, 22 fluid ounces): 91%
  • Liberty 280 (in-crop, 29 fluid ounces): 92%

Some recommendations include:

Select Max: Up to 6 ounces of Select Max with a nonionic surfactant at 0.25% v/v plus AMS at 2.5-4 pounds per acre. Do not use a COC or MSO. Wait a minimum of 6 days from time of application until planting corn due to risk of crop injury. (Select Max will also control corn hybrids containing Roundup Ready and LibertyLink stacked traits.)

Gramoxone SL: 1.5-4.5 pints per acre in combination with metribuzin  (2-6 ounces per acre), Lorox (1 pint per acre) or atrazine (1 pound per acre). These photosystem II inhibitors (group 5) are not added to control the corn but are used to slow down the Gramoxone activity, which helps provide more consistent control. (Be aware that not all metribuzin products have a label for use in corn in Pennsylvania.)

Liberty 280: 22-29 ounces Liberty has not been as consistent for control corn as Gramoxone. (Keep in mind, Liberty will not control hybrids that have stacked Roundup Ready/LibertyLink traits.)