Frost in Northern Iowa Last Week

Last week, Joel DeJong, Iowa State University Extension field agronomist near LeMars, had thermometer readings of 32 degrees. H has observed some damaged corn plants, and would like to address that in this blog.

He says some corn plants at the VE to V1 stages of development were damaged above ground, with leaves already drooped over and turning greenish-black. Such damaged leaves will slowly bleach to a straw color as the tissue dries out. As the frosted leaf tissue in the whorl dries, the whorl will often develop a constricted ‘knot’ that may restrict expansion of the undamaged whorl tissue later on. Usually, knotted corn plants will successfully recover as the expanding whorl tissue breaks these knots. Once in a great while, it may be necessary to mow a frosted corn field to cut off severely knotted leaf tissue. The key to deciding whether to mow or not is to allow the damaged field three to five days to show you how well it is recovering. As with most early-season injuries to corn, the recovery of frosted corn depends greatly on whether the internal growing point region was damaged. The good news is that the growing point region of corn younger than growth stage V6 is below the soil surface and protected from above ground frost damage.

The bottom line on diagnosing the severity of frost or low temperature injury to corn or soybean is that you generally need to wait three to five days after the weather event before you can accurately assess the extent of damage or recovery. Injury can look very serious the day after the event, but recovery may be possible if the growing points are not damaged. These three to five days will be better spent continuing to plant the remainder of your crop acres, assuming that you are not yet finished with corn and soybean planting. After three to five days, surviving corn plants should be showing new leaf tissue expanding from the whorls, while dead corn plants will still look dead. Yield loss to frost damage in corn younger than V6 is related primarily to the degree of stand loss, not to the degree of leaf damage.

Replant Decisions for Corn: Perhaps in five days you have a field that is not recovering well from frost damage, or is not the stand you had hoped. You need to remember to take the emotion out of a decision to replant and use research-based information for making a replant decision.

Follow the steps in this Replant Checklist
http://www.agronext.iastate.edu/corn/production/management/planting/replanting.html from ISU Extension to help you make a decision. In that checklist there is a link to a worksheet to help you calculate the yield loss of uneven heights. Don’t forget to ask for help if you need it.

Read more updates from ISU Extension and Outreach field agronomists here. http://www.iowafarmertoday.com/app/blog/