Cold weather stresses corn; herbicide injury possible

Recent cold weather and frost has placed corn under significant stress. Emerged corn subjected to these conditions likely has considerable tissue damage; it will take more favorable growing conditions for recovery to occur. Corn that had not yet emerged by the time of the frost, but had germinated, has not developed very much. Cool soil conditions also will stress seedlings.

Many factors influence corn growth and development. Some are controllable; others, such as weather, are not. Earlier planting is one of the "controllable" factors that corn producers can use to increase yields. Although earlier planting has many benefits, such as better corn plant development and a higher yield potential if most of the vegetative development occurs in the cooler, moister weather of May and June, there are risks. These risks include cool, wet soils that can decimate stands, difficulties in weed control by tillage and cultivation, and the pos-sibility of frost injury.

Temperature has a profound effect on corn growth and development. Temperature directly affects how the plant functions in photo-synthesis, respiration, absorption of water and nutrients, transpiration, and other metabolic processes. The temperature range for growth of most agricultural plants is quite narrow, possibly 15-40° C (59-104° F). At temperatures much above or below these limits, growth decreases rapidly. Optimum temperatures for plant growth are quite dynamic and will change with the species or variety of plant, duration of exposure, age of the plant, and stage of development.

Young corn plants are relatively resistant to cold weather. An air temperature near -1° C (30° F) generally will kill exposed above-ground parts. The growing point of the young corn plant remains below the soil surface until approximately the V6 stage of development (six collared leaves visible). Therefore, recovery from a moderate freeze, when the growing point is below ground, usually is rapid and almost complete. A late-spring freeze, however, occasionally may kill early planted corn whose growing point is at or above the soil surface (the V6 stage of development or larger).

Frost-damaged corn seedling. Note new leaf emerging from whorl.
Frost-damaged corn. Note damaged plant (V2+) on right and undamaged plant (V1) on left.

Frost injury on very young corn plants surprisingly has very little effect on yield. The bottom four or five leaves on a corn plant never get very large and soon are shaded heavily by the larger leaves above them. Therefore, their contribution to the food supply for growth of the corn plant is small. In the event that young corn plants are damaged by early season frost, a hasty decision to replant would be ill-advised. Wait a few days to see if growth resumes. If the growing point is not damaged, a new leaf should emerge in three or four days.

When poor growing weather follows an early season frost, corn seedlings sometimes may die. Check for rot by splitting the seedling and looking for dark, water-soaked tissue. If this condition is evident and widespread, replanting may be necessary.

Corn seedlings that are stressed from cold weather are particularly sensitive to herbicide injury. Postemergence applications should be evaluated carefully until corn begins to demonstrate "normal" growth and development. If weeds are small and growing slowly, there is likely sufficient time to allow delayed application timing. However, if weed pressure is high, and if weeds are large relative to the optimum size for control, growers should consider the risks of crop injury and the benefits of weed control before making a herbicide application.

Growth regulator herbicides such as 2,4-D and dicamba are of particular concern. If these herbicides are applied in combination with other translocated herbicides such as sulfonylureas that require surfactants or crop oils, the potential for injury increases significantly. Contact herbicides such as bromoxynil also may have a higher risk of significant corn injury if applied to stressed seedling plants.

If weeds are small, rotary hoeing or careful cultivation may be a better management option than a postemergence herbicide application. Each field must be evaluated to determine the best weed control strategy.

This article originally appeared on pages 76-77 of the IC-478(10) -- May 26, 1997 issue.

Updated 05/25/1997 - 1:00pm