Dr. Ronnie Schnell

Cropping Systems Specialist – College Station

Introduction

Recent cold weather has affected newly planted, emerging or emerged corn or sorghum throughout south and central Texas. Corn and sorghum will experience similar types of injury although tolerance to low temperatures does differ between the crops to some degree. Sorghum generally requires warmer soil temperatures. Three types of injury may be observed, depending on stage of growth and temperatures experience above and below ground. This includes imbibition injury, cold stress, and frost/freeze damage. Damaged can be assessed by inspecting plants as temperatures warm up. If plants survived, they will show signs of regrowth after five to seven days of warm weather. Replant decisions must consider yield potential of current stands and yield potential of later plantings.

Imbibition Injury

Imbibition chilling occurs when cold soil temperatures, <50 °F, are encountered by germinating corn seed within 24-72 hours of planting. Sorghum can be sensitive to soil temperatures below 60 °F for extended periods of time. Imbibition injury is characterized by the rupture of cells in seeds after imbibing cold water. When seeds imbibe water, they swell. Cold water/soil can reduce the elasticity of cells making them prone to rupture. Seeds will appear swollen with no sign of germination, meaning the radicle and coleoptile abort.

Seedling Cold Injury

Cold stress affects earlier planted corn and sorghum where germination began during warm temperatures but emergence was not complete when cold temperatures arrived. Symptoms of cold injury during emergence include stunting, death of seminal roots, deformed mesocotyl elongation (corkscrew), and leafing out underground. Soil compaction and other factors can result in similar symptoms.

Frost or Freeze Injury

Leaf tissue is likely to show symptoms of injury during any frost-freeze event. Damaged leaf tissue will be discolored, turning gray to greenish-black within a few hours and eventually drying up and turning brown. More importantly, the status of the growing point should be evaluated. A healthy growing point should be a light cream-color and tissues remain firm. Damaged growing points will be discolored and have a water soaked or mushy appearance. This is a clear indication that the plant is unlikely to recover. However, the best method for determining the extent of damage on a whole field basis will be to wait 3 to 5 days or more to evaluate tissue damage and regrowth.

Regrowth following freeze damage will be evident as new leaf tissue emerges from the whorl. Regrowth will be slower if cool conditions persist following the freeze event. Several days with temperatures above 70 °F is needed for regrowth. The key is to be patient. Yield loss for plants exposed to freezing temperatures earlier than the V-5 stage will result from stand loss, not the extent of damage to aerial plant parts. Up to 50% defoliation at the V10 stage can reduce yield about 6%, with insignificant yield losses at earlier growth stages. Frost damaged leaf tissue can sometimes develop a restrictive knot around the whorl which is disruptive to regrowth but the plant usually recovers without incident. If stand losses are not severe, the original planting will likely out-yield later plantings.

Assessing Damage

1. Determine planting date in relation to cold weather. Check fields planted on different days and with different hybrids.
   1. Cold soil temperatures occur less than 72 hours of planting, imbibition injury possible.
   2. Cold soil temperatures occur greater than 72 hours of planting, cold stress injury possible.
   3. Freezing temperatures for emerged crops will kill exposed tissue but may not kill the growing point. Exact temperature and duration will determine if growing point was killed.
2. If seedling injury is observed, make note of hybrid affected for future reference. Some hybrids have better cold tolerance.
3. Give plantings 5-7 days of warm weather and evaluate seedling germination and emergence or recovery of emerged plants. New leaves will emerge from the whorl if the growing point survived.
4. Final plant stands will determine if replanting is necessary. Check with insurance adjusters before replanting. Yield potential of current plant populations must be weighed against yield potential of later plantings.