Cold Damage In Corn And Winter Wheat Concerns


   The unseasonably cold, wet spring has presented numerous challenges across the state, especially in areas where corn has already been planted or where wheat has hit the jointing stage. In particular, freezing temperatures and excessive rains have created concern for the survival of corn seed(ling)s and also some winter wheat stand concerns.
   Effect on stands
   With the vast majority of the state's corn acreage not yet planted but with calendars indicating that it is that time of year, there are a few important points to remember before rushing out to plant this season. The soil is still very, very cold and poses a significant risk to yield potential. The first water the corn seed takes in (imbibes) is very important. Imbibition is the process by which seeds absorb water for the initiation of germination. A chilling effect occurs when water colder than 50°F is imbibed. The imbibition of cold water disrupts the reorganization of cells during rehydration and can result in the loss of seed vigor or seed death. The most critical time for imbibition is within 24 hours of planting. Therefore, imbibitional chilling effects are more severe when seeds are planted into soils 50°F or colder compared to planting into warmer soils followed by a drop in temperature.
   Sprouted corn can generally withstand a frost or freezing conditions because the growing point is beneath the soil. However, we are experiencing some dramatically low temperatures. Temperatures between 28°F and 32°F are damaging, but can become lethal when they fall below 28°F. Though the air temperature has been colder than this, soil temperatures are likely at or above 28°F and may have helped to insulate the seed and seedlings during the cold spells. If freeze damage to corn is suspected, it is important to allow several days to evaluate whether the effects were lethal.
   Temperature fluctuations over the winter also likely affected a small portion of the winter wheat acreage, especially in areas of a field where planting depths may have been shallow. Generally, winterkill is not a common occurrence in Missouri, but it is not unheard of. To assess the extent of the damage in a potential winter kill situation, take note of the affected acreage and determine if the root systems are still developing (healthy, white crowns and roots). In most areas, winterkill is not a primary concern, but stands may be thin due to late planting and dry fall conditions. For grain production, wheat stands should ideally have about 25 plants per square foot with 3-5 tillers per plant. Wheat stands of 12-15 plants per square foot, or less, are good candidates for replanting to corn, soybeans or grain sorghum. A hula hoop is a good way to measure stands of wheat or you can count plants down a row using the following chart as a guide for length of row equal to 1 square foot:

   Row Width Length of Row to Equal 1 sq ft
      7” 20.5”
      7.5” 19.25”
      8” 18”
      10” 14.4”

   Impact of a late freeze on wheat
   Wheat in the central to northern portions of Missouri are typically not yet jointed and appear to have tolerated the freezing temperatures very well. The impact of freezing temperatures is of most concern on the jointed wheat in the southern portions of the state.
   Wheat development in southern Missouri is at Feekes 7 (2nd node) in many fields and even approaching Feekes 8 (flag leaf) in the very southern Bootheel counties. Freeze injury in wheat depends on stage of development and duration of freezing temperatures. Wheat freeze injury at jointing (Feekes 6) is possible when temperatures fall to 24 degrees or lower for longer than two hours. Wheat freeze injury at boot (Feekes 9) has the potential to occur when temperatures fall to 28 degrees or below for longer than two hours. Feekes 7 and Feekes 8 are somewhere in between. The best time to monitor any type of freeze injury is several days after the freezing event. Split stems and carefully monitor heads for damage. The Kansas State Wheat Freeze Injury Guide provides further information on freeze damage in winter wheat and how best to evaluate the severity of the damage.
   Disease impacts
   Freezing temperatures and prolonged moisture can predispose young corn plants to seedling diseases, especially when germinating seed and seedlings are damaged early in the season. This is especially true when pathogen levels are high and cold, wet conditions persist for several weeks. Cellular damage from freezing wounds the plant and may allow for the infiltration of pathogenic bacteria or other disease-causing pathogens into the young plants. Pythium likes wet soil around 50°F and can still be damaging even when seed treatments are applied. Make sure soil temperatures are adequate and soil is not too heavily saturated with water prior to planting this spring.
   Key take-aways
   Freeze damage in corn – wait a few days to evaluate the impacts of freeze damage on stand in corn. Not all freeze damage is lethal.
Suspected winterkill in wheat stands – Examine the roots of several affected plants for new root growth (healthy, white crowns and roots). In thin stands, it is helpful to quantify the number of plants per square foot to determine if a replant is necessary.
   Wheat that is not jointed can withstand extremely cold conditions. Jointed wheat should be given several days after the freeze before determining if there is any significant damage.
   Most importantly, plant when soils are warmer and not heavily saturated.
   Luckily, warmer days are almost upon us. Keep an eye on those soil temperatures before jumping into your planter to maximize your yield potential. ∆
   DR. KAITLYN BISSONNETTE: Assistant Extension Professor, University of Missouri
   GREGORY A. LUCE: Adjunct Instructor in Plant Sciences, University of Missouri
MidAmerica Farm Publications, Inc
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