Agronomy Dept., Purdue Univ.
While farmers are free to plant corn whenever they choose to, there are risks associated with “early” planting (Nielsen, 2020). The primary risk is that associated with “cold” soil temperatures. Soils that hover around 50 degrees (F) for days or longer after planting delay germination and slow emergence of the young seedlings. More importantly, soil temperatures lower than about 50F increase the risk of “imbibitional chilling” injury to germinating seeds.
Herein lies the concern with fields that were planted during the past week, especially in central and northern Indiana. “Imbibition” refers to the initial uptake of water by seed during the first 24 to 48 hours after being planted into moist soil. The resulting rehydration causes the seed to swell and the germination process to begin. Imbibition occurs naturally, with no physiological processes involved (e.g., dry wood will imbibe water). It also occurs whether soils are cold or warm and therein lies the potential for “imbibitional chilling” injury.
When the seed swells as it rehydrates, its internal cell membrane structure is damaged. When seeds (and soil) are warm, the membrane damage is quickly repaired by the physiological activity associated with germination and “life goes on” normally. When seeds (and soil) are cold, their cell membranes are less elastic, the cell membrane damage due to swelling is more severe, and the physiological repair of the damage is slowed or stopped. Left unrepaired, this damage to cell membranes and the subsequent leakage of cell contents can result in death of the seed.
Past research on the nature and causes of imbibitional chilling injury to seed does not clearly identify the environmental conditions “in the real world” that result in a high probability of the problem. The literature implies that soil temperatures simply lower than 50F are a key factor. It is not clear from past research whether the injury can occur with only a few hours of exposure to sub-50F soil temperatures or whether it requires lengthier exposure to cold temperatures. What is known is that this type of chilling injury is most likely to occur during the first 24 to 48 hours after planting seed into moist soil because that is when imbibition (and corresponding seed swelling) occurs.
Identifying and Diagnosing the Problem in the Field
Identifying and the diagnosing the problem in the field is often challenging for several reasons. First of all, germination and emergence of corn in cold soils will naturally be slow. The first visual indicator of germination (other than the seed swelling) is the appearance of the radicle root between 35 and 60 Growing Degree Days (GDD) after planting (Nielsen, 2019).
Tip: Calculating GDDs using soil temperatures is preferred over air temperatures for predicting corn development progress prior to about the 6-leaf growth stage (V6). The reason is that the seed & young seedling responds more directly to soil temperature as long as the main growing point of the corn plant (apical meristem) remains below ground (until about V5-V6).
When soil temperatures hover around 50F for days or longer after planting, accumulating 35 to 60 GDD may take 1 to 2 weeks. Initially, dead seed due to imbibitional chilling injury do not look much different than live seed taking their normal “sweet time” to germinate in cold soils. However, once 60 GDD or more have accumulated, then seed that seems to be “dormant” compared to others that exhibit radicle roots, coleoptiles, and lateral seminal roots may well be the result of imbibitional chilling injury. Sometimes, instead of immediate cessation of the germination process (i.e., “dormant” seed symptom), the radicle root and coleoptile emerge from the seed coat before ceasing further development (Fig. 2).
Another challenge in diagnosing imbibitional chilling injury as the cause of poor stands of corn is that eventually the dead seed or seed that germinated but simply ceased further development will naturally begin to decompose. Consequently, if you wait too long to investigate a problem field, you might be tempted to diagnose seed or seedling disease as the cause of the poor stand.
Daily, or hourly, soil temperature records coupled with knowledge of a field’s planting date are useful for “pointing the finger” at imbibitional chilling injury. Because imbibition occurs within the first 24 to 48 hours after planting into moist soil, one can imagine that timing of planting relative to the onset of several days of cold soil temperatures influences the risk of imbibitional chilling injury. Anecdotal stories abound in the coffeeshops about fields planted 3 days ahead of a cold snap emerging just fine… fields planted 2 days ahead of the cold snap experiencing some emergence problems… fields planted 1 day ahead of the cold snap having more problems… and fields planted the day of the cold snap having major problems.
Factors Influencing Risk of Imbibitional Chilling Injury
Intensity and Duration of Cold Soils. Obviously, 40F soil temperatures represent a higher risk than 50F temperatures. A single day of cold soils is likely less risky than multiple, consecutive, days of cold soils.
Soil Moisture. Daily soil temperature fluctuation is more dramatic in dry soils than in moist soils. That means higher daily maximums and lower daily minimums.
Plant Residue Cover. Daily soil temperatures fluctuate less in no-till fields that have a lot of surface residue from previous crops or current cover crops. In particular, soil temperatures in such fields will not drop as rapidly or dramatically in response to a cold snap as will bare fields. That’s the good news. The bad news is that soil temperatures in fields with heavy surface residues are generally lower to begin with than bare soils early in the season and so early planting of corn in no-till fields is somewhat more risky in general.
Seed Quality? One can speculate that seed lots with lower than desirable cold germination ratings might be more susceptible to imbibitional chilling injury.
Bottom Line: If you were among the ambitious souls who chose to plant more than a few acres of corn during the past 14 days or so, I encourage you to scout those fields over the next week or so to assess the success of germination and emergence. Emergence success is usually lower for early planting versus later, warmer, planting. However, the risk exists for unusually lower emergence success this year because of the unusually low soil temperatures of the past week.
An Interesting Question: Bill Cox, Extension Corn Specialist at Cornell University in New York, raised a question several years ago about whether modern hybrids are still susceptible to imbibitional chilling injury (Cox, 2014). Based on a small set of trial data from Cornell’s Aurora Research Farm in Cayuga County NY, Bill concluded the answer was “maybe not” and that “…the timing of the adverse conditions would have to be so unique and so time-dependent in the first 48 hours after planting that it may not be worth worrying about”.
I admit that Bill may have a point. I have not encountered many clear-cut examples of imbibitional chilling injury in all the years I have worked with corn in Indiana, as exemplified by the fact that I have so few photos of the problem in my image collection of various corn problems. Nevertheless, the risk for imbibitional chilling injury with cold soils is real. That risk plus the indisputable fact that that cold soils are simply not conducive to desirable rapid germination and emergence of corn should always be weighed when choosing to plant corn early in soils that are cold or likely to become cold.