Factors Influencing Cold Stress in Corn and Soybean
Soil Temperature Data. . .
on CropWatch are updated each morning and include the seven-day average and how it compares with the long-term average. To predict developing trends the table also includes the lowest soil temperature during the period and when it occurred and the highest soil temperature and when it occurred.
Warmer weather allowed for field work and planters to start rolling the last few weeks. Corn planting progress in Nebraska was at 4% as of April 20, according to USDA's National Ag Statistic Service, and will most likely jump by the next report.
With the anticipated cold snap this weekend, producers may be wondering how cold soil temperatures affect newly planted corn and soybean seeds. Ultimately, neither corn nor soybeans should be damaged from imbibitional chilling if soil temperatures do not dip into the low 40s within 48 hours of planting. The following provides additional information to consider.
Imbibitional Chilling Injury of Corn
When corn seeds imbibe (take up) water, cell membranes stretch and cells expand. When a damaged cell membrane rehydrates, it may not return to its normal shape and size. This can create a "leaky" cell. Water is at its densest at about 39°F so when cold water is imbibed, it may result in additional membrane damage. These ruptured membranes may occur in the cell walls and in the mitochondria. In the plant this action may disrupt the embryo/endosperm enzymatic conversion to energy, but mostly results in leakage of cell solutes and sugars. This, in turn, is likely to reduce growth rate and interfere with growth of the emerging seedling.
- Debate exists about what specific temperature and timing causes imbibitional chilling. However, corn plants that imbibe cold water (in the low 40s) in the first 48 hours after planting undoubtedly are affected.
- Planting when soil temperatures are above 50°F alleviates concerns of imbibitional chilling affecting corn emergence. (April 11 CropWatch article on early-planting soybeans) (CropWatch Soil Temp Information). Some scientists suggest that corn will not be injured at soil temperatures as low as 41°F; however, there is certainly some risk of injury from imbibitional chilling at those low temperatures.
- Some inbred lines (seed parents) are clearly more susceptible than others, and the seed industry has been actively eliminating the most sensitive ones from use. However, if environmental conditions are extreme, many fairly tolerant parents may still show damage—imbibitional chilling is a physical phenomenon that can override biology and genetics.
- For best results, begin planting corn when soil temperatures are in the high 40s and the short-term forecast calls for warm days that will continue pushing soil temperatures higher. If soil temperatures are in the high 40s and the weather forecast calls for cold wet conditions within the next 48 hours — that will likely reduce soil temperatures, so refrain from planting.
How Cold Stress Affects Soybean
Soybean germination consists first of a very fast uptake of water (imbibitional phase) followed by a much slower uptake of water (osmotic phase). Chilling during the first phase can cause severe problems because the imbibed water is needed to rehydrate the cotyledons and embryo to the point that cell membranes become functional. Cold temperatures interfere with proper hydration of those membranes.
The imbibitional phase is typically not very long (less than 24 hours) and can occur with relatively little soil moisture since the seed is dry at planting. Thus, getting a cold rain within 24 hours after planting can lead to chilling injury in soybean and thus lower stands. A study by UNL researcher W.J. Bramlage and colleagues showed that when the soybean seed coat was removed, imbibition injury occurred within 30 minutes. With a seed coat, imbibition is slower and a longer exposure is needed before chilling injury occurs.
- Chilling injury is likely greater if soil temperatures were cold (less than 50° F) at planting rather than becoming cold 24 or more hours after sowing. Chilling injury occurs with temperatures of less than 50°F within 24 hours of planting; germination failure and seedling death occur at soil temperatures around 40°F. The longer the seed is in the ground at warm soil temperatures before cold temperatures occur, the less chance there is for chilling injury.
Bottom line: Plant your soybeans if you think the soil temperatures won't get cold (less than 50°F) for at least 24 hours. If you planted two or more days before the cold rain, there should be no imbibitional injury due to cold temperature.
- During the second phase of germination, the fully functional membranes (after imbibitional hydration) create an osmotic situation in which water diffuses into the living cells. Osmotic water uptake is slow with cold temperatures. Chilling during this phase causes little direct injury to the germinating seedling. Cold temperatures will, however, slow emergence.
- Saturated soil with cold temperatures significantly hurt germination, thus fungicide seed treatments are recommended if planting in April or early May.
Be Aware of Soil Moisture Content
More important than cold soil temperatures after imbibitional water uptake is the soil moisture content. Cold soil delays the time between germination and emergence, but cold soil plus saturated soil conditions can substantively reduce soybean and corn emergence because soil-borne pathogens thrive in water-saturated soil. Since the period of seedling germination to emergence takes longer due to cold temperatures, those pathogens will have more time to infect the soybean seedling cotyledons and access their carbohydrate, protein, and lipid reserves. Soybean seedlings need those cotyledon reserves to live on until the unifoliolate leaves start photosynthesis and form more carbohydrates. This is why fungicide seed treatments are crucial if planting soybeans in April or early May and in cold, wet conditions. Ultimately, water-saturated soil with cold temperatures reduces both soybean and corn germination.
"Optimum" Planting Dates
Our use of the terms "optimum" planting dates here implies that both corn and soybean yield responses to planting date is a curve — lower yields early, an optimum date, followed again by lower yields. Many producers, however, see yield responses as a downward sloping straight line; thus, the later they plant, the lower their probable yields. In fact, planting before that "optimum" date incurs the likelihood of yield loss, at least in some environments. At this time year, soil temperature, moisture, and condition at planting as well as the weather that follows planting — plus all the other variables and unknowns — will dictate the optimum planting date. We determine that by looking in the rear-view mirror.
Additional Considerations and Recommendations
Corn hybrids and soybean varieties vary in their characteristics, including cold tolerance and resistance to fungal pathogens. Timing and duration of weather events and other random environmental factors can lead one to draw different conclusions about which hybrid or variety was best. Sometimes the risks we take with earlier planting are exactly that — risks — and sometimes loss will occur as a result.
If you have early planted corn or soybeans, it will be important to scout those fields early for any potential problems since poor stands may need to be replanted. Continue to monitor soil moisture temperatures at our CropWatch Soil Temperature page.
For More Information
For additional research see Dynamics of Imbibition of Soybean Embryos by C.W. Vertucci et al. in Plant Physiology, the journal of the American Society of Plant Biologists.Greg Kruger, UNL Extension Cropping Systems Specialist, West Central REC
Jim Specht, UNL Professor of Agronomy and Horticulture
Roger Elmore, UNL Extension Cropping Systems Agronomist
Jenny Rees, UNL Extension Educator
Tom Hoegemeyer, UNL Agronomy and Horticulture Professor of Practice