With this week’s forecasted low temperatures projected to dip into the high 30s (°F) with potential rain events, growers have asked if/how germination will be affected for corn and soybean planted this week. 

Summary

Imbibitional (fast) water uptake occurs within the first 48 hours after a seed is planted. Once planted, corn seeds need a two-day (48-hour) window and soybeans need at least a 24-hour window when the soil temperature at planting depth does not drop much below 50°F.  When the soil temperature drops much lower than 50°F within that time frame, there is potential for chilling injury to affect seed germination and seedling growth. Soil temperature decreases after this time are less likely to affect seed germination.  

Key Considerations

Check the weather forecast and soil temperatures for your area. It's also important to check the soil temperature of each field the morning you intend to plant. (This can be done with a meat thermometer.)

Second, check on your seed tag or with your seed dealer regarding the cold tolerance of your corn hybrids/soybean varieties. Hybrids and varieties vary in cold tolerance and company rating scales differ. However, be aware that imbibitional chilling is a physical phenomenon that can override genetics.

Cold Stress in 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. 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.
• 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, soil temperatures will likely drop and planting should be delayed until temperatures warm.

An inexpensive meat thermometer can be used to check soil temperature in an individual field. (Photo by Nathan Mueller)

of the field(s) you want to plant that day and then assess the temperature forecast for that same evening and the next day and evening. If the morning soil temperature is at or above 50°F and is not likely to fall below 50°F in the next 48 hours, consider planting. 

Cold Stress in 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 (usually 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 soybean chilling injury and thus lower stands.
• 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.
• Saturated soil with cold temperatures significantly reduces germination rate, thus fungicide seed treatments are recommended if planting in April or early May.
• 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.

In conclusion, check the weather forecast, soil temperature, and hybrid/variety cold tolerance before planting.  The first 24- and 48-hour periods are critical for soybean and corn, respectively, if soil temperatures dip much below 50°F.  Monitor your fields based on planting date throughout the year to determine any affects on plant stand and yield.

References

Kruger, G., J. Specht, R. Elmore, J. Rees, and T. Hoegemeyer.  Factors Influencing Cold Stress in Corn and Soybean.  UNL CropWatch.

Specht, J., J. Rees, P. Grassini, and K. Glewen.  2016.  Three Key Considerations for Planting Corn and Soybeans.  UNL CropWatch.

Specht, J. 2013.  Tracking Air and Soil Temperatures in Soybeans Planted April 29.  UNL CropWatch.

Vertucci et al.  1983.  Dynamics of Imbibition of Soybean Embryos. Plant Physiology 72:190-193.