Planting is anticipated to begin for many growers in Nebraska the next few weeks. Based on formal research reports showing increased yields, we have recommended early soybean planting beginning mid- to late- April. This article will take a deeper look into the research on the soybean germination process, particularly when chilling injury can occur, and factors to consider when planting soybean early.

Soybean Germination Phase: Imbibition

The first soybean germination stage is called the imbibitional phase in which a very fast uptake of water typically occurs in less than 24 hours. It can also occur with relatively little soil moisture. A study by UNL researcher W.J. Bramlage et al. 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 would be needed before chilling injury occurs. McDonald Jr. et al. found that the seed coat extended the duration of the imbibition phase to 8 hours. Table 1 from their study has been amended by Specht and Rees to show the rate at which water uptake occurred in the seed. Please refer to it for additional comments. An Ontario (Canada) Extension Report (Bohner, 2003) suggests the imbibitional phase in soybean may be completed in just 6 to 24 hours if the soil temperature is kept at 45°F.

Table 1 Table 1 (amended from McDonald et al. 1988). A comparison of water uptake (g/kg fresh weight) in the axis, cotyledon, and whole seed of soybeans soaked in moistened paper towels for 48 hours without (WO) and with (W) the seed coats and the ratio (without/with) of the values at an alternating 16hr/8hr 68F/84F thermoperiod in a 100% RH germination chamber. Amended: *RATE (referring to rate of water uptake for the whole seed) was added to this table by Specht and Rees. It was calculated just for the ‘whole seed with the seed coat’. Example: The seed water content change from 0 to 2 hours was 204 g – 73 g=131/2 hours=65.5 g/kg/hour. Based on this data at these temperatures with paper towel germination, the rate of water uptake slowing over time may indicate that the imbibition phase was complete by 4 hours of this experiment. It would be helpful to have included 6, 10, and 12 hour increments to better understand the rate changes of water uptake. It would also be helpful to have conducted this experiment at 50F and in soil of a stated soil moisture content. Axis Cotyledon Whole Seed Time (h) WO

g/kg fresh wt. W

g/kg fresh wt. Ratio

WO/W WO

g/kg fresh wt. W

g/kg fresh wt. Ratio

WO/W WO

g/kg fresh wt. W

g/kg fresh wt. Ratio

WO/W *RATE

g/kg/hr 0 104 88 1.18 79 72 1.1 81 73 1.11 2 318 283 1.12 229 179 1.28 239 204 1.17 65.5 4 422 402 1.05 342 302 1.13 396 362 1.09 79 8 503 470 1.07 462 413 1.12 459 410 1.12 12 16 561 569 0.99 510 524 0.97 525 540 0.97 16.3 24 653 632 1.03 563 566 0.99 556 572 0.97 4 48 809 839 0.96 578 584 0.99 596 611 0.98 1.6 LSD (0.05) 29t NS 14t 0.14 11t 0.18 t LSD value used to compare any two means within the same time for the seed part with and without seed coats.

Imbibing cold water can lead to “chilling injury”. Thus, getting a cold rain within 24 hours of planting can lead to chilling injury in soybean and lower stands. The first visible symptom of this injury will be cotyledons displaying outer layers of dead tissue (Tully et al., 1981). Chilling can cause severe problems because the imbibed water is needed to rehydrate the cotyledons and embryo to the point that cell membranes become functional. This is a requirement before any subsequent growth can start.

It is known that imbibitional water uptake is faster with warmer compared to colder temperatures (Roskruge and Smith, 1997). This suggests the imbibitional phase reaches completion sooner if seed is planted mid-day into warm temperatures rather than at sunrise into cooler temperatures. In an experiment in 2019, Specht planted soybean at mid-day into moist soil on various dates in April. The soybean planted on April 10, 2019 in the afternoon prior to a cold snap had no difference in germination and emergence compared to the other planting dates. This suggests the imbibition process was completed prior to the cold snap.

Research has also shown the seed moisture and seed coat quality (damaged, wrinkled from wetting/drying) can also impact the imbibitional phase. Soybean seed at 6% and 16% beginning moisture content was exposed to cold (41°F), warm (77°F), or warm followed by exposure to cold during imbibition. In their study, Obendorf and Hobbs (1970) found the higher beginning seed moisture content allowed for no reduction in survival, dry matter accumulation or seedling height at the cold temperature imbibition. However, a reduction in all these characteristics was observed for low moisture seeds and was variety dependent. Yet, there was no reduction in these characteristics when the low moisture seeds imbibed water at the warm temperature before being followed by cold temperature. Because the seed at 16% moisture did well in all three situations, they suggest that a higher moisture content soybean seed (16%) can improve seedling establishment in cold soil. Other studies resulted in similar findings with beginning suggested soybean moisture range of 13-16%. This moisture increase may occur naturally after seed in cold storage is transported and exposed to higher humidity and temperatures in the outside air and in seed storage buildings.

After the imbibitional phase ends, the risk of chilling injury also ends. It’s just difficult to pinpoint length of the imbibitional phase as soybean seed quality, moisture, variety; soil moisture and temperature; and cold snaps/cold rain within 24-48 hours can all impact this phase based on the research. After the imbibitional phase ends, the risk of chilling injury also ends.

Soybean Germination Phase: Osmotic

In the osmotic phase of soybean germination, a much slower uptake of water occurs. Seedlings in this phase are quite tolerant of soil temps as low as 35-40°F, although extended low soil temperatures can be expected to lengthen the germination to emergence timeframe. Moreover, delaying emergence can lead to greater soil-borne pathogen problems, particularly if the soils are both wet and cold.

Early Planting Considerations

With what is known regarding the soybean germination process and chilling injury, we offer the following early soybean planting considerations.

Planting Conditions:

Avoid planting soybean when a cold rain and cold snap is expected within 24 hours (safer within 48 hours). Cold rain can increase the imbibitional uptake of cold water into the soybean seed causing chilling injury.

When possible, consider planting soybean in the warmer part of the day or when air temperatures are generally warming.

Plant into moist but not saturated soils. The imbibitional phase may complete without chilling injury in cooler soils (45°F) if the soil is moist but not saturated and no cold rains/cold snaps occur within 24 hours. Some soil-borne pathogens can thrive in saturated soils and can impede germination and/or cause seedling death.

Plant 1.5-2” deep to allow for buffered soil moisture and temperature conditions. This also can delay emergence before a frost. Emergence of pre-May planted soybeans can be delayed by up to 14 days compared to 7 days or fewer in May plantings. UNL research found lowest yields when soybean was planted 1.25 inches deep or less or 2.25 inches or greater with the highest yield at 1.75 inches deep.

Planting Protection:

Use a fungicide + insecticide seed treatment. UNL doesn’t have early-planted soybean research relative to a cost-benefit analysis of these treatments. The fungicide treatment helps protect against seedling diseases and the insecticide treatment protects against bean leaf beetles.

No need to increase seeding rates. UNL planting date X seeding rate research showed no yield benefit to increasing seeding rates from 120,000 to 180,000 seeds/acre with early planting.

Crop insurance is available for soybean fields planted on or after April 25. The cost-benefit of potential higher yield for early planting must be weighed against your level of risk for potential seedling frost or other crop damage.

Know the long-term probability of freezing air temperatures of 32°F and 28°F for the month of April using weather data available from a nearby NOAA weather station. SoyWater is a tool to estimate when seedling emergence can be expected (based on historical weather station records) for several April planting dates you are considering this year. Soybean seedlings can only be injured or killed by freezing temperatures after they emerge from the soil. They are most susceptible to death from frost when the hypocotyl hook is emerging at the soil surface. They are fairly frost tolerant in the cotyledon stage.

The bottom line is: Consider planting soybeans if you think soil temperatures won't get cold (conservatively less than 50°F) for at least 24-48 hours after planting. If you planted two or more days before the cold rain, there should be no imbibitional injury due to cold temperature. The longer the seed is in the ground at warm soil temperatures before cold temperatures occur, the less likelihood there is of chilling injury. Replicated field research on the interaction of soil temperature (40-50°F) and the duration of hours needed for imbibitional water uptake is needed. Such research could provide valuable data for soybean producers scheduling April planting dates.

References:

Bohner, Horst. 2003. Do Soil Temperatures at Planting Impact Soybean Yield?, Ontario Ministry of Agriculture, Food and Rural Affairs.

Bramlage, W.J., A.C. Leopold, and D.J. Parrish. 1978. Chilling Stress to Soybeans During Imbibition. Plant Physiology 61:525-529.

Obendorf, R.L. and P.R. Hobbs. 1970. Effect of Seed Moisture on Temperature Sensitivity During Imbibition of Soybean. Crop Science 10:563-566.

McDonald Jr., M.B., C.W. Vertucci, E.E. Roos. 1988. Seed Coat Regulation of Soybean Seed Imbibition. Crop Science 28:987-992

Roskruge, C.L. and M.T. Smith. 1997. Journal of Plant Physiology 151:620-626.

Specht, James, Jenny Rees, Roger Elmore, Nathan Mueller, Keith Glewen. Soybean Germination/Emergence with April Planting Dates Relative to Coincident Air and Soil Temperatures in April and May, University of Nebraska Extension CropWatch, May 16, 2019.

Specht, James, Patricio Grassini, Jenny Rees, Nathan Mueller, and Roger Elmore. Amplifying Positive Impacts of Early Soybean Planting, University of Nebraska Extension CropWatch. April 19, 2018.

Specht, Jim, Greg Kruger, Jenny Rees, Roger Elmore, Patricio Grassini, Keith Glewen, and Tom Hoegemeyer. Corn, Soybean Planting Considerations for this Week’s Cold Snap, University of Nebraska Extension CropWatch. April 24, 2017.

Specht, Jim, Jenny Rees, Roger Elmore, Nathan Mueller, Keith Glewen. Considerations when Planting Soybean Early, University of Nebraska Extension CropWatch, April 25, 2019.

Tenorio, Fatima Amor, Patricio Grassini, Jenny Rees, Keith Glewen, Nathan Mueller, Laura Thompson, and Jim Specht. April 2016. Early Bird Gets the Worm: Benefits of Early Soybean Planting, University of Nebraska Extension CropWatch. April 20, 2016

Tully, R.E. et al. 1981. Crop Science 21:312-317.