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Assessing Frost Damage in Soybeans

Duane R. Berglund, NDSU Extension Agronomist

Soybean tops are easily damaged by frost in the 30 F to 32 F range. Temperatures under 30 F for any extended period of time can completely kill soybean plants (stems and leaves). Generally speaking, the soybean fields planted to narrow row spacing (6-7 inches to 15 inches), may have slightly more tolerance to light frosts than soybeans planted in wider rows (30-36 inches). Also soybean plant populations that have thin stands are more affected and injured by frost.

The thick plant canopy of the solid-seeded, closely drilled beans tends to hold the soil heat better and protects the lower portion of the plants and developing pods to some degree. Pods in the lower portion of the plant should continue to fill beans and develop normally. Some maturity delay may occur. Some small pods in the upper area of the plants may not fill normally and can abort in some cases.

Beans that are still green and soft will shrivel. Stalks rapidly turn dark green to brown and will not recover. Beans in pods that have turned yellow will mature normally. Some green beans will turn yellow after 30-40 days of storage.

Growers and researchers over the years have tried color keys of yellow soybean leaves, yellow pods and brown pods to estimate soybean maturity and safety from frost. Usually these methods didn't work because of differences in varieties of indications of maturity.

However, studies do show that "yellow" pods sprinkled with brown are the best clue of physiological maturity. It is suggested to open pods and check shrinking of beans and look for separation of beans from the white membrane inside the pod. This indicates the soybeans are physiologically mature and fairly safe from frost injury. All pods do not mature evenly.

It's been noted that if one or two pods on any of the upper four nodes have turned brown in color, and other pods are light yellow to tan, the soybeans are fairly tolerant to a killing frost.

Research information from Wisconsin has shown that all varieties tested had reduced yields when frost occurred at or before R6. Earlier maturing varieties sustained economic yield losses from frost at more advanced growth stages than later maturing varieties. The greatest yield losses occurred when frost occurred at stage R5. The number of beans per plant and reduced bean size all contributed to overall yield loss. Maturity was hastened by some frost treatments and was not delayed in any of the trials studied.

The leaves do remain on the frost damaged soybean plants. Seed moisture may be slightly higher and seed size usually is reduced as the soybeans dry and shrink. A frost will not hurt soybean yields if the soybean growth stage is beyond R7. A frost between R6 and R7 may or may not affect yield, depending on the temperature and duration of the freeze.

In the event of a leaf-killing frost, when pods are still light green or yellow, wait until the pods are mature in color before combining.

The most significant effect of an early frost on soybeans may be in the reduction of quality to use as a future source of seed.

Table 1 shows growth stages and potential yield losses of a killing frost on soybeans.

Table 1.  Percent of yield produced by various soybean growth stages and calendar days between growth stages.

Growth Stage

Days after bloom begins

Days to maturity

Percent of total

Begin Pod (R3)

15

68 --

Full Pod  (R4)

24

 59 --

Begin Seed  (R5)

33

 50 25

Full Seed  (R6)

48

 35 47

Begin Maturity (R7)

73

 10 95

Full Maturity (R8)

83

 0 100

**Note this is  for full, late maturity soybeans in southern Minnesota.

-Source: University of MN  reported at http://www.ag.ndsu.edu/procrop/syb/soymat09.htm

If all leaves on a soybean plant are killed between full seed stage and beginning maturity, 53 percent or less of yield can be lost. A freeze before maturity has less effect on yield the closer the freeze date is to mature date.

Air temperatures of 29 F are necessary to completely kill corn and soybean plants.

If frost is so severe that the majority of the grain yield has been lost, then harvesting for soybean hay should be considered. Check with your insurance agent and FSA-USDA office before harvesting or destroying the frozen crop.

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