Do I always need to replant soybeans after a frost event?

Patience is the farmer’s biggest virtue when it comes to making replanting decisions due to freeze.

Emerged soybean
Emerged soybean after the May 9, 2020, frost event. Photo by Ricardo Costa, MSU Extension.

In 2020, spring officially started on March 19 but for many regions across southeast Michigan through mid-May, that didn’t translate to warmer temperatures. According to the Michigan State University Enviroweather station in Deerfield/Blissfield, prior to the recent warmup there were only five days since the beginning of this years’ equinox where average temperature raised above 60 degrees Fahrenheit.

Everybody could agree that a higher number of warm days would be helpful for farmers planning to put their soybeans in the ground. It is also worth noting that according to the weekly Michigan Crop Report, 2020 is the year where the highest percentage of soybeans (35%) have been planted by the second week of May when compared to reports from 2015 to 2019.

While Mike Staton, MSU Extension soybean educator, states that early soybean planting can increase yields due to the increase in the number of pods and seeds per plant and the larger canopy built by early planted soybeans when compared to later-planted ones, no one could expect that on May 9 Michiganders in southeast Michigan would experience below-freezing temperatures (as low as 21.2 F) lasting for over 7 hours.

Frost damage can occur to soybeans when exposed to freezing or below-freezing temperatures. The critical temperature for freeze injury to emerged soybeans is 28 F. Temperatures at or below 28 F for more than a few hours at a time can cause plant death. While plant death after the occurrence of below-freezing temperatures is a possibility, that is not always the case.

Taking some time to scout your fields after a freezing event is a must. Check the health situation of the plants. Water-soaked lesions on the cotyledons, leaves and hypocotyl are signs of freezing. These parts will most likely dry and turn brown after several days.

Staton’s recommendation is when inspecting the plants, look for signs of new growth at the main growing point or the axillary buds located at the base of the cotyledons. If no new growth is seen after 10 to 12 days, the plants are most likely not viable and should be considered dead. Soybeans that are just emerging are the most sensitive to freezing weather, while the not emerged have a better chance to survive.

Still, finding damaged or dead plants after a freeze event is not what will tell you if replanting is necessary. The most important thing is to take stand counts to find out the plant population after the freeze event. That is why patience is so important. There is no way to find out what your final plant population will be right after a frost event. It will take days to see how the affected plants will react.

After you determine the outcome of the affected plants, you will need to take at least 10 random stand count samples from different areas of each field and calculate the average. According to Staton, you will need to count the number of emerged seedlings in a length of row equal to 1/1,000 of an acre. The length of the row you will use will depend on the row spacing (Table 1).

Table 1. Length of row required to equal 1/1,000 of an acre.

Row width

Row length

30 inches

17 feet 5 inches

20 inches

26 feet 2 inches

15 inches

34 feet 10 inches

10 inches

52 feet 3 inches

7.5 inches

69 feet 7 inches

For example, to estimate the number of soybean plants per acre in 7.5-inch rows, count the number of plants in 69 feet 7 inches of row at 10 random locations in the field. Simply multiply the average count for the 10 locations by 1,000 to get plants per acre.

If your seeding was 120,000 seeds per acre and after your stand counts you realized that only 80,000 plants emerged, you shouldn’t jump inside of your tractor cab to start replanting just yet. Data from 48 planting rate trials conducted in Michigan from 2015 to 2019 shows that stands of 60,000 to 70,000 plants per acre can produce high yields. The fact that thin stands can still produce high yields, and that soybean yields decrease by 0.3 to 0.4 of a bushel per acre per day when planting after the first week of May, should be considered before making any replanting decision.

On May 20, after assessing two soybeans fields located in southeast Michigan, it was observed that soybeans were not presenting any signs of freeze damage. Even though plant population was lower than expected, both places showed an average plant population superior to the minimum of 60,000 to 70,000 plants per acre necessary to produce high yields. On top of that, with the low temperatures that followed the freeze, there is also the possibility that not all the seeds have germinated yet. So, the final plant population could be even higher and as a result replanting was not recommended for either field. You can watch the assessment video below.

In summary, if your fields have more than 80,000 plants per acre, replanting is not advisable. Just remember that replanting has costs, and in the end, your final goal is always to get the maximum returns with the minimum use of inputs.

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