David Moseley, LSU AgCenter Soybean Specialist

Article Highlights:

• The Louisiana soybean planting season began early in 2023.
• Depending on several possible factors, some soybean fields survived a freeze on March 20^th, but others were completely lost.

The USDA-NASS survey reported that two percent of the Louisiana soybean crop was planted by March 19, 2023. The warm and dry weather during early March allowed producers to plant their corn crop early and then begin planting soybean. Unfortunately, on March 20^th, the temperature dropped to approximately 28 degrees Fahrenheit in Northeast Louisiana. Soybean tissue death can occur if the temperature falls to 28 degrees Fahrenheit for more than four hours, especially if the unifoliate leaves are exposed (Vann and Stokes, 2020).

On March 21^st and over the following few weeks, the condition of three fields was documented. Two fields were at the LSU AgCenter Northeast Research Station and the other field was near Clayton, LA. The planting dates were March 1, 15, and 6, respectively. For the March 1^st and 6^th planting dates, with only 30 miles between the two fields, almost 100 percent of the soybean plants at the Northeast research station were killed, but approximately 50 percent of the soybean plants at the field near Clayton, LA survived. One explanation could be that the plants near Clayton, LA were less developed as they were planted five days later. At least 50 percent of the unifoliate leaves were likely still insulated by the cotyledons. The soybean in the field planted on March 15^th at the Northeast Research Station was not emerged on March 20^th and survived the freeze.

The producer near Clayton, LA planted 160,000 seeds per acre. With an approximate 50 percent survival rate, the final stand was approximately 80,000 plants per acre. According to data from the LSU AgCenter Dean Lee Research Station in 2020, a final stand of approximately 61,000 plants per acre may result in similar yield as a stand of over 100,000 plants per acre. Most University soybean agronomist would agree that 70,000 – 75,000 plants per acre that are healthy and uniformly distributed will allow approximately 95 percent of the full yield potential as compared to a more full stand.

Figure 1. Soybean seedling damage on March 21, 2023. The seeds were planted on March 6^th and were in the VE growth stage (the cotyledons had emerged past the soil surface, but the unifoliate leaves had not completely unrolled and in some cases the growing point was still protected by the cotyledons) when the freeze hit Louisiana on March 20^th. The cotyledons had suffered damage from the freeze, and it was unknown if the growing point had been killed.

Figure 2. On March 27, 2023, the original main stem on some plants had ceased growing after the apical meristem was killed during the freeze. However, after the apical meristem was killed, two axillary branches had begun to develop trifoliolate leaves from the cotyledon node.

Figure 3. By April 5, 2023, both auxiliary branches had developed two open trifoliolate leaves and had another trifoliolate leaf beginning to open on each branch. Both axillary branches can fully develop and replace the original main stem.

Figure 4. It was observed on April 5, 2023 that the apical meristem was not damaged on most plants. The soybean plants were progressing normally and had reached the V2 growth stage (two open trifoliolate leaves). The plants had reached the V2 growth stage approximately 30 days after planting which is within the normal average duration between planting and V2.

Figure 5. The soybean seed was planted on 7.5 inch spacing with a seeding rate of 160,000 plants per acre. After the March 20, 2023 freeze, the final stand count on April 5 was estimated to be 80,000 plants per acre which can still support close to full yield potential of a field with a full stand.

Reference:

Vann, R., D. Stokes. 2020. Soybean Cold Damage. North Carolina State Extension.