Spring 2014 has been quite challenging with wet soil and cold temperatures. We’ve received several calls and e-mails regarding soybean seedling damage (from those who have actually been able to plant). It appears that some soybean fields were hit with a “trifecta” of stress issues. What should we be looking for in terms of frost, PPO herbicide injury, and disease?
Frost can occur at air temperatures between 32 to 36 degrees F while a freeze requires temperatures less than 32 degrees. From the weather records I have looked over, air temperature dropped as low as 34 degrees F in northern Ohio on May 16. Soybean plants should be assessed for frost damage at least five days after suspected injury to inspect for regrowth. If damage occurs above the cotyledons, the plant will likely recover. If damage occurs below the cotyledons, the plant will not recover. Look for a discolored hypocotyl (the “crook” of the soybean that first emerges from the ground) which indicates that damage occurred below the cotyledons. If soybeans were not yet emerged at the time of the frost, they should be fine. I did observe some frost damage last week in west central Ohio; however, just the tips of the cotyledons were damaged, so a full recovery is expected.
Many of the situations we were made aware of involved the use of flumioxazin (Valor) products, which are widely used in the state, along with sulfentrazone (Authority) products. Both of these PPO-inhibiting (group 14) active ingredients can injure soybeans, but primarily when applied after planting. Injury is most likely when the herbicide is applied closer to the time of soybean emergence and rain occurs during this period also. This is the reason for the statement on flumioxazin product labels specifying that the herbicide can be applied only up to two days after planting. Even this restriction does not completely prevent injury, since we have observed it in our research plots from applications on the day of planting. Most of the residual soybean herbicides used in Ohio are applied a week or more before planting with burndown herbicides, which greatly reduces the risk of injury.
Symptoms of soybean injury from soil-applied group 14 herbicides include necrosis of cotyledons and early leaves, distorted leaf growth, and reduced rate of growth (stunting). The severe injury and death of plants that has been observed in some fields would be atypical of this type of herbicide injury. However, the herbicides were applied after planting in these fields followed by substantial rain and cold, so herbicides could be a contributing factor. This situation has been observed in other soybean-growing areas, and addressed in university newsletters and blogs. Two examples:
“Soybean Injury from Soil-applied Herbicides” U of Illinois, http://bulletin.ipm.illinois.edu/?p=2183
Soybean Seedling Emergence Issues: Environmental Stress Compounding Factors, Mississippi State http://www.mississippi-crops.com/2014/05/06/soybean-seedling-emergence-issues-environmental-stress-compounding-factors/
Heavy rains on poorly drained soils will provide the perfect environment for Pythium spp. Add to this situation injured soybeans, several other types of fungi will move in and take advantage as well. We were able to receive plants from several of the affected fields, and the pathogens are present. Pythium as well as Fusarium are the two that we recovered from affected seedlings. Seedlings with soft brown to dark tissue will not recover. The lesions will continue to grow. Seedlings with some small lesions will recover but may not perform to the fullest potential through the season.
Optimum Soybean Stand
During the past ten years, the AgCrops Team has conducted several seeding rate trials which are summarized below. Generally, soybean yield is maximized when there is at least 116,000 plants/acre at harvest when planting in May. When planting in June, generally at least 155,000 plants/acre is necessary at harvest to maximize yield. To quickly estimate stand, count the number of plants in 69’8” of row for 7.5 inch row spacing, 34’10” for 15 inch row spacing, or 17’5” of row for 30 inch row spacing. These counts represent 1/1000th of an acre (i.e., 120 plants in 69’8” of row in 7.5 inch row spacing = 120,000 plants/acre). Also, keep in mind soybean yield is decreased by approximately half a bushel per acre every day when planting later than mid-May. When considering replanting soybean, make sure to take into account existing stand, yield loss due to late planting, and the cost of additional seed (we recommend higher seeding rates when planting in June).