Most of the soybean acreage in South Carolina is now planted and some of the earlier soybeans are beginning to enter reproductive development. Again this year, much of South Carolina has experienced some period of drought over the first five months of the year which resulted in replanting of some acreage because of emergence problems associated with lack of moisture. Some areas of the state most affected by early-season drought received rainfall from mid to late June whereas other areas have yet to receive an appreciable quantity of rainfall. Many producers planted soybeans in dry soil, hoping that preceding rainfall would ensure emergence. The prolonged drought resulted in poor emergence for soybeans planted during the early portion of the planting season. Research has shown soybean emergence is "poor" when seed remain in the soil for 12 or more days after having imbibed water but not having enough moisture to allow radicle elongation. Under these conditions, seedling emergence also decreases as soil temperature increases.
Stand establishment and early growth of the crop were hampered by dry soils and extremely high daytime temperatures, especially in May. There are two periods during the growing season that moisture is crucial to the success of soybean and other crops. The first is at planting and the second during reproductive development. Following stand establishment, soybean can tolerate a high degree of moisture stress during vegetative development. Conversely, soybean is very sensitive to moisture stress during the reproductive period. For dryland soybean, weekly rainfalls are needed throughout flowering and podfill in order to obtain above average yields. Stress during reproductive development leads to pod shed and reduced seed size which in turn has a direct impact on seed yield. There are several ways that the effects of drought can be reduced. The following are a few recommendations for South Carolina soybean farmers to consider as they plan management strategies for the remainder of the season and additional suggestions for upcoming years:
1) Late-season weeds can further rob yields and profits. There are several postemergence herbicides that can provide good control, if the weeds and crop are not significantly stressed (e.g., Pinnacle for 8- to 10-inch pigweed). If farmers are using Roundup Ready varieties, the addition of First Rate to Roundup Ultra will assist with morningglory control. Refer to the 2000 Pest Management Handbook for control recommendations, crop and weed growth stages for application, and efficacy ratings for soybean herbicides.
2) Insects can be more difficult to control under drought conditions. This is due to the reduced occurrence of beneficial insects and diseases of destructive insects. Begin scouting each soybean field at first bloom and repeat every five to seven days. Economic thresholds for every destructive insect are listed in the Clemson Pest Management Handbook. Remember, after first bloom the crop can tolerate a 15 to 20 percent foliage loss without losing yield potential. For pod feeders, the thresholds are two large corn earworms or one stinkbug per row foot in wide rows. When scouting drilled soybeans, use sweep nets and count the average number of insects per 10 sweeps in 4 different spots in each field. The threshold for stinkbug is 2, while for corn earworm, it's 4 per 10 sweeps.
3) Nematodes, nutrient deficiencies, and other problems can be masked by drought. Farmers are urged to walk their fields and look for problem spots that don't appear to have a weather-related cause, and obtain soil, tissue, or whole plant samples from both "good" and "bad" spots and have agents send the samples to either the Plant Problem Clinic or to the appropriate lab at Clemson for analysis.
4) Drought during reproductive development can sometimes be avoided, especially when planting some of the later maturing varieties. It is recommended to plant a range of maturity groups (late V to early VIII) with more percentage of the acreage planted to the later maturing varieties. A range of maturity groups can spread podfill over seven to eight weeks, increasing the likelihood that some soybeans will receive adequate rainfall during this critical period. Currently, research is being conducted by other universities in the southeast to identify soybean lines with a high degree of tolerance to drought, and breed this tolerance into some of the higher yielding varieties. However, this drought tolerance may only result in slight increases in yields and does not replace the need of water. Further information on variety selection can be found in Soybean Leaflet No. 8, Selecting Soybean Varieties
5) Collect soil samples for nutrients and nematodes in the fall following harvest. Applying nutrients according to soil test recommendations will result in a healthier crop which in turn promotes better root growth and increased tolerance to drought. Nematodes are commonly found in soybean fields throughout South Carolina. Nematode populations may be sufficient to severely stress plants in some areas of a field. Stress resulting from nematodes and drought may be magnified when nematode susceptible varieties are grown. Therefore, if fields are not tested for nematodes, one should assume that nematodes are present within a field. If tests are conducted and nematodes found, a management program should be established to reduce the potential for yield loss as a result of multiple stresses from nematodes and drought. Rotation systems for managing soybean cyst nematode can be found in the South Carolina Soybean Production Guide. Other information addressing nematode management can found in Soybean Leaflet No. 2, Soybean Nematode Control
6) Use some form of deep tillage prior to or at planting. Conservation tillage is one way in which moisture can be conserved. Most soils in the Coastal Plains of South Carolina have a naturally occurring E horizon (hardpan) that prevents deep root rooting. The weight of most tractors and equipment compacts the upper portion of the soil, preventing adequate soybean growth and speeding up the onset of drought and stand loss associated with stress during drought. Deep tillage increases the rooting depth and volume from which nutrients and soil moisture can be extracted. With frequent disking prior to planting or cultivation following soybean emergence, the soil surface experiences a high rate of evaporative loss. Another benefit of surface residue and reduced tillage includes greater infiltration of rainfall, resulting in greater water storage following periods of heavy rainfall.
Additional drought related information is available in Soybean Leaflet No. 11, Managing Drought Stressed Soybeans in the Southeast. There are extensive discussions of soybean growth stages and other management pointers in this publication.
Prepared by Dr. Jason Norsworthy with excerpts from an earlier version prepared by Dr. Jim Palmer.
Last updated 7/7/00