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 Drought Stress in Soybean Minimize
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Posted by: WCM Staff 8/9/2007 6:56 AM
Moderate to severe drought stress is afflicting much of Wisconsin’s soybean crop.

Drought Stress in Soybean

Shawn P. Conley  and John M. Gaska, State Soybean Extension Specialist and Soybean Outreach Specialist

 
Moderate to severe drought stress is afflicting much of Wisconsin’s soybean crop. As of 8/06/07 soybean across the state is developmentally in the R5 (beginning seed) to R6 (full seed) growth stages. Though weekend rains were beneficial to seed fill, no new pods will be formed as flowering in soybean has ceased. Below is a brief discussion regarding the impact of drought stress on soybean.   
 
In soybean there are two growth periods for which soil moisture is critical for optimum growth and development: at planting and during the reproductive stages from bloom through pod fill. The time period from stand establishment to bloom is not as critical. Drought stress during this time period will often shorten internodes; however yield loss rarely occurs (Image 1). In Wisconsin the main reproductive growth in soybean occurs from early July to mid-September. Soybean in this stage use about 1/4 to 1/3 inches of water per day. Lack of sufficient water can cause flowers and young pods to abort reducing the number of seeds per plant (Image 2). Also, soybean plants reduce the size of their leaf pore openings to reduce the loss of water vapor. This also reduces the intake of carbon dioxide and the manufacturing of photosynthates which slows plant growth. When normal soil moisture returns, normal growth is resumed. This ability to reduce metabolic activity allows plants to tolerate dry spells without dying or harming their ability to resume growth when normal moisture returns.
 
In most years, water is not a major factor limiting the yield of soybean on medium and fine textured soils in Wisconsin. Research conducted between 1996 and 2000 at the Arlington Research Station showed no yield difference between irrigated and non-irrigated soybeans. However there was significantly more biomass (total plant weight) per acre in the soybeans that received regular irrigation. The extra biomass was concentrated in the leaf and stem portion of the plants, and not in the seeds.
 
Managing soybeans for drought tolerance involves using the same sound growing practices that would normally be used for high yields. Soil fertility, especially pH levels are important for good root growth and proper nodulation. Low soil pH inhibits nodulation and uptake of essential micronutrients which make soybeans more susceptible to drought injury. Healthy soybean plants will also have deep root growth which enables to plant to take advantage of deeper moisture supplies.
 
Where hardpans or compacted zones are a problem, deep tillage should be used to break these up and allow root growth into subsoil moisture. Conservation tillage can help the crop withstand the effects of drought by providing residue cover to reduce soil moisture evaporation. Long term conservation tillage also improves soil tilth which helps rainfall infiltration and water movement.
 
Finally, narrow row spacings should be used since the canopy formed by the plants increases competition with weeds and acts as a barrier to evaporative soil moisture losses.
 
If drought has severely affected pod set and seed fill, and if livestock feed is needed, soybeans can be harvested as a forage for ensiling. Highest protein and yields are obtained from soybean harvested at the R6 to R7 growth stage. Harvesting soybeans for forage between the R1 and R5 stage will result in very high quality silage, but dry matter yields will be reduced significantly. Forage quality will be reduced from R5 soybean forward if a conditioning process is used during harvest. Conditioning will cause significant seed shattering.
 
References:
Managing Drought-Stressed Soybeans in the Southeast. North Carolina Cooperative Extension Service. 1999. Virginia Soybean Update. Virginia Agricultural Experiment Station. Volume 2, No. 4, July 1999.
 
 

Image 1. Soybean height reduced by drought stress. Soybean plants near Chippewa Falls Wisconsin averaged 15” in height at R5/R6 soybean. 

  

Image 2. Pod abortion in soybean caused by drought stress.
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