Agricultural News
Current Weather Conditions Causing Drought Stress on Soybean Crop
Tue, 28 Jun 2011 08:40:44 CDT
With the recent string of hot and dry days the 2011 Oklahoma soybean crop has started to show signs of drought stress. Some areas have caught timely rains but many need moisture at a critical time of soybean development, blooming and pod fill. Relationships among air temperature, soil water content, and soybean development are complex. The ideal temperature for soybean growth and development is around 86° F. Day temperatures above 95° F, coupled with low humidity, have been shown to reduce seed set and potential seed vigor. Drought symptoms appear early as leaf wilting and reduced growth.
In addition, nodule formation, development and nitrogen fixation are reduced when soil temperatures rise above 90° F. In general, soybeans can tolerate short periods of high temperatures if supplied with adequate moisture but the crop cannot tolerate high temperatures indefinitely. With temperatures routinely running near or above 100° combined with the lack of rainfall, yield potential for the soybean crop is in jeopardy of being lost.
Although the current weather conditions are extreme even for vegetative growth, all is not necessarily lost for June planted soybeans which are in vegetative growth stages. With a return to more favorable weather conditions, these fields still have a change to make reasonable yields. Some of these fields have been planted for only a little over 30 days and have not reached the critical decision stage. Soybeans in the vegetative stage require less water compared to later growth stages. The greatest problem for these fields is where little or no moisture has been received.
Soybean response to current conditions will be decided by the following three things:
1. planting date and crop development state
2. maturity group
3. how long hot and dry conditions persist
Soybean fields in reproductive stages (bloom, pod set, or pod fill) will have increased flower and pod abortion and in later reproductive stages, pods will be small with fewer and smaller (or shriveled) seeds than normally expected. If the crop has been in the blooming stage for 3 to 4 weeks, has set no or very few pods, and drought conditions persist, it is likely that yields will be very low. The crop will not bloom indefinitely and may be unable to wait for a return to favorable weather conditions. If temperatures remain above 95°F and moisture in unavailable, seed set and development most likely will not occur.
Below is a more complete description of stress periods on soybean reproductive stages:
Stage R2: Full Bloom
Soybeans are better able to escape the effects of hot, dry weather conditions than other crops such as corn. Corn flowers over a short period but soybeans produce more flowers than pods and flower over a long period of time. Even under ideal conditions, soybean plants do not form a pod for each flower set. Up to 75% of the flowers or pods produced by a plant may abort. It is the 25% of the blooms that would ordinarily be expected to set pods and further develop but instead continue to abort that is of great concern.
Stress during flowering reduces the length of the flowering period. We could have expected a blooming period of 3 to 4 weeks or so under excellent conditions but that period will be shortened with these conditions. The exact critical high temperature is not documented but there are many observations that temperatures above 95°F result in very little or no pod set. Flowers and small pods are aborted. If the current weather conditions persist, it is possible that no pods at all will be set. If the crop has already been blooming for 3 to 4 weeks, it is very near the end of the blooming period and little or no crop will be set.
Stage R3: Rapid Pod Growth to Stage R4: Full Pod Elongation
Stage R3 is the end of the flowering period and the beginning of rapid pod formation. The bean filling period lasts 3 to 5 weeks. Hot, dry conditions at this time cause greater yield reductions than the same stress earlier in the season (vegetative or flowering stages).
Stage R5: Beans Beginning to Develop
Beans are filling rapidly at this stage. The demand for moisture is great. Moisture and heat stress greatly reduces yields. Stress during rapid pod growth reduces the number of beans per pod and reduces bean size. Pod filling is the most susceptible time for drought injury to the soybean crop.
Stage R6: Beans Full Size
Stress at this time will reduce dry matter accumulation and seed size. Dry matter begins to accumulate in the beans at stage R6 and continues at the same rate through stage R8 (full maturity), about 35 days later. Seed yields are affected by the rate of dry matter accumulation in the seeds and by the length of time that dry matter accumulates. The rate of dry matter accumulation ranges from 60 to 90 pounds per acre per day (1 to 1½ bushels per acre per day). Stress affects both rate and length of time that dry matter is accumulated but the length of time is affected to a greater extent. With drought conditions, a rule of thumb is that soybean yield potential can be decreasing by 1 to 1½ bushels per acre per day.
State R8: Physiological Maturity
Beans are ready to harvest.
Hopefully, we catch some rain soon and get back to near normal temperatures.
Parts of this article were modified from Sholar and Keim. 1998. Effect of drought on soybeans.
Our thanks to Dr. Chad Godsey, Oklahoma State University Assistant Professor, for providing this article through the Extension News. More articles from the Plant and Soil Science Extension News can be found by clicking here.
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