1998 Drought Information - Soybean

Effects of Drought on Soybeans

Ron Sholar and Kent Keim
OSU Department of Plant and Soil Sciences



THE CURRENT drought is having a profoundly negative effect on Oklahoma row crops. While no crop is escaping the grip of this weather, irrigation where available is a benefit. However, even with irrigation, the extremely high temperatures are limiting crop yield potential.

Relationships among temperature, water, 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. 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 above 95 combined with the lack of rainfall, yield potential for the soybean crop is being lost.

The greatest concern for growers is whether soybeans still have a chance to make a crop or if serious consideration should be given to cutting the soybeans for hay. The ability of the soybean crop to tolerate these drought conditions depends on:

    a. planting date and crop development state
    b. maturity group
    c. how long drought conditions persist

Making a decision on whether to stay with this crop or cut it for hay requires an accurate assessment of the situation.

QUESTION - SHOULD THIS SOYBEAN CROP BE CUT FOR HAY NOW?

SHORT ANSWER FOR TWO SITUATIONS:

Crop in Vegetative State - June and early July planted soybeans which are in vegetative or early bloom stages still have a chance to make reasonably acceptable yields. Yields will be greatly influenced by a return to more favorable weather conditions.

Many 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 are hardy. However, growers must weigh the likelihood that weather conditions will improve.

Crop in Reproductive Stages - Under drought conditions, soybeans in early reproductive stages 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. As long as temperatures remain above 95F and moisture in unavailable, seed set and development will not occur.

The following is a more detailed explanation of factors to consider in determining how to handle the soybean crop.

A. Fields in Vegetative Stages

Although the current weather conditions are extreme even for vegetative growth, all is not necessarily lost for June or early July planted soybeans which are in vegetative or early bloom stages. With a return to more favorable weather conditions, these fields still have a change to make reasonably acceptable 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. With irrigation, these fields still are holding up very well. Cooler temperatures and moisture will be required to set a crop when these fields move into reproductive stages.

B. Fields in Flowering, Pod Set, and Pod Filling Stages

Moisture stress at every stage of plant development can reduce seed yields but the extent of yield reduction from water stress varies with stage development.

Fields planted in April and early May should be in pod filling stages and weather conditions are critical for further development. Except in northeast Oklahoma where rainfall has been somewhat normal, many fields of early planted soybeans have very few pods. Some fields have no pods set at all and have reached the end of a blooming period. The extreme temperatures and dry conditions are taking a tremendous toll on this crop. A temperature of around 86 F is desirable and temperatures have been running from 10 to 15 higher than that. Growers thinking about taking a hay crop rather than risk waiting for more favorable conditions should assess crop growth stage as they make management decisions.

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. This decreases the possibility of total crop loss from a single drought period during the reproductive period. Oklahoma is suffering from a lengthy drought and the soybean crop will not bloom immediately.

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 95F 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 1 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 1 bushels per acre per day.

State R8: Physiological Maturity

Beans are ready to harvest. Soybeans in Oklahoma have not yet reached this stage.



Oklahoma Cooperative Extension Service
Division of Agricultural Sciences and Natural Resources
Oklahoma State University