Soybeans And Hot, Dry Weather

The continued hot, dry conditions in the southern portion of the province (north shore of Lake Erie) have not only had an affect on corn but it has also begun to impact soybeans. The full extent of the hot, dry conditions on soybean development in these regions will not be known for a few more weeks. How the extended hot, dry conditions will influence soybean development will also be affected by other factors such as cultivar, Maturity Group, plant population, spacing, and planting date.

Visual symptoms on soybeans?

Soybeans are well adapted to deal with fluctuations in temperature and moisture. In fact soybean growth and development is usually optimized when temperatures are in the mid 80's. The concern arises when temperatures remain above 90°F for extended periods of time since soybeans cannot tolerate high temperatures indefinitely. Unofficially there has already been 25-28 days above 30°C this summer. High day-time temperatures can result in leaf sun scald, flower abortion, reduce pod set, reduce seed size and even affect the seed vigour potential of the seed. Nodule formation, development and nitrogen fixation ability are all reduced when soil temperatures are above 90° F.

The effect on soybeans is increased further when dry or drought conditions coincide with high temperatures. One of the first obvious signs of dry conditions is leaf wilting and reduced growth.. In some fields the hot, dry conditions have may already have impacted the soybean varieties yield potential.

Impact On Growth Stage

Although extended hot, dry conditions can influence yield at any stage of development, it is the reproductive stages that are most susceptible. In the vegetative growth stage soybeans are rather hardy and if the environmental conditions become more favourable the impact on yield will be minimal.

Soybean varieties grown in Ontario and the northern U.S. are referred to as having an indeterminate growth habitat whereas southern U.S. varieties have a determinate growth. What does this mean? The indeterminate cultivars flower from the base of the plant upward as the plant grows vegetatively as compared to determinate cultivars which flower after vegetative growth stops. Therefore, soybean varieties adapted for Ontario have an overlapping vegetative and reproductive growth periods. The result is that indeterminate varieties have an extended flowering period. This extended flowering period gives us added built-in protection or insurance against stressed conditions.

Another soybean developmental trait that gives us protection from periods of stress is the over production of flowers and pods. In most cases 60 to 75% of all soybean flowers either abort or don't make yield. Yield that may be lost due to flower abortion early in the reproductive stages can be compensated for by flowers and pods that will be produced later in the reproductive stages. In addition, stress at these stages may result in an increase in the number of beans per pod and weight per bean, which also help compensate for the aborted flowers and young pods

Although the hot, dry weather has had an impact on soybeans in the Lake Erie north shore region it may not have a dramatic impact on yield if more favourable conditions (rain) come soon. The concern is if the 25% to 35% of the blooms that would ordinarily set pods and further develop begin to abort because of continued hot, dry weather or the soybean plants become more susceptible to insect and disease pressure which may already be starting (see articles on soybean diseases and mites in this issue).

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