August is a critical time in the life of a soybean plant. Although many of the flowers have pollinated, the plants are still setting new flowers and the tiny pods and seeds inside them are beginning to develop. Depending on when they were planted, soybeans are generally in the R3 (beginning pod) or R4 (full pod) growth stage at the beginning of the month. It takes an average of 24 days for soybeans to advance from R4 to the R6 (full seed) stage. This period is the most important time for determining final yield of the field.
The presence of a pod means that the flower has been successfully fertilized, but that doesn’t mean that the pod will be full at harvest time. There are numerous factors that can cause the seed to abort or pod to drop. Stress during pod fill can also cause reduced seed size, which lowers final yield.
One factor that can reduce yield is excessive heat. The ideal daytime temperature for growing soybeans is 85°F. At higher temperatures, the plants become heat stressed. Heat stress while seeds are forming (between R4 and R6 growth stages) is detrimental to yield, as seeds can abort. The average daily high temperature in August in Illinois ranges from the low 80s in the northern part of the state to the high 80s in Southern Illinois. This means that there are plenty of potentially harmful days over the course of the month that can set back yield.
Just as high temperatures can decrease yield, abnormally low temperatures during this critical time in the plant’s lifecycle can also be detrimental. Low night temperatures (upper 40s to low 50s) interfere with the metabolism of the plant, leading to lower photosynthetic rates the following day. After several days in these conditions, pod numbers will be reduced.
Lack of soil moisture in August is also harmful. Drought stress can cause flowers to abort and small pods to drop. The most sensitive pods are those that are newly formed, less than ¼ inch long. There are plenty of pods of that delicate size on the plants in August. In most cases growers are at the mercy of the weather, but in irrigated fields properly timed watering can cool the crop, provide plenty of water and lessen the effect of some stresses.
Soybean growers can’t do much about the weather, but there are a few factors that can be controlled. Disease and insect pressure can drag down yields, and a timely fungicide or insecticide application can help preserve plant health which pays big dividends in some instances. Anything that can be done to decrease stress increases yield potential. Always be sure to read and follow label directions; some fungicides can’t be applied after the soybeans reach the R5 growth stage.
Late season nutrient deficiencies can stress plants and lead to reduced seed set. However, university research has consistently shown little or no benefit from foliar nutrient application. The best way to prevent nutrient deficiencies in soybeans is with adequate fertilization prior to planting.
On average, every increase of one pod per plant across an acre of soybeans is worth about 2 bushels. The flipside of this is that when one pod per plant aborts across an acre, 2 bushels are lost. This means that seemingly insignificant factors can lead to significant yield increases or decreases. However, most of those factors are out of the grower’s control. Try not to worry about the weather and scout your fields to see if an insecticide or fungicide application is warranted. As a producer, your objective is to set a healthy plant and protect it so that it can handle any stresses nature throws at it.