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David Moseley, LSU AgCenter Soybean Specialist
The Louisiana soybean crop has been severely affected by extreme heat and drought stress. Factors such as availability of irrigation, planting date, and soil type have all had a significant impact on the condition of plants.
In some irrigated fields, plants that did not receive water matured early with little to no marketable seed. Figure 1 shows three irrigated fields in the central region. The left and middle pictures show fields with either a center pivot or furrow irrigation system. In both fields, the plants outside of the irrigation water matured early with small seeds (Figures 1, 2, and 3). The picture on the right also shows a field with furrow irrigation (Figure 1). The height of the beds did not allow the irrigation water to reach the middle soybean rows, which created droughty conditions.
Figure 1. Soybean plants in irrigation production systems were showing signs of drought stress where the water did not reach.
Figure 2. Two soybean plants from the same field. The irrigation water did not reach the plant shown on the left.
Figure 3. Two pods from the same field. The pod on the right originated from a plant that did not receive irrigation water.
The extreme heat also caused stress in irrigated fields where some plants matured early with little to no marketable seed (Figure 4). The pictures show increased heat stress either between different sections of fields, between different rows of fields, or within the same row of a field.
Figure 4. Plants within irrigated fields in Richland Parish show various levels of heat stress and damage. (Left) All plants in the front had a greater effect from heat stress than plants in the back. (Middle) Some rows within a field had a greater effect from heat stress than other rows. (Right) Some plants within a row had a greater effect from heat stress than other plants within the same row.
Figure 5. The soybean plants from the irrigated fields shown in figure 4 expressed necrotic symptoms before maturing early. (Picture from Keith Collins)
On August 11, the USDA-NASS survey projected 1.09 million acres of the 1.12-million-acre soybean crop would be harvested, and the statewide yield would be 49 bushels per acre. This yield projection would be up two bushels from 2022. The survey report stated the projection was considering conditions as of August 1. With only approximately 25% of the crop at least coloring by July 30, the final yield of most of the soybean acres was still subject to heat and drought stress.
The USDA-NASS survey reported the soil moisture level began to decline rapidly after July 16. As of August 13, between 65-69% of the topsoil and subsoil moisture levels were rated as short to very short (Figure 6). The crop condition also declined after July 30. For the week ending on August 13, the USDA-NASS survey indicated 20% of the crop was rated as poor to very poor (Figure 6).
Figure 6. The percentage of the Louisiana soil moisture level rated as very short and short; and the percentage of soybean acres rated as good to excellent and poor to very poor.
According to the Louisiana Agriclimatic Information System, operated by the LSU AgCenter, the daily average temperature at the Dean Lee Research Center from July 27 – August 14 was seven degrees Fahrenheit higher compared to the daily average temperature during 2020-2021. Furthermore, in 2023, the last recorded rainfall at the research station was on July 22. In 2020-2021, the average rainfall between July 23 – August 14 was 2.3 inches. (Figure 7)
Figure 7. The average temperature and total rainfall in 2020-2021 and in 2023 at the Dean Lee Research Station between March 2 and August 14. Note: Available data from the Louisiana Agriclimatic Information System for the Dean Lee Research Station was used to compare to 2023. Data from 2022 was excluded due to excessive rainfall in August.
For more information on drought and heat stress, read the article Soybean Drought and Heat Stress from the LSU AgCenter Louisiana Crops Newsletter Volume 12, Issue 6 – July 2022.
