LA Soybean Crop Progress and Condition Week of July 6, 2020

As of July 5, the USDA rated the 2020 soybean crop as 87% good – excellent, five percentage points higher than the previous week (Figure 1). For the second straight week, the good – excellent rating is the highest of the 2020 season. There have been reports of heavy rainfall in some parts of the state in the last few days. The core-block trial in the Natchitoches parish was flooded in some areas on July 7 (Figure 2).

Statewide, 86% of the soybean crop is blooming with 45% setting pods. At the Dean Lee Research Station, we are evaluating the development of soybean varieties of different maturity groups across six planting dates. The planting dates were: March 19, March 30, April 21, May 11, June 1, and June 16. Some of the MG 3 varieties in the March 19 planting date reached the R6 growth stage as early as June 24; and as of July 1, there were two varieties between MG 4.5-4.9 that have also reached the R6 growth stage. The varieties in the May 11 planting date are between the R3 and R4 growth stages. For the June 1 planting date, the early MG varieties reached the R1 growth stage on July 2.

Plants are considered in the R6 growth stage when the immature seed (still green) fills the pod cavity at one of the four uppermost nodes with a fully developed leaf on the main stem (Figure 3). Plants at the R6 growth stage are entering the point of physiological maturity and the leaves will begin to senesce by the R7 growth stage. The R3 (beginning pod) growth stage is reached when a pod on one of the four uppermost nodes with a fully developed leaf on the main stem measures 3/16 of an inch (Figure 4). Plants are in the beginning bloom (R1) growth stage when there is one open flower at any node along the main stem (figure 5). Keeping track of the reproductive growth stages is critical for applications such as irrigation, supplemental fertilization, and fungicides.

Figure 1. As of July 5, the 2020 soybean crop was rated as 87% good – excellent.

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Soybean Response to Flooding

The recent rainstorms have brought heavy precipitation to some soybean fields in Louisiana. The core-block demonstration trial in Natchitoches parish has standing water that covers part of the plants, especially in low-lying areas. The damage to soybean plants due to flooding depends on several factors including the plant’s growth stage, the duration of the flood, and other environmental conditions.

The long-term effect of a flood is difficult to predict because the following environmental factors is unknown. In General, reports suggest the reproductive stages, and possibly early vegetative stages, are more sensitive to flooding. Depending on the ambient and soil temperatures, soybean plants may be able to resist damage in flooded conditions for 2 – 3 days. The soil oxygen concentration will begin to drop in sustained flooded and saturated soils. In sunny and high temperature conditions, the respiration rate of the plants and soil microbes will increase, further depleting the soil oxygen concentration. If debris is covering soybean leaves after a flood, the yield can be reduced due to a decrease in photosynthesis.

If a field is flooded, there are a few items to consider. If possible, clear any obstructions in areas such as water furrows or ditches to promote surface drainage. However, to prevent soil compaction, it is good to limit unnecessary traffic on saturated fields. When possible, reduce applications that can burn leaves. Continue to protect the yield potential by scouting for pest and evaluate activity of the nitrogen-fixing nodules. A sustained flood can reduce the activity of nodules; however, the activity can recover after the flooded or saturated conditions subside.

Figure 2. The core-block demonstration trial in Natchitoches parish with standing water that covers part of the plants
(Photo taken by Assistant Extension Agent Randall Mallette from the Natchitoches parish).

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Figure 3. A soybean pod with immature seed that fill the pod cavity.

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Figure 4. A soybean pod measuring 3/16 of an inch to demonstrate the beginning pod (R3) growth stage.
(Photo taken by soybean agronomy associate John Upton from the Dean Lee Research Center).

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Figure 5. A soybean plant with an open flower on the main stem to demonstrate the beginning bloom (R1) growth stage.
(Photo taken by soybean agronomy associate John Upton from the Dean Lee Research Center).

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7/13/2020 1:08:06 PM
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