Early-Season Flooding and Soybean Survival
Sunday night areas of south central and eastern Nebraska received more than 5 inches of rain. Soils became saturated, resulting in flooding and ponding. Other areas of Nebraska received little if any rain and need some.
According to the National Agricultural Statistics Service Nebraska Crop Progress and Condition Report, 36% of the state's soybean crop had been planted as of May 11. That is up from 11% as of May 4. Conditions last week encouraged planting and most finished up planting corn.
We know that prolonged flooding affects plant growth as a result of inadequate oxygen to sustain root respiration and growth. Excessive water also is detrimental to soybean nodulation formation and function. The amount of injury depends on variety, growth stage, duration of waterlogging, soil texture, fertility levels, and diseases present; but it is clear that the longer soil is saturated, the greater the impact on growth and yield.
A good share of Nebraska's planted soybeans likely have not emerged. What are the impacts of flooding on germinating seed? Unfortunately, few research reports discuss the impact of flooding on germinating seeds and emerging seedlings. Wuebker et al., 2001, discussed and investigated early-season soaking effects on seeds and seedlings. Their growth chamber study showed that the timing of flooding relative to the days after imbibition affected germination. They found:
- Soaking seed beginning one day after imbibition reduced germination by more than 20%, but there was little difference in germination among soaking times (ranging from 1 to 48 hours).
- When soaking began two or three days after imbibition, germination was reduced — by 50% to 70%, respectively — with 48 hours of soaking. Soaking seed for 1 to 24 hours beginning two to three days after imbibition reduced germination by 25% to 33%, respectively.
- Other sources cited by Wuebker et al. stated that most soybean varieties subjected to four days of soaking prior to germination had severe germination losses; thus, varieties may differ in their responses to flooding.
- Temperature also affected responses. Seed soaked from 1 to 48 hours at 59ᴏ F reduced germination by an average 39% less than the control. Germination was similar no matter how long the seed was soaked. However, when seed soaked for 48 hours at 77ᴏ F, germination fell over 60%; when seed soaked for 1 to 24 hours at 77ᴏ F, germination averaged 27% less than the control. Cooler temperatures improve soybean viability.
Early-Vegetative Development Stages
Impacts of flooding during early-vegetative development stages of soybean are better documented, as noted in an earlier CropWatch article based on a KSU news release:
"Vegetative growth stages. Excess water during vegetative stages usually causes less injury than waterlogging during the reproductive and grain filling stages. Short-term waterlogging (two to three days) at the V2 to V4 stages can cause yield reductions of 0 to 50%, depending on soil texture, variety, and subsequent weather. Yield reductions from waterlogging during the early vegetative stages have been attributed to reduced plant population and shorter plants with reduced branching and fewer pods per plant. As the duration of soil saturation increases, researchers have documented greater reductions in population, height, pods per plant, yield, and leaf tissue nitrogen…"
Other scientists compared soybeans subjected to waterlogging for seven days at development stages ranging between V2 and R 6.3 in a growth chamber (Linkemer et al., 1998). The most sensitive stages in terms of yield reductions were V2, R1, R3, and R5. Yield losses at V2 were 29% below the control.
As with corn, decisions on replanting are not easy.
First, assess the stand by counting viable plants. Dig in gaps to determine if seeds are viable. Fortunately soybeans compensate when planted at low populations. Use data from the last week's CropWatch article to help make replant decisions based on remaining population.
Second, consider the date when you might be able to plant. Use the guideline discussed in the previous CropWatch on soybean planting dates: Yields decrease ¼ to 5/8 bushel each day planting is delayed in May.
Waterlogging Effects on Growth and Yield Components in Late-Planted Soybean by Geoffrey Linkemer, James E. Board, and Mary E. Musgrave. 1998. Crop Science. 38:1576-1584. (Growth chamber flooding at V2, V3, V7, R1, R3, R5, R6).
Wuebker, Eileen Feilmeier, Russell E. Mullen, and Kenneth Koehler. 2001. Flooding and Temperature Effects on Soybean Germination. Crop Science. 41:1857-1861.
Extension Cropping Systems Agronomist
UNL Professor of Agronomy and Horticulture