Soybeans in some areas of the state have experienced stress due to below-average rainfall and a lack of subsoil moisture this growing season. Will drought stress such as this--early in the growing season--influence yield if precipitation levels approach average later in the season, say in July or August?
"Winter, spring and early summer precipitation this year has been below the long-term average, and soybeans in some areas of the state are experiencing periodic water stress due to inadequate available water content in the upper soil depths," notes Andy Lenssen, Iowa State University Extension soybean agronomist. "Farmers have asked whether drought stress early in the season can influence yield if precipitation levels approach the long-term average later in the season." To answer that question, Lenssen provides the following explanation of what happens.
The soybean plant is susceptible to yield loss from water deficit or drought stress at two key stages--germination and reproduction-seed development. Soybean must imbibe about 50% of its weight in water to germinate and begin to develop the radicle and hypocotyl, the primary root and shoot tissues. Seed planted into dry soil, or not placed into the soil, will be unable to imbibe water at all until adequate precipitation has occurred.
Soybeans can start to be affected by moisture deficit soon after germination
Soybeans can respond to water deficit as early as two days after germination. Water deficit at this time results in poor hypocotyl elongation, while root elongation may be unaffected. Drought stress at later vegetative stages of development has similar results: shoot growth is decreased or stopped, but roots can continue to grow.
This evolutionary response in soybeans allows the plant to search for additional soil water while having an overall low water use rate. Assuming adequate rainfall occurs again, soybeans have the ability to reinitiate shoot growth, and the shoot growth rate may be greater than that observed prior to the onset of drought stress. This is called compensatory growth.
Short-term, moderate drought stress during vegetative growth stages generally does not impact soybean yield. Conversely, longer-term severe drought stress can cause irreversible plant cell death causing low growth yield.
Soybean yield is most sensitive to moisture deficits during reproductive stage
Soybean yield is most sensitive to water deficits during reproduction. Soil water deficits during the reproductive growth phase results in increased flower abortion, reduced pod number, reduced seed per pod, and small seed.
Nitrogen fixation is a key biochemical pathway for soybean yield and nitrogen fixation can be severely limited or completely halted by even moderate drought stress. Once nitrogen fixation has been stopped, substantial precipitation and soil water accumulation is required to reinitiate the process. Compensatory reproductive growth rarely will occur in soybean under moderate drought stress at reproductive growth phases.
Management practices that leave low amounts of crop residue on the soil surface or cause soil compaction can reduce soil water infiltration rate. Excessive or poorly timed tillage can cause soil compaction and increase water runoff from high intensity rain storms. Reduced compaction and increased water infiltration rate can increase soil water content, nitrogen fixation, and soybean yield--particularly during growing seasons with less than adequate precipitation.