Figure 1 – Stressed corn symptoms or corn rolling taken by Soy Envoy, Leo Rocha in White Co. IL
Lately, the panic of a looming drought has caused great concern. We have observed the hardening of soils in recent replanted fields, slow germination, stunted crop growth, herbicide effectiveness concerns, possible nitrogen losses, and nutrient deficiency symptoms, especially in corn, because of potential inability to take up nutrients without water. The good news is that compared to corn, soybeans can better adapt in times of moisture stress. Indeterminate soybeans can handle moisture stress over a longer period and range of growth stages because they continue to grow during their reproductive stages and can compensate for drought if growth has been reduced at previous stages. We can easily see corn showing signs of drought or heat symptoms; however, signs of soybeans under drought stress aren’t as obvious.
Figure 2 – Long root development of soybean during drought with a possible lack of nodulation taken by ISA At-Large Director, David Wessel in Cass Co. IL
When soybeans suffer from heat and water stress, their responses vary due to their cellular structures, metabolic processes, and physiological development – this can directly and indirectly cause soybeans to exhibit symptoms based on their response. Drought stress impacts yield most during germination and later during reproductive seed development. Early season soybean stress may cause leaves to be smaller or limit vegetative growth; therefore, more energy and efforts may be given to root development. Ultimately, nitrogen fixation can be severely limited or completely halted by even moderate drought stress.
Figure 3-Stressed soybean symptoms from drought or leaf flipping taken by Soy Envoy, Leo Rocha in White Co., IL
A more obvious sign of heat stress in soybeans may be leaf flipping, which is like corn rolling. Soybeans flip leaves to reflect more sunlight by exposing the silver-green leaf underside. This process can conserve water by reducing plant temperature stress and photosynthetic rates. Ultimately, the earlier and longer the leaf flipping occurs, the worse the severity. Lastly, extreme drought-stressed soybeans will exhibit leaf clamping. In other words, the leaves all fold in like a taco and this can occur anytime throughout the growing season when stress is severe enough.
The soybean’s genetic basis of drought tolerance is not well understood. The key to understanding this might be how plant growth and development or adaptive traits such as rooting depth, water use efficiency, nitrogen fixation, and differences in leaf wilting, can provide drought tolerance. Research is ongoing and most soybean types have been found to lose more water via their leaves under dry conditions than otherwise. More water-efficient soybean lines have been identified and can be used in breeding to limit water use early in the growing season in dry conditions. Previous research showed that nitrogen fixation is the most important drought tolerant trait and was predicted to help increase yield in dry years in the U.S. Breeding now has brought us new soybean varieties with drought-tolerant nitrogen fixation and higher yields under drought conditions. Research continues to seek out other genetic resources for specific traits to further increase yields during drought.
Later pod formation is the most critical growth stage because flowering has stopped and there is no longer any compensation for lost pods. The main source of yield reduction at this time is a decrease in total pods per plant, because younger pods will be more susceptible to abortion under stress compared to older pods/seeds that set earlier. If rain occurs after R5 (pod fill), depending on soybean genetics, seed size could still compensate for yield reductions.
Figure 4- Irrigation starting on June 1st due to drought taken by ISA At-Large Director, David Wessel in Mason Co., IL
We still can hold on to some hope late into the season because in 2012, corn in Illinois died in the field while soybean growth stalled to conserve water. When rains returned in mid-August of 2012, soybean plants were able to bounce back, fill out remaining pods that survived, and yields were not too far below normal.
Dry weather can be one of the greatest threats to soybean productivity and ultimately, profitability. It is something you can’t control unless you irrigate.
There are management practices such as less tillage, increased soil residue, improved soil health, or reduced compaction that can help to increase the soil moisture content, nitrogen fixation, and soybean yield during growing seasons with drought.