Dry weather stress in Missouri corn and soybeans this summer is intensified by spring planting conditions, said a University of Missouri agronomist. Poor seedbeds resulting from cool, wet weather prevented full development of root systems.
“Drought stress is always a concern,” said Bill Wiebold, MU Extension specialist. “However, six weeks of wet spring weather and the poor tillage and planting conditions have greatly increased our plants' vulnerability to this stress.”
A long period of wet weather kept producers from planting their fields, Wiebold said. As a result, farmers rushed to plant before soil was ready to work. “Many acres of corn and soybeans were planted before soil conditions were optimum for equipment operations.”
Working the soil when it is too wet destroys soil structure. That inhibits root growth. “Compacted soils have soil pores too small for roots to enter. Cloddy soils have large spaces that dry quickly and into which roots won't grow,” the agronomist said.
In addition, slow-growing roots are more susceptible to diseases, insects and herbicide stress.
“The results of accumulated stresses are small, poorly functioning roots,” Wiebold said.
Lack of water in the soil this summer across much of Missouri, especially in the northwestern regions, puts extra demands even on normal roots.
Corn and soybean crops in August require about 2 inches of water a week. That water must be supplied from the soil by the roots.
“Root systems that are smaller than normal are less likely to meet the demand,” Wiebold said. With normal development, strong roots should grow deep to reach subsoil water.
If roots are not meeting the plant's water needs, the leaves go into a defensive mode. Normally, water is transpired through small leaf openings called stomata.
When water is short, plants begin to close their stomata.
In further defense, corn leaves roll or “twist,” to reduce the sunlight energy absorption. The rolled leaf conserves humidity. In soybean plants, the leaves change orientation to be parallel to the sun rays to reduce energy absorption.
Both defenses reduce photosynthesis, the plant process of using the sun energy to convert water and carbon dioxide from the air into sugars. Lower photosynthesis reduces yield.
There are other stresses. If a plant cannot maintain water pressure, or turgor, stem elongation and leaf expansion slow. This causes drought-stunted plants. “This stunting leads to less photosynthesis and less yield,” Wiebold said.
The biggest stress comes when drought hits at the time of reproduction. Female parts of the plant must be fully hydrated to function.
With too little water, pollen tubes in corn will not grow and fertilization will not occur. Any interruption of pollination reduces the number of corn kernels on the cob.
Continued drought causes seeds to abort. “And, if dry weather persists through seed-filling, seed size is reduced,” Wiebold said.
In its defensive mode a plant will strive to make a few seeds to reproduce the species. In that mode, however, bumper crops are not achieved.
Duane Dailey is the news coordinator, Extension & Ag Information, University of Missouri.