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Drought stress affects plants in different ways at different times, depending on what growth stage the plants are in when stress is experienced. Water loss happens through evapotranspiration, which is water loss from both the soil and plant tissue.1 Without sufficient moisture, root13;10;growth may be restricted and nutrient availability, uptake, and transport are limited,13;10;leading to potential nutrient deficiency. Plants weakened by13;10;moisture stress may also be more susceptible to disease and insects.5 Limited plant growth and smaller leaf size can13;10;also lead to slow row closure and increased weed competition. Actual yield loss13;10;from moisture stress varies depending on the timing and duration of drought13;10;conditions.
Moisture stress in corn during the early vegetative growth13;10;stages can reduce ear size and potential yield. Research has shown that four consecutive days of visible wilting during13;10;the vegetative growth stage can reduce potential corn yield by 5 to 10 percent.513;10;Corn plants experiencing drought stress may show symptoms of nutrient deficiency,13;10;wilting, or rolled leaves (Figure 1).
Row number and kernel number are important components of yield in corn. Each of these begins development between the V6 and V8 growth stages, shortly after the ear shoot is formed. The potential number of kernels per row (ear length) is determined by V15 and possibly as early as V12.3 Row number is largely determined by genetics, but the number of kernels per row is strongly influenced by environmental conditions. Therefore, any stress during these early growth stages, including drought, can reduce the number of kernels per ear. If ear size is reduced during this period, it cannot be corrected later in the season.5
Early season moisture stress can also affect plant population and cause other problems in the plant. The final plant population at harvest is largely determined during the first 30 to 45 days after planting. Stand loss may occur due to excessively dry soils, particularly in later planted fields where young plants experience moisture stress before their nodal root systems develop well enough to reach deeper moisture. Rootless corn syndrome or “floppy corn syndrome" may develop in dry conditions; affected plants lack all or most of their nodal roots. Desiccation of the nodal roots is increased by hot and sunny weather conditions. The above-ground appearance of plants with rootless corn syndrome may seem normal until a wind event occurs and the corn plant simply falls over.2,4
Vegetative growth in soybeans may be restricted by early13;10;season moisture stress. Signs of13;10;moisture stress in soybeans include leaf flipping and leaf clamping. Soybeans flip their leaves to expose the13;10;silver-green underside, which helps the plant to conserve water by reducing temperature13;10;stress. While not as visually dramatic, it is analogous to leaf rolling in corn.613;10;As moisture stress increases, the leaves will begin to clamp together to13;10;reduce the leaf area exposed to sunlight.
Early drought stress can cause young soybean plants (V3 to13;10;V5) to begin flowering sooner than normal. Node production and spacing may also be reduced, resulting in fewer and13;10;lower set pods on the plant. Seeds per13;10;pod and seed size may also be reduced due to drought. Since soybeans flower continuously during the13;10;reproductive stage, they have a wide time frame to compensate for early season13;10;stress. Yield loss, reduced pod count,13;10;and small seed size can still occur where early season stress was severe.6
If available, irrigation should be used to help mitigate early season drought stress in corn and soybeans. Management13;10;practices that reduce other stresses like weed competition, insect pressure,13;10;and disease may also help lessen the effects of drought stress.