Weather conditions play a huge part in determining crop yields. One environmental action that is often encountered by growers is drought conditions. Water is important for many metabolic and physiological processes within the crop. Corn and soybeans can often suffer from reduced available water.
Evaporation and transpiration are the two ways a crop loses water, which together is called evapotranspiration. Evaporation is the loss of water from the soil and plant surfaces. Transpiration is the movement of water from the soil into plant roots, up through stems and leaves, then out the stomata and into the atmosphere. The stomata are openings in the plant leaf that regulate air and moisture movement into and out of the leaf. The opening is controlled by two “guard cells” which serve as gatekeepers.
Corn uses a fair amount of water as it develops with the greatest amount required the week of tassel, and the three ensuing weeks. If the weather is hot and dry during this period, corn can use up to two inches of water per week. But as Figure 1 shows, corn water use can fluctuate greatly from day to day. In the midst of a dry spell, corn leaves will roll up in an attempt to reduce the amount of sunlight captured and conserve water. The guard cells push to close the stomata to do so. This leads to a reduction of photosynthesis.
Soybean water use increases throughout the season and peaks at mid to late reproductive stages. Soybeans will react to drought by reducing leaf size and flipping their leaves. Flipping leaves exposes the silver-green underside of the leaves which will reflect more sunlight thus lowering plant temperature in an attempt to reduce transpiration. This will also reduce photosynthesis which will conserve water. When drought is extended and severe, the leaves will fold up. Moisture stress during pod formation is the most critical time for affecting yield as younger pods may abort and seed size reduced. The good news is that soybeans will recover better than corn if we get later rains. Rains after R5, can increase seed size.
Two-spotted spider mites are the main insect problem in soybeans when it is dry. There is a beneficial fungal pathogen, Neozygites floridana which under conditions cooler than 85° with 90% relative humidity will keep the two-spotted spider mites in check. However, under hot, droughty conditions, the fungi will not produce spores and the spider mites are free to multiply.
Soil water holding capacity of a grower’s soil can have a substantial effect on the crops ability to get through hot, dry weather. Heavy soils with a high organic matter can retain more water than light, sandy soils with a low organic matter. The use of no-till practices can benefit crops under drought conditions by helping the soil seize and preserve more moisture by the residue reducing evaproation.
Crops under drought conditions can have greatly reduced yields, depending on the duration of the dry period. Helena does however have products which will help in times of crop stress, like drought. Megafol® and Orbix™ are great products which help the crops react to the stress and help the crop overcome the negative consequences induced by dry conditions.
— Dr. Randy Simonson, AGRIntelligence Agronomist & Dr. Greg Willoughby, Technical Service Manager NBU