Risk management

The combination of high temperatures and inadequate moisture has created severe stress in many corn fields. As dry weather continues, more corn fields are showing signs of moisture stress with leaf rolling evident during midday hours. In many stressed fields, soil moisture is available but it appears beyond reach of most corn roots. Current weather conditions are inhibiting root development. In addition to water deficits, high soil temperatures are limiting root growth near the soil surface. The corn canopy shades the soil surface and moderates soil temperatures, but many corn fields have yet to canopy. Plants with root systems restricted to the upper four to five inches of the soil profile are stunted. Scattered thunderstorms during the past weekend brought relief to some localized areas, but cooler weather and the possibility of more storms forecast for the coming week may ease the stress somewhat.

To estimate the impact of dry hot weather on corn yield potential, let's review the effects of moisture deficits on corn growth and development from the late vegetative stages, prior to pollination, to the dent stage of kernel development. Yield losses to moisture stress can be directly related to the number of days that the crop shows stress symptoms during different growth periods. The following summarizes findings of Iowa research by Claassen and Shaw on effects of drought on grain yields in corn. This Iowa data is widely used in estimating the potential impact of water stress on yield potential.

Vegetative Stages: Drought stress during early vegetative growth usually has a negligible impact on grain yield. However, during later vegetative stages, when kernel numbers per ear are determined, plants are more sensitive to stress. According to Claassen and Shaw's findings, four days of stress (i.e. corn wilted for four consecutive days) at the 12th-14th leaf stage has the potential of reducing yields by 5-10 percent. Kernel row numbers on the ear are determined by the 12th collared leaf stage and the potential number of kernels per row is complete about one week before silking.

Tassel Emergence: As the tip of the tassel begins to emerge from the whorl, the upper stalk internodes rapidly elongate and the ears begin to expand. Silks from the base of the ears are also rapidly elongating. Four days of moisture stress at this stage has the potential to reduce yields 10 to 25%.

Silk Emergence to Pollen Shed: At this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. This is the most critical period in terms of moisture use by the plant. Four days of moisture stress at this stage has the potential to reduce yields 40-50%.

Blister Through Dent Stage of Kernel Development: About 12 to 36 days after silking, the cobs, husks and shanks are fully developed and the kernels are increasing in dry weight. Moisture stress will reduce kernel fill from the ear tip down. Four days of drought at the blister stage has the potential of reducing yields 30-40%, and at dough stage, 20 to 30%.

Where dry weather has contributed to corn stands with uneven emergence, and development, yield loss may range from 5 to 20% depending on various factors such as the length of emergence delays and the percentage and distribution of later emerging plants. Where there is considerable variability in plant size, smaller, stunted plants will be at a competitive disadvantage with larger plants for nutrients, water and sunlight.

Effect of drought on corn yield

Stage of development	Percent yield reduction (from 4 consecutive days of visible wilting)
Early vegetative	5-10
Tassel emergence	10-25
Silk emergence, pollen shedding	40-50
Blister	30-40
Dough	20-30