Many areas of Wisconsin have not had rain for three to four weeks. Corn growing on lighter soils is starting to show signs of stress early in the morning. Many are concerned about how this drought might affect corn yields. To begin talking about water influences on corn growth and development and yield we must begin with the concept of evapotranspiration.

Evapotranspiration is both the water lost from the soil surface through evaporation and the water used by a plant during transpiration. Soil evaporation is the major loss of water from the soil during early stages of growth. As corn leaf area increases, transpiration gradually becomes the major pathway through which water moves from the soil through the plant to the atmosphere.

Yield is reduced when evapotranspiration demand exceeds water supply from the soil at any time during the corn life cycle. Nutrient availability, uptake, and transport are impaired without sufficient water. Plants weakened by stress are also more susceptible to disease and insect damage. Corn responds to water stress by leaf rolling. Highly stressed plants will begin leaf rolling early in the day. Evapotranspiration demand of corn varies during its life cycle (Table 1). Evapotranspiration peaks around canopy closure. Estimates of peak evapotranspiration in corn range between 0.20 and 0.39 inches per day.

Corn yield is most sensitive to water stress during flowering and pollination, followed by grainfilling, and finally vegetative growth stages (Table 1). Water stress during vegetative development reduces stem and leaf cell expansion resulting in reduced plant height and less leaf area. Leaf number is generally not affected by water stress. Ear size may be smaller. Water stress around flowering and pollination delays silking, reduces silk elongation, and inhibits embryo development after pollination. Moisture or heat stress during this period can result in lack of synchronization between pollen shed and silk emergence. Kernel number is reduced. Water stress during grainfilling increases leaf dying, shortens the grainfilling period, increases lodging and lowers kernel weight.

In Wisconsin, soil moisture reserves from winter and early spring precipitation were adequate to surplus. Corn roots can grow between 5 and 8 feet deep, and soil can hold 1.5 to 2.5 inches of available soil water per foot of soil, depending upon soil texture.

Table 1. Estimated corn evapotranspiration and yield loss per stress day during various stages of growth.
Growth stage	Evapotranspiration	Percent yield loss per day of stress (min-ave-max)
	inches per day	%
Seedling to 4 leaf	0.06	---
4 leaf to 8 leaf	0.10	---
8 leaf to 12 leaf	0.18	---
12 leaf to 16 leaf	0.21	2.1 - 3.0 - 3.7
16 leaf to tasseling	0.33	2.5 - 3.2 - 4.0
Pollination	0.33	3.0 - 6.8 - 8.0
Grainfilling	0.26	3.0 - 4.0 - 5.0
Maturity	0.23	0.0 - 1.0 - 2.0
derived from Rhoads and Bennett (1990) and Shaw (1988)