The last two weeks have been extremely warm and dry in Wisconsin. Many are concerned about how this early season drought might affect corn yields during 1995. Many will remember that the same thing occurred during 1994 when extremely dry weather was observed during May and into mid-to-late June. Well timed rains finally began to arrive by the end of June and we were on our way to a record year. An early season warm, dry spell is characteristic of years in Wisconsin when the state yield record jumped by 15% over the previous record year.

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. 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. 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. Water stress around flowering and pollination delays silking, reduces silk elongation, and inhibits embryo development after pollination. Kernel number is reduced. Water stress during grainfilling increases leaf dying, shortens the grainfilling period, increases lodging and lowers kernel weight.

Table 1. Estimated evapotranspiration of corn during various stages of growth when grown on a fine sand.
Growth stage	Evapotranspiration
	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
16 leaf to tasseling	0.33
Pollination	0.33
Grainfilling	0.26
Maturity	0.23

Correlations between yield and both June temperature and June rainfall are low. In Wisconsin during 1995, 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. So far during 1995, we have had dry weather conditions only during growth stages with relatively low evapotranspiration demand. There is reason for concern about weather patterns, but not panic yet over corn yields.