Jerry Clark
Crops & Soils Educator for Chippewa, Dunn, & Eau Claire Counties
UW-Madison Extension

Many questions are being asked about the effect the dry weather is having on corn yield. Recent rain showers have reduced concern in some areas of Chippewa, Dunn, and Eau Claire counties and the scattered precipitation has been just that, scattered. It is a start to a growing season where one neighbors gets ample rainfall and the next neighbor, nothing. 

For corn, we know the pollination period of tasseling and silking is important for grain yield. Other stages of corn growth and development are just as critical to reach maximum yield potential. Plant stress at any of these development stages can reduce yield even if favorable weather occurs during the pollination period. The chart below indicates critical stages of a corn plant and what specific development processes are occurring. 

Critical stages of a corn plant* 

Stage	Growing Degree Days	Potential Yield	Actual Yield
VE	125	Ears/area
V6	470	Kernel rows/ear	All plant parts present
V12	815		Kernel rows/ear
V18	1160	Kernels/row
R1	1250	Kernel weight	Kernel number and ears/area
R6	2350		Kernel weight

*Darby, H. and Lauer, J. 

Normal corn plants follow the same general pattern of development. Specific time intervals between stages and total leaf numbers developed may vary between different hybrids, seasons, planting dates and locations. The rate of plant development for any hybrid is directly related to temperature, so the length of time between the different stages will vary as the temperature varies. Environmental stress may lengthen or shorten the time between vegetative and reproductive stages. Most hybrids will develop between 17-21 leaves, silk about 65 days after emergence, and reach physical maturity or “black layer” 60 days after pollination. 

Dry Soil Conditions and Herbicide Activation 

The extended dry weather has helped get the corn and soybean planting season wrapped up and provided excellent conditions for harvesting first crop alfalfa and make hay (while the sun shines). An issue created when topsoil becomes dry is activation of preemergence herbicides. To be effective, the herbicide must be located within the soil profile at the depth where weed seeds germinate (primarily the upper inch of soil). There must be sufficient water to keep the herbicide dissolved in soil water. Both factors can be adversely affected by limited rain early in the season.  

It typically takes at least a half-inch of rain to activate the herbicide and move it to the depth where germinating seeds are located. If the upper inch or two of soil is dry, the quantity of rain for activation will be slightly higher since the soil first must be wetted before water will move the herbicide into the profile. Rainfall of 1 to 2 inches may be needed to move herbicides to an effective depth if the upper two inches of topsoil is dry. Even if an herbicide is activated in a timely fashion, performance may be reduced when there is limited rain in the weeks following application and activation. Preemergence herbicides move from the soil water to adsorption sites in the soil as the soil dries therefore reducing the amount of herbicide available to germinating weed seeds. Herbicides less tightly bound to soil may provide more consistent control during dry years. 

If a field fails to receive an activating rain within 5-7 days of application, consider using a rotary hoe or harrow to control weeds escaping the residual herbicide. These tools are most effective before germinating weeds emerge (white root stage). If this isn’t possible, begin to scout fields within two weeks of planting to determine the appropriate time for postemergence applications or a cultivation.