Will short corn come up short on yield? As you gaze across many fields in Iowa this year, that question comes to mind. There are a lot of fields in early August with corn plants that are shorter than normal for this time of year, and in some fields there is a lot of variability in plant height.

WILL SHORT CORN BE SHORT ON YIELD? Cool, wet weather this spring reduced corn plant heights and has increased the variability of plant heights across fields. Shorter plants are not necessarily lower yielding. Whether corn is knee-high or elephant-eye high by the 4th of July is less important than whether the crop intercepts near maximum light at silking, and is also less important than the timing of the first major fall frost.

Cool, wet weather this spring has resulted in reduced corn plant heights and increased variability of plant heights across fields this summer, observes Roger Elmore, Iowa State University Extension corn agronomist. Shorter plants are not necessarily lower yielding, he says. Whether corn is knee-high or elephant-eye high by the 4th of July is less important than whether the crop intercepts near maximum light at silking and the timing of the first major fall frost.

But what about that old saying, the one about "Knee high by the 4th of July!" Elmore says, "Some of us grew up with that old saying which either foreshadowed disaster or forecast bumper yields. If corn was knee high by the 4th, most thought it would mature before the first fall frost. That was good news! As we look back, the statement perhaps was valid back when the first target corn planting date was mid-May."

Elmore, along with ISU colleagues Elwynn Taylor and Mahdi Al-Kaisi, provide the following information to help you figure out what you are seeing in your fields now, and what it may mean for harvest this fall.

Crop conditions can change and corn plants respond to those changes

Now farmers begin to plant corn in mid- to late April and most wrap up planting by mid-May. These changes were driven by many factors including reduced tillage systems, better herbicides, more cold-tolerant hybrids, improved seed treatments, improved planter systems, larger farms, climate change ….and we've probably missed others. And now, we're used to seeing corn "…as high as an elephant's eye, An' it looks like its climbin' clear up to the sky" (Rodgers and Hammerstein, 1943, musical, Oklahoma) by the 4th of July. But that didn't happen this year.~~~PAGE_BREAK_HERE~~~

Many agronomists report not only shorter corn plants than normal this year (at least in some parts of Iowa) but also more variability across what usually appear as uniform fields. Why?

Consider the effect of this year's late planting dates for corn

Remember when 2013 first rolled in? The impact and lingering effects of the 2012 drought still haunted us. Several articles in ISU's Integrated Crop Management newsletter reflected the concerns of ISU agronomists this past winter. By April 28 only 2% of Iowa's corn was planted -- sitting in seed furrows, soaking up water, and bracing for up to several inches of snow. Then, after the snow, wet conditions stymied planting. By May 12th, only 15% of Iowa's corn had been planted (USDA data). A week later though, another 56% was planted! Iowa farmers planted about 1.5 million acres per field work day the week of May 13, probably the most they've ever accomplished in one 'field work day' (Figure 1).

Figure 1. Corn planting progress in Iowa, 2004 and 2008 to 2013. Data compiled from USDA. The "blue bar" in the figure represents half (50%) of the acres planted. In 2010 (yellow) the crop was half planted April 18th; in 2013 the half-planted date was about May 15th. The very high yield year of 2004 was 50% planted by April 30th.

Iowa finally reached 99% of Iowa's corn planted on July 1. That ranks 2013 alongside 2008 in recent history; two of the slowest and most drawn out planting seasons of modern times. Incidentally, 2010 reflects the earliest we've ever planted corn in Iowa, and 2004 and 2009 reflect years with the highest statewide average yields recorded.

But later planting dates usually result in taller corn than earlier planting dates since plants develop during periods with long days. Plant height increases because of elongated internodes during such days. So, what happened this year?

Soil temperature has an effect on corn height

We know that small differences in soil temperature and moisture affect both above- and below-ground corn growth. Root depth follows the downward progression of temperature increases. Generally, roots do not grow in soils much colder than 50 degrees F.~~~PAGE_BREAK_HERE~~~

Most reports dealing with corn height and development and correlations with soil temperatures are found in crop residue management research. More residue cover on the field surface increases soil moisture content and decreases soil temperatures. Associated with the reduced soil temperatures is a decrease in plant height and decreased dry matter at specific calendar dates during vegetative development. However, corn grown with the same residue amounts had similar heights and dry matter when compared at the same vegetative development stages. Since seedling emergence is correlated with temperature, cold soils result in slow emergence.

Upper canopy leaves on corn plant contribute more to yield than lower canopy leaves

Illinois researchers compared corn growth and development by altering soil temperatures (±9  degrees F from ambient soil temperatures) with an underground heating system used through the V5 -- five collared leaves growth stage. In addition to delaying development in terms of calendar days, grain yield declined with the coldest soil temperatures. With these coldest soils, lower canopy leaves (leaves one to 13) had greater leaf area than upper canopy leaves (leaves 15 to 21). The opposite was true with the warm soil treatment. Upper canopy leaves contribute more to yield than lower canopy leaves.

All leaf development initiates while the growing point is still underground, that is, before V6 growth stage of the corn plant; soil temperatures up to V6 thus affect early crop development. Noticeable stalk elongation begins at around V5. Root development and leaf initiation have priority before that time. Cell expansion occurs near the internode bases. This process of cell expansion is influenced not only by the amount of light and day length (the latter discussed above in relation to late planting) but also by temperature. Early-season cold temperatures increase internode rigidity and limit cell expansion and internode elongation.

How do soil temperatures affect 2013 corn growth?

Soil temperatures this year fell below normal during the five weeks when most of Iowa's corn was planted and considerably below what we experienced in 2012 (Figure 2). We speculate that abnormally cool 2013 soil temperatures affected corn internode elongation and thus resulted in shorter corn plants, especially those planted from April through mid- to late-May.~~~PAGE_BREAK_HERE~~~

Figure 2. Soil temperatures in 2013 compared to those of 2012 and the 10-year average at Gilbert, Iowa in central Iowa. Data compiled from MESONET. Iowa soil temperature was 5 to 8°F colder than usual in the late April to May 5 period. Between May 6th and June 10th soils were again several degrees colder than usual.

In addition, "within field" variations in plant heights are seen in fields this year and that may be due to uneven crop residue distribution across the field. Differing residue levels alters both soil temperatures and soil moisture, in turn affecting corn growth and development. Other factors may affect within field plant height variations as well.

Is plant height a good indicator of corn yields? If light interception is near complete at silking time

Plant height itself is not necessarily a good indicator of corn yields if light interception is near complete at silking. As mentioned previously though, if upper canopy leaf areas of corn plants were reduced by the cool temperatures this year, then grain yield will be reduced. However, yield potential in 2013 is likely compromised by the delayed planting and possibly cool and wet soil conditions experienced early in the growing season. In this case, an early frost will be detrimental.

Cool, wet weather this spring reduced corn plant heights and increased variability of plant heights across fields. Shorter plants aren't necessarily lower yielding. Remember, whether corn is knee-high or elephant-eye high by the 4th of July is less important than whether the crop intercepts near maximum light at silking and the timing of the first major fall frost.