In better-watered areas of Illinois where the corn crop is well established, the return of warm temperatures has caused very rapid growth. Corn planted in central Illinois in mid-March and not damaged by frost has accumulated about 900 growing degree days (GDD) by now and thus, has reached stage V9 or V10, the point at which stem elongation accelerates. Such fields will likely show tassels by mid-June.
Corn planted in early April has accumulated about 650 GDD and is at V7. Corn planted in mid-April is at V5, having accumulated approximately 520 GDD.
Though the crop is in good condition in most areas, the dry weather pattern is causing some concern. Water use accelerates as corn reaches V7-V8, but it is still only about an inch per week. Dry surface soils have meant low evaporation rates, so most water is moving out through plants, which is an efficient way to use water. In most areas, plants are still growing well by extracting water from the soil, and the drying of surface soils is encouraging deeper root growth.
"In deeper soils that can provide 8 to 10 inches of water to a crop, there should be enough water to keep the crop growing well into June," said Nafziger. "At some point, of course, we will need rain to keep the crop growing up to its potential."
While the state's corn crop is nearly all planted and much of the crop is growing well, there are reports of "floppy" corn plants in western and northwestern Illinois and into southeastern Iowa. "Given the better-than-average planting conditions this year, this was not a problem that we expected to see," said University of Illinois crop sciences professor Emerson Nafziger.
"It's an easy problem to spot," said Nafziger. Plants develop using water provided by the seminal (seed) root system up to the 3- to 5-leaf stage, after which the nodal roots -- those that develop from the base of the stalk -- take over and become the main root system for the rest of the season. If the nodal root system fails to develop, the plant become wobbly and may fall over; hence the name "floppy" corn. This problem has also been called "rootless" corn due to the absence of nodal roots.
This year, the problem appears to be most common in corn that was planted in the last week of April and that is now at or just past the V3 stage. It has been observed in both no-till and tilled fields, though it is probably more common in no-till fields. This is because the no-till planting furrow can act as a barrier to nodal root growth.
Another source of difficulty for nodal roots is what has been dubbed the "high-crown syndrome" (HCS). This is a relatively rare phenomenon in which the base of the stem (the crown) ends up positioned at or very near the soil surface instead of at its normal placement about three-fourths of an inch deep in the soil. As a result, the plant ends up perched atop the soil. Because nodal roots of such plants emerge above the soil surface, they often have great difficulty penetrating the soil, especially under dry conditions.
Why plants end up perched above the soil like is not clear. The crown (base of the stem) is usually set when light strikes the tip of the coleoptile as it emerges above ground. At this point, the coleoptile and the mesocotyl stop growing and the crown depth is set as the first leaf breaks through the coleoptile.
One possible disruption of this process can be rapid growth in warm soils, when the tips of coleoptiles that emerge in the evening do not stop growing until the next morning, at which point the crown is already near the soil surface. But because soil temperatures were only in the 50s in late April this year, that is not a likely explanation.
Another cause of high crown placement is soil subsidence due to rainfall after planting into dry soils fluffed by tillage. If the planting furrow opens as soils dry after planting (this is most common in no-till), the crown will be set near the seed, placing the seed and seedling above the soil. Finally, PGR herbicides such as 2,4-D can, if they reach the seed or seedling during this process, cause rapid growth of the mesocotyl and push the crown to the soil surface.
"Once plants can no longer stay upright due to a lack of anchoring roots, water uptake and photosynthesis slow down and the supply of sugars starts to decrease, limiting the ability of the plant to grow or to form roots," Nafziger said. "If the plant is lying on soil that stays dry, it may break off its mesocotyl anchor and die."
In some areas that stayed dry in May, even fields where the crown is at normal depth have plants that are struggling to establish nodal roots due to dry surface soils. Roots cannot grow into soils from which they cannot extract water.
"The usual first sign of inadequate root systems is curling of leaves in the afternoon," explained Nafziger. "As the water shortage progresses, leaf curling takes place earlier each day and plants may start to lose their green color."
Plants growing in dry soil often show some degree of purpling as well. Having plants turn purple may be preferable to having them turn pale green. "This is because purpling results from sugar accumulation in the pIant, and sugars cannot accumulate without photosynthesis," said Nafziger.
Sugars accumulate when there is no place (such as roots) where they can move, or when there is not enough phosphorus to help them move. Roots that are growing poorly do not need much sugar and have difficulty reaching the phosphorus in the soil, so plants with poor roots often turn purple. Some hybrids do this faster than others, but a return to normal root growth quickly alleviates the purpling, usually with no harm done.
"However," Nafziger said, "there is little to be done once the lack of nodal roots causes corn plants to fall over." There are reports that some of this corn is already being replanted.
The only source of quick relief from problems with nodal roots is rainfall, and even that needs to happen before plants start to fall over. Root tips can dry out and suffer damage by contact with hard, dry soil.
In theory, moving soil into the row to keep plants standing until it rains will help, but it would have to be done before plants start to fall over. Watering down the row might help; however, wetting the soil in a band 4 inches by 2 inches over 30-inch rows requires approximately 1,500 gallons of water per acre and may not be practical.
One question is whether plants perched on top of the soil will recover to become fully productive even if rain wets the soil enough to allow nodal roots to penetrate it and grow. Such roots tend to grow downward at a steep angle, which might give the plant a small advantage. But there may be fewer roots, and rapid water uptake might be a challenge. The risk of lodging will also increase due to less anchoring by the root system.
"With their delayed start and the fact that some roots initiated above the soil surface often do not penetrate the soil surface very well, such plants may become fully productive only if the season turns out to be relatively free from stress," Nafziger said. "Even though we're pleased with the early planting into generally good conditions, 2012 has not been stress-free so far, at least in some areas."