Hail trouble: Protect your corn
Storms during early June brought more unwelcome rain, damaging winds and destructive hail. Variation currently exists in corn development across Iowa, ranging from emerged to the sixth leaf stage. Vegetative stages are determined and most often referenced based on the leaf-collar method developed by Iowa State agronomists.
In contrast to soybean, corn has an advantage early season when hail damages the aboveground plant because its growing point remains below ground until approximately the sixth-leaf stage. The sixth-leaf stage of the ISU leaf-collar system correlates to the seventh-leaf stage used by hail adjusters. Several fields that received hail damage are beyond this point, with the growing point at soil level or above.
Two different methods exist for assessing damaged fields based on the developmental stage of the crop when it incurred the damage:
In fields where the corn was at the fifth leaf or smaller, re-growth is expected and yield impacted negligibly. This is true regardless of the amount of defoliation.
In fields where corn was near or beyond the sixth leaf stage, evaluate injured plants to determine whether the growing point is viable. Make assessments of plant survival three to five days after the storm so that surviving plants have a chance to recover. If weather is not conducive for plant growth for a prolonged period after the storm, assessing the remaining stand may require waiting up to a week. It may take that long before it is clear which plants will survive and which will not.
Assessing a damaged field requires that the growing point is located and evaluated. Use a sharp knife and cut lengthwise down the stem in order to cross-section the stem. Assess the viability of the growing point; it should have a white to cream color. Plants with a healthy growing point should survive, especially if the growing point lies below the soil surface.
Be cautious about two issues unique to 2008:
Soil crusting, soil temperature variations and planting depth variability result in uneven emergence and variable early season growth across many fields. If this is the case in a damaged field, then perhaps not all plants are at the same stage of development.
Plant heights this year appear short relative to what we expect for a certain developmental stage. This is due to condensed internode lengths likely caused by the cool spring conditions we have experienced.
Many agronomists are finding that although plants have heights similar to two- or three-leaf plants, they are actually at four or five-leaf. Therefore, height may be deceiving and not an accurate representation of plant viability.
Nodal roots form approximately one inch below the soil surface when planting depths are greater than 1.5 inches and soil conditions are normal. The coleoptile, nodal roots and growing point all emerge from the same node. The location of the nodal roots, and root structure in general, may vary from normal this year because of abnormal planting conditions. Situations experienced in 2008 are: