In much of Iowa, storms on June 4 and 5 not only brought more 'unwelcome' rain but also damaging winds and destructive hail. Across the state, this year's corn crop is currently in various stages of early development--ranging from plants that have just to the sixth leaf stage.
Vegetative stages are determined and most often referred to based on the leaf-collar method developed by Iowa State University agronomists. ISU Extension corn agronomist Roger Elmore, and ISU Extension specialist Lori Abendroth, offer the following guidelines to farmers and hail insurance people who are assessing hail damage to cornfields.
In contrast to a soybean plant, corn has an advantage in the early season when hail damages the aboveground plant, because corn's 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 in the past several days are beyond this point, with their growing point at soil level or above.
Determine the leaf stage of corn plants
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, regrowth is expected and yield is impacted negligibly. This is true regardless of the amount of defoliation or leaf loss. The plant will grow more leaves.
• In fields where corn was near or beyond the sixth leaf stage, evaluate injured plants to determine whether the growing point is viable. You're your 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.
Keep two key things in mind this year
Elmore and Abendroth advise you to be cautious about two issues unique to 2008: uneven growth and variation in plant height.
Soil crusting, soil temperature variations and planting depth variability resulted in uneven emergence and variable early season growth across many fields. If this is the case you are running into now 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.
Height of plants may be deceiving
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, the nodal roots and the 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:
* Wet planting conditions prompted some farmers to plant shallower which may have placed the nodal roots and growing point higher.
* If conditions were wet at planting (many farmers were trying to plant between rain events), the seed furrows may have reopened once soils dried.
* Moderate to severe rain events have compacted soils and/or caused soil erosion, thereby creating shallow nodal root systems even if planted at a normal seeding depth.
Each of these three scenarios will cause the plants to form nodal roots closer to the surface than normal. If this happened, the growing point on very young plants may lie close to the soil surface and have experienced hail damage.
When does hail injury impact yield?
The grain yield impact on young plants (less than sixth-leaf) injured by hail is negligible if their growing points remain healthy. Certainly, leaf defoliation or the loss of entire leaves will slow growth rates, but this leaf loss will not significantly impact yield. However, plants with unhealthy growing points will die, and therefore reduce the plant population of that field. Grain yield losses will result.
Information in Figures 1 and 2 (see below) is from the USDA and Federal Crop Insurance Corporation) and identifies expected yield reductions based on surviving population (Figure 1) or percent of leaf area destroyed (Figure 2).
Figure 1 displays the estimated yield loss associated with two different starting populations (what existed before the damage occurred). Data represented by the squares, show yield reductions with an initial population of 28,000 plants per acre. Data represented by triangles show yield reductions with an initial population of 32,000 plants per acre. It is clear that small reductions in plant survival do not impact yields severely.
For example, in both original populations a reduction of 10,000 plants per acre reduced yield by less than 20%. Thus, it is not a 1:1 relationship, neighboring plants are able to compensate (to a degree) for non-surviving neighbors.
Figure 1. Plant survival and the impact of stand reductions on grain yield for original stands of 28,000 and 32,000 plants per acre (ppa). Adapted from USDA and FCIC, 2005.
Leaf defoliation from hail will not affect plants if they are less than the sixth leaf stage. Plants with six leaves or greater will experience yield losses depending on the extent of the defoliation. For example, 75% leaf defoliation of sixth- and seventh-leaf plants will result in 5% and 6% yield losses, respectively (Figure 2). Leaf losses less than 40% do not affect corn yields if they occur at these early-growth stages.
Figure 2. Percent leaf area destroyed and the impact on grain yield for plants at the sixth- or seventh- leaf stage. Adapted from USDA and FCIC, 2005.
Fields with corn less than the sixth-leaf stage and fields with sixth- or seventh-leaf corn that has less than 40% leaf area destroyed are expected to recover with little yield loss. Evaluate fields that are outside of either of these two parameters and assess expected yield reductions.
Remember that the key to responding to hail is to assess plant viability thoroughly once the plants have had a good chance to recover. Contact your crop insurance company before destroying the crop or replanting.