By Jeff Coulter and Seth Naeve, Extension Agronomists
Recent storms left several areas affected by hail damage. Especially hard hit was an area from Lyon County through Renville County. Throughout this area, corn was at the V7 to V8 stage (7 to 8 leaf collars) when damaged, and soybean had three to five fully developed trifoliolate leaves (V3-V5 stage).
In early July, assessing hail damage and making replant decisions can be difficult, with many variables to consider on your way to making a final decision to replant or maintain the existing stand. Many of the answers to questions regarding crop yield loss and the need for replanting can be found in the following online guides:
Corn Damage and Replant Guide: www.soybeans.umn.edu/pdfs/CornGuide.pdf
Soybean Damage and Replant Guide: www.soybeans.umn.edu/pdfs/SoybeanCropDamage.pdf
Survivability of Corn Plants:
Yield potential of hail-damaged corn depends primarily on the number of plants per acre with healthy growing points and the amount of leaf area lost on these plants. To determine whether a corn plant will survive and regrow, split stalks and examine the growing point. In corn, the growing point remains below the soil surface until the V5 stage (5 collared leaves). Growing points located at or below the soil surface can be damaged by large hail stones landing on soft soil, and by freezing from hail accumulation around the base of the plant. At the V7 stage, the growing point is located about 1-2 inches above the soil surface and looks like an arrow head. Healthy growing points will be firm and white to yellow in color. If damaged, the growing point will be watery and orange to brown in color. Plants with damaged growing points should be considered dead. Even if a growing point is slightly damaged, the plant has a high chance of bacterial infection and death.
Another consideration is stem bruising. Plants with stem bruising near the growing point should have their stalks split in order to determine the severity of the stem bruising, and whether the growing point has been injured. Stem bruising not only limits the plant's ability to translocate water and nutrients to the developing ear, but it also reduces standability. Fields with severe stem bruising should be harvested early to avoid significant losses from stalk lodging.
Plants with damaged whorls can often regrow, but sometimes the leaves become tightly bound in the whorl and have difficulty growing out. Such plants are referred to as buggy whips (Figure 1). Plants exhibiting severe buggy-whipping should not be counted as living plants, as their contributions to yield will be limited.
Figure 1. Buggy-whip corn plant. Photo by Dave Nicolai.
Survivability of Soybean Plants:
Soybean plants with significant amounts of green tissue remaining (more than one green cotyledon and/or remaining leaf tissue) are likely to survive early season hail damage, as they can regrow from axillary buds located at the juncture of the stem and leaves. Soybean plants cut below the cotyledons will not recover.
Estimating Yield Loss Due to Reduced Plant Population:
Hail damage is highly variable within fields. Thus, accurate estimation of the surviving plant population requires that surviving plant counts be made in as many locations within a management unit as possible. Making a replant decision based on a few stand counts near the field entrance will likely lead to a poor ultimate decision. Count several areas that are equal to 1/1000th of an acre and take the average. Then, multiply by 1000 to determine plants per acre.
Table 1. Length of row equal to 1/1000th of an acre for various row widths.
Soybean can tolerate low populations very well, with only small reductions in yield potential across wide ranges in plant loss. For instance, populations near 100,000 plants per acre are likely to produce maximum yields, and those around 80,000 will yield about 90% of the maximum. However, expected yields drop more rapidly in stands below 50,000, with 39,000 plants per acre likely to produce about 75% of the normal yield. Yield potential for corn at various plant populations is listed in Table 2. When gaps of two feet or more are present throughout the field, assume an additional 5% reduction in corn grain yield.
Table 2. Relationship between corn plant population and yield. Data from 2005-2008 at Lamberton and Waseca, MN. Source: Coulter (2009).
Estimating Yield Loss Due to Leaf Loss:
In addition to yield loss due to a reduced stand, growers should also consider added yield reductions due to leaf loss. Any green leaf area remaining on a plant will contribute to yield. Only consider leaf area lost if it is removed or brown in color. Information on corn yield and leaf loss is given in Table 3. For soybean, leaf loss up through the V4 stage (4 fully developed trifoliolate leaves) has little effect on yield.
Table 3. Relationship between corn grain yield and leaf loss. Source: Hicks et al. (1999).
Replanting should be considered only in fields where the crop is a total loss. Replanting corn at this time is not an option as the crop will not mature in time to make grain or silage. Soybeans may be replanted, but yields will be limited at best. Plating date studies have traditionally been cut off at about July 1, where resulting yields are about one half of a normally planted soybean crop. Experience with soybeans after peas has shown that planting a soybean variety that is at least one maturity group earlier than adapted to the region before July 4 will occasionally produce respectable yields. Producers are seldom content with yields from crops planted a week or more after July 4th. By this time, yield potentials fall to 40% of normal or less. Therefore, in fields that are a complete loss, replanting soybeans should happen as soon as possible, if at all.
Seed availability may be the primary determinant for replanting or not. Producers may wish to contact their seedsmen before laboring over this decision. In many cases, the choice may have been made for them already.
Coulter, J.A. 2009. Optimum plant population for corn in Minnesota [Online]. Available at http://www.extension.umn.edu/distribution/cropsystems/M1244.html. Univ. of Minnesota, St. Paul.
Hicks, D.R., S.L. Naeve, and J.M. Bennett. 1999. The corn growers field guide for evaluating crop damage and replant options [Online]. Available at http://www.soybeans.umn.edu/pdfs/CornGuide.pdf. Univ. of Minnesota, St. Paul.