By Jeff Coulter and Seth Naeve
Recent storms left several areas affected by hail damage, including south-central and southeast Minnesota along interstate 90, and central Minnesota near St. Cloud. In southern Minnesota, corn was at the V7 stage (seven collared leaves, including the lowermost leaf with a rounded tip) when damaged, and soybean had two to three fully developed trifoliolate leaves (V2-V3 stage). In central Minnesota, the crops were not quite as far along when damaged (Figures 1 and 2).
Figure 1. Hail-damaged corn in central Minnesota. Photo by Dan Martens.
Figure 2. Hail-damaged corn in central Minnesota. Photo by Rick Gilbertson.
In late June, assessing hail damage and making replant decisions for corn and soybean 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 following online guides for evaluating hail damage and making replant decisions:
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 physically damaged by large hail stones landing on soft soil, and by freezing from hail accumulation on around the stalk 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 the growing point is slightly damaged, it will likely be invaded by bacteria and the plant will die.
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 pre-harvest 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 3). Plants exhibiting severe buggy-whipping should not be counted as living plants, as their contributions to yield will be limited.
Figure 3. 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 (Figures 4 and 5). Soybean plants cut below the cotyledons will not recover.
Figure 4. Soybean damaged at the cotyledon (VC) growth stage in Sibley County, MN. Photo taken on June 10, 2008 by Dave Nicolai.
Figure 5. Hail-damaged soybean (same field as Figure 4) in Sibley County, MN. Photo taken 16 days later on June 26, 2008 by Dave Nicolai.
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 potential, based on numerous trials 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. The relationship between corn grain yield and leaf loss. Source: Hicks et al. (1999).
Replanting Considerations: Determine replant costs. These will include time, fuel, and seed costs. Additional costs may include penalties associated with variety selection when the very best genetics are no longer available. For corn, most agree that it is risky to plant in mid- to late June for grain. In general, the latest recommended planting dates for corn grain are June 15 in southern Minnesota and June 5 in central and northern Minnesota (Table 4). Reasonable silage yield can be obtained when planting corn as late as June 25 in southern Minnesota. Yield potential of silage corn planted in late June is uncertain. Corn silage planted in late June may produce reasonable dry matter yields in central Minnesota, but it will most likely have low quality (milk/ton) due to very limited grain (starch) production.
Table 4. Corn maturity guidelines for late planting in Minnesota. Source: Hicks et al. (1999).A number of growers in southern Minnesota have indicated that they will try replanting hail-damaged corn fields to very early-maturing corn hybrids (around 80-day relative maturity) because they cannot plant soybean due to the herbicides already used for corn, and because they have already made their nitrogen fertilizer applications. Information on available growing degree days and hybrid maturity are listed in Tables 5 and 6. Most early-maturing corn hybrids, however, do not have transgenic resistance to corn rootworm. According to Ken Ostlie, Extension Entomologist with the University of Minnesota, there is a little potential for larval corn rootworm damage when planting corn after June 10, and thus the use of soil-applied insecticide for corn rootworm control is generally not necessary.
Table 5. Estimated growing degree days (GDD) available in southern (Albert Lea) and central (St. Cloud) Minnesota from June 25 to the average first frost date.
Table 6. Growing degree days (GDD) to physiological maturity for various corn hybrids when planted on May 1 or June 25.Mid-June soybean replants should utilize varieties that are approximately 0.5 maturity units shorter than full-season varieties adapted to your region. Planting in late June will require a shift of one full maturity unit. Yield potential for late-planted soybean drops rapidly throughout June. When compared to the yield from an early May planting, yield potential for a June 24 planting date is only 57%.
Only those soybean fields with remaining stands below 30-40,000 plants per acre are likely to produce greater yields when replanted. Due to increased fuel and seed costs, the economics of replanting will often be cost prohibitive with remaining soybean stands above 30,000 plants per acre. Take many stand counts throughout the field and carefully weigh the economics of the situation. Be sure to speak with your insurer and leave appropriate check strips if needed.
Coulter, J.A. 2009. Optimum plant population for corn in Minnesota [Online]. Available at http://www.extension.umn.edu/distribution/cropsystems/M1244.html (verified 22 June 2009). 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 (verified 22 June 2009). Univ. of Minnesota, St. Paul.