By Jeff Coulter and Seth Naeve, Extension Agronomists
Recent storms left several areas affected by hail damage. Especially hard hit was a large area centered around Nicollet and Sibley Counties. Throughout this area, much of the corn was at the V8 stage (8 collared leaves) when damaged, and soybean had three to five fully developed trifoliolate leaves (V3-V5 stage).
In late June, 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 remaining plants per acre with healthy growing points that will recover, the amount of leaf area lost on these plants, and the growth stage at which the crop was damaged. 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 V8 stage, the growing point is located about a foot above the soil surface and has a small tassel at the top of it. 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 will not recover.
Another consideration is stem bruising. Severe stem bruising not only limits the plant's ability to translocate water and nutrients, but it also reduces standability. Plants with stem bruising should have their stalks split in order to determine the severity of the stem bruising, and whether the growing point has been injured. Plants with stem damage extending beyond the leaf sheaths and into the pith of the plant will either not recover or will likely have large reductions in yield. Fields with severe stem bruising should be harvested early to avoid significant losses from stalk lodging.
When there is whorl damage to corn, new leaves can have difficulty emerging through damaged tissue and can become tightly bound in the whorl. Plants with leaves that are tightly bound in the whorl can sometimes break free after about one week of growth. However, many of these plants are unable to recover. This makes it difficult to assess final plant population within just a few days after hail.
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 or entirely stripped of leaf tissue 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 get 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 in southern Minnesota. Source: Coulter (2009).
Estimating yield reductions due to leaf loss:
In addition to yield loss from a reduced stand, growers should 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 for grain at this time is not an option as the crop is not expected to reach black layer (maturity) before the first killing freeze. Replanting corn for silage may work if the corn is replanted very soon and a hybrid that is 15 or more relative maturity units earlier than a full-season hybrid is used. However, planting corn silage after about June 25 is not recommended in southern Minnesota.
Soybeans may be replanted, but yields will be limited by the short season remaining. Soybeans planted around the first of July routinely produce a yield of about half of that of normal planting dates. 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 reasonable yields. Producers are seldom content with yields from crops planted a week or more after July 4. By this time, yield potentials fall to 40% of normal or less. Therefore, in fields that are a complete loss, replanting of soybean should occur as soon as possible.
Seed availability may be the primary determinant for replanting or not. Producers should contact their seed suppliers as soon as possible to confirm that early-maturity seed is still available.
Caring for your hail-damaged crop:
For those producers who choose to keep their existing crops, care should be taken to ensure that these fields produce as much as possible. Because the crop has been placed under tremendous stress, it is important to reduce the level of future stresses. The most important and difficult challenge in hail-damaged crops often revolves around weed control. Good weed control must be maintained season-long; however, contact herbicides that stress the crop should be avoided where possible.
While it is crucial to avoid further stress to your crop, foliar fungicides are not likely to improve crop recovery and yield. The most damaging diseases affecting corn and soybean after hail are bacterial in nature. Fungicides have no effect on these bacterial diseases. Furthermore, the defoliated crop is not likely to effectively take up the fungicide. Producers should instead focus on pests that they can control (like weeds and insects) and avoid costly applications of inputs that are not likely to increase yield.
Coulter, J.A. 2009. Optimum plant population for corn in Minnesota. 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. Available at http://www.soybeans.umn.edu/pdfs/CornGuide.pdf. Univ. of Minnesota, St. Paul.