Slugs cause significant economic injury to corn and soybean crops in Virginia every year. Symptoms of slug feeding will vary depending on the size or the growth stage of the crop, and the size of the slug. In corn, slug damage is typically limited to defoliation of emerging leaves. While extensive leaf feeding will have detrimental impacts to yield, rarely will slugs actually kill the corn plant because the growing point will remain below ground until the V6 growth stage. Slug damage to soybean is different however because the growing point of the soybean plant is within the emerging cotyledons. If slugs are actively feeding when germination occurs, they can feed on the cotyledons and cause death of the soybean plant. Luckily slug damage is typically localized in a field, and if scouting has caught the infestation early further damage can be prevented with targeted applications of the various slug baits that are available. If however, feeding injury has been severe enough to result in significant stand losses, then the difficult decision of whether to replant or not must be made.
Making the decision to replant requires careful consideration, and when all is said and done the economics must be positive in order for a replant decision to move forward. Replanting should only be contemplated if the cause for the sparse stand can be corrected. Generally in Virginia, planting later into warm soils will promote rapid early growth in the young plant, which should then be able to outgrow the slug pressure. Typically replanting will require a complete removal of the existing failed stand, rather than filling-in with new plants. This is particularly true with corn, because plants of uneven sizes and maturity perform poorly. Filling-in may be acceptable in a few cases, specifically: 1) If you can fill-in within two weeks of the original planting date; and 2) If you can get a uniform plant spacing within the row between the old and new plants. Unfortunately with slug injury, 2 weeks is probably too early to indicate a problem. Waiting longer than 3 weeks between plantings for fill-ins generally reduces yield potential by at least 10%.
The following provides a set of step-by-step procedures that may be followed when considering whether to replant a damaged stand:
- Estimate the possible yield of a full stand at the original planting date.
This is the “best guess” estimate of what a realistic yield for the crop would have been prior to damage.
- Determine population and uniformity of the existing stand.
Count the number of live plants in the damaged areas to determine the plant population of these areas. This should be done in at least 3-5 spots in the affected area since severity of slug damage can vary considerably. An easy way to do this is to count the plants in a length of row equal to one-thousandth of an acre (table 1). Determine the number of plants in the specified row length and multiply by 1000 to determine plants per acre. In drilled soybeans it may be easier to determine stand density with the “hula hoop” method. Using any perfectly round hoop with a known diameter, toss the hoop in at least five randomly selected locations within a damaged area. Average the number of plants counted within each hoop, determine the hoops area and convert to plants per acre using the following formula:
Plants per Acre = (average count * 43,560) / (3.14 * hoop radius in inches2)
- Estimate the yield potential of the existing stand.
Estimating how much a damaged stand will likely yield is the most difficult decision in the process. This is in many cases an educated guess, because potential yield is strongly affected by environmental conditions. Table 2 provides estimated corn and soybean yield potential at various plant populations (yield as % of normal). Uniformity of the stand is also critical. Virginia research in soybean has indicated that large gaps cause as much yield loss as low plant populations. To determine the percentage of gaps in the damaged stand, pace off sections of row 20 paces long in at least 6 areas of the field. Determine (by counting the number of paces) the total length of row lost to 2-3 foot gaps. Then determine the percent of stand lost to gap spacing. Table 3 provides some guidance for estimating how much yield loss (as % of normal) you may expect in the soybeans from gaps.
- Estimate the potential yield of replanted full stand, on the replant date.
The delay in planting date caused from deciding to replant crops injured by slugs may have a significant affect on the final yield. Table 4 gives some guidance on the potential corn yield loss based on delaying the planting date. Remember too that if the decision was made to fill in instead of kill the entire field, later emerging plants cannot effectively compete with the remnants of the original population for sunlight water and nutrients.
- Estimate the costs of replanting.
Even if the yield from replanting would be greater than that from the damaged field, the cost of replanting may exceed the value of the additional yield gained from replanting. Estimate as closely as possible seed cost (unit cost by seeding rate), fuel, machinery and labor costs associated with replanting, pesticide costs (usually no additional pre-emergence herbicide will be required, but a burndown will), and any other costs including interest, drying later maturing corn or beans, or the cost of delaying planting the fall small grain if desired.
- Compare the value of leaving the reduced stand to a replanted stand.
The following worksheet (Table 5) provides a guideline for comparing the costs of replanting versus leaving a damaged stand.
Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.
May 14, 2009