# HPPD carryover in soybeans: plant-back and forage decisions URL: https://leafwrit.com/us/herbicide-plantback-grazing-restrictions/ Reviewed: 2026-10-29 HPPD carryover in soybeans shows up as chlorosis, bleaching and tissue death. Group 27 herbicides interfere with pigment synthesis, and injury can come from reduced dissipation during dry years or from overlapping applications. Carryover injury can appear as crinkling or strapping of leaves with bleached tips and plant stunting. Carryover from Group 27 products is uncommon in most years. Late applications combined with a dry summer raised the number of cases in 2019. ## Separate symptoms from the suspected route of exposure HPPD injury is most often visible on the newest soybean leaves. Mesotrione carryover can show up when soybeans reach the V2 growth stage. Carryover injury can be confused with growth regulator injury, and drift from a nearby corn application can also injure soybeans. Crop injury comes from herbicide applied directly to the crop, carryover from the previous year, contaminated sprayers or drift from adjacent fields. Field history, field topography and field edges distinguish these sources. Each route leaves its own pattern: - Carryover is typically associated with soil types, or it appears in streaks or oddly shaped field areas where applications overlapped. - Tank contamination usually injures the whole field, with worse symptoms where passes overlapped. - Drift injury declines with distance from the source, and symptoms at the field edge point to drift. - Symptoms scattered throughout a field suggest a soil-applied herbicide. Sprayer overlap injury looks like parallel streaks of symptomatic plants. HPPD inhibitors move through plants via xylem, phloem or both, depending on the chemistry and plant species. Note the newest affected leaves separately from the older foliage when you compare symptoms. Look at the leaf position, the tissue pattern and the distribution across the field together with the application history, and do not name a suspected product from a photo of yellowing alone. Whoever helps you interpret the injury should be able to see whether symptoms follow a boundary, an overlap, a soil zone or the route of a machine. Compare the newest leaves with the [HPPD injury photographs](https://cropprotectionnetwork.org/encyclopedia/hppd-hg-27-inhibitor-herbicide-injury-in-soybean) to learn the symptom vocabulary, and keep your own notes factual. ## Why dry weather raises the risk of carryover Soil moisture is often the most critical factor governing the persistence of soil-residual herbicides. Soil microorganisms degrade many herbicides, and their populations can be greatly depressed when soil moisture is limited. Dry soils also enhance herbicide adsorption to soil colloids, which leaves the herbicide unavailable for plant uptake and microbial degradation. Herbicide carryover is generally seen in isolated areas of a field. Areas with low organic matter, high pH, or high herbicide rates from application problems are likely to show injury. Residues in the soil do not mean the crop will be injured. Extremes in soil moisture or temperature that stress the crop increase the chance of carryover injury, and planting into favorable soil temperature and moisture helps keep carryover symptoms from developing. ## Treatment history: what a carryover investigation needs Herbicide records from previous seasons are the basis of a carryover investigation, and up to three or four years of records may be needed. The labels of the herbicides previously applied give the plant-back intervals. For a specific application, the record covers the products used and their active ingredients, the date of application and the weather at that time, and the other products in the tank mix, including insecticides, fungicides, fertilizers and adjuvants. Herbicides can interact with other pesticides or adjuvants to increase the risk of non-target injury. Uneven or late-season applications increase the potential for interactions with the herbicides of the following year. The persistent products involved include atrazine, clopyralid and the HPPD inhibitors. Start with the actual product names and EPA registration numbers, then the dates, rates and the area each treatment covered. Include repeat treatments, tank-mix components and work done on only part of a field. Where a treatment went can matter as much as the whole-field total. An overlap, a headland or a corner that was treated repeatedly may have a different history from the middle of the field, so compare the injury pattern with soil zones and application overlaps and keep those distinctions in your notes. Before you plant, list every earlier treatment that could matter and read the current rotational-crop section of the label for those exact products. If you plan to graze or cut forage, read the livestock and harvest directions separately. ## Callisto: the 10-month soybean interval Pesticide labels are legally enforceable. Nearly all herbicide labels have rotational crop intervals that set the time between application and planting of a rotational crop. For Callisto, EPA registration **100-1131**, booklet **SCP 1131A-L1S 0426**, Table 3 sets soybeans in the 10-month group, with alfalfa, canola, cotton, peanuts, potato, rice, sunflowers and tobacco. Other Callisto intervals in Table 3: corn, oats, flax and sorghum anytime; wheat, barley and rye 4 months; cucurbits, dry beans, red clover, sugar beets and all other rotational crops 18 months. Peas and snap beans sit in the 10-month group only when all of these hold: at least 20 inches of rainfall plus irrigation between application and planting, soil pH of 6.0 or greater, an application of 3 fl oz/A or less no later than June 30 of the preceding year, and no other HPPD herbicide applied the year before planting. Otherwise the minimum is 18 months. When Callisto is tank mixed with other products, the most restrictive product's crop rotation interval applies. Read the whole rotational-crops section together with the table. Read each footnote with the row it belongs to: do not carry the conditions for peas or snap beans over to soybeans, and do not assume that every crop listed near soybeans has the same conditions. ## Mesotrione-tolerant soybeans are a separate use Callisto can be applied preemergence to soybeans identified as mesotrione tolerant. Applications to soybeans that are not mesotrione tolerant result in significant crop injury, and Syngenta Technical Representatives supply the list of tolerant varieties. On tolerant soybeans, apply Callisto prior to soybean emergence at 6.0 fl oz/A. The maximum is 6.0 fl oz/A (0.19 lb ai/A) per year, with one application per year, and application to emerged soybeans is prohibited. Registered soybean herbicides such as Dual Magnum can be tank mixed. If weeds are already emerged, add a non-ionic surfactant (NIS) at 1 qt/100 gallons or a crop oil concentrate (COC) at 1 gallon/100 gallons. Before relying on this use, confirm that the variety you planted is on the mesotrione-tolerant list. ## Flexstar, Classic, Sonic and Stinger herbicide carryover in corn and soybeans Carryover is read from the rotational-crop table of the label of the exact product, because nearly all herbicide labels have rotational crop intervals. Flexstar herbicide (EPA reg. no. 100-1101) contains fomesafen, and bronzing or speckling of older soybean leaves is the symptom of a fomesafen application. Sonic herbicide (62719-680) contains sulfentrazone and cloransulam-methyl. Stinger (62719-73) contains clopyralid, and clopyralid in the soil may cause carryover injury to soybean. Classic herbicide (5481-681) contains chlorimuron. For Flexstar carryover in corn, Stinger carryover in soybeans or any other pair, find the corn or soybean row in the rotational-crop table of that product's label and its conditions, and do not carry the interval over from another product. For Flexstar and other fomesafen herbicides (Flexstar GT, Marvel, Rumble) the planting interval to corn is 10 months, and a tank mix takes the longest interval of any partner. Sonic is also 10 months for field corn, but 18 months for sweet corn. Sweet corn and popcorn are more sensitive to herbicide residues than field corn, so check their rows separately. These figures are a starting point: the rotational-crop table of the label in your hands, with its conditions, decides. ## Plant-back and preharvest intervals measure different things A rotational crop interval is the time between herbicide application and planting the next crop. These intervals are set to reduce the likelihood that residues persist in amounts that harm the rotational crop. Some are based solely on time, while soil pH and precipitation after application change the length of others. Check both questions separately: when the next crop can be planted, and when the current crop can be harvested or the last application made. Keep earlier applications in the field's history after harvest. A preharvest interval is the time between application and harvest of the treated crop. Failing to observe it may leave residue levels in the harvested crop above established limits. The labels of almost all postemergence soybean herbicides state a preharvest interval or a soybean developmental stage beyond which applications cannot be made. ## Grazing, forage and hay: separate restrictions A plant-back interval says nothing about livestock. The Callisto soybean directions state: do not graze or feed soybean forage or hay to livestock. Livestock grazing or foraging on treated soybean is not allowed on the labels of many postemergence soybean herbicides. Read the section for the crop you are actually growing instead of carrying a livestock allowance over from another crop or from an older summary table. Callisto sets different limits by crop. In corn, do not feed or harvest forage, grain or stover within 45 days after application. In oats, do not graze or feed forage from treated areas within 30 days following application. For grass crops grown for seed, do not graze or feed forage within 14 days following harvest of seed or straw and at least 74 days after application. ## Other causes that resemble HPPD injury Atrazine and metribuzin belong to Group 5, which interferes with photosynthesis and causes chlorosis and necrosis of leaves. Atrazine injury can come from carryover or drift, and metribuzin injury from direct application. Both are more likely to injure soybeans on high-pH soils. The injury is typically most noticeable on unifoliate and first trifoliate leaves as interveinal chlorosis and necrosis. Growth regulator injury is symmetrical: all three leaflets of a soybean leaf are injured similarly, and low doses typically cause strapped or cupped leaves. HPPD injury can look like very minor growth regulator injury, and the bleached tissue distinguishes the two. Non-herbicide causes also mimic herbicide injury. Distorted leaves on new growth can come from frost or hail, or from thrips, aphids or leaf-roll caterpillars. Chlorosis and necrosis from drought stress or nutrient deficiency also resemble herbicide injury. Similar symptoms in areas managed by different people point to an environmental or biotic cause, and a single symptomatic species among several suggests the cause is not a herbicide. ## Locating symptoms on the plant Describing symptoms means naming them (epinasty, stunting, chlorosis, necrosis) and where they appear: old or new growth, leaves, stems or roots. Healthy plants from the same area and nearby fields serve as the comparison. On grasses, new growth is where the smallest, youngest leaves come out of the leaf sheath. In broadleaf plants, new growth occurs at the end of the stem (apical bud) or sometimes between a leaf and the stem (axial bud). A herbicide's mode of action is how it injures the plant, for example by disrupting photosynthesis or blocking production of certain amino acids. The WSSA site of action group number is often listed on the herbicide label. Write the active ingredient and the site of action group into your notes next to the symptoms. ## Bioassays and laboratory analysis Because carryover is spotty, the accuracy of bioassays is questionable, and the results are often false positives or negatives. Take samples from the affected patches and from comparison areas, so the bioassay reflects the different histories of those places. Soil and plant tissue can be analyzed for herbicide residues at a laboratory for a fee, and such an analysis may be necessary to confirm a diagnosis. Tell the laboratory which chemicals and exposure routes your field investigation points to, and keep the report together with your field notes. ## Contaminated compost, manure and topsoil Carryover can happen when contaminated materials are moved from one area to another. Manure contaminated with persistent synthetic auxin herbicides, for example, can cause herbicide injury symptoms. Compost, manure and topsoil brought to a site are routes by which carryover arrives. Ask whether compost, manure or imported soil was brought to the field. An investigation that looks only at the last sprayer pass can miss this route. ## If drift is the more likely cause Write down the field-edge pattern, the time of your observation and any nearby spraying, without assigning blame in your notes. Compare the drift pattern with the mapped treatment history of the field. ## Next step Assemble the field treatment history and read the exact product label; how a label is organized, including the rotational and grazing statements, is covered in [reading a pesticide label](/us/read-pesticide-label/). Use the [injury investigation checklist](https://nextgeneration.montana.edu/extension/Full_HTML_Pubs/herbicideinjuryguide/diagnose.html) to record symptoms and competing explanations. ### What does HPPD carryover in soybeans look like? Group 27 injury can include chlorosis, bleaching, tissue death, crinkling or strapping and stunting. Other injuries can look similar, so the application history and the field pattern are read together with the symptoms. ### Why does drought increase herbicide carryover concerns? Dry conditions can reduce herbicide dissipation and microbial degradation. Injury also depends on the product, crop, soil and subsequent growing conditions. ### What plant-back interval applies to Callisto before soybeans? For Callisto, booklet SCP 1131A-L1S 0426, wait 10 months before planting ordinary soybeans. Direct preemergence use on mesotrione-tolerant soybeans has separate directions. ### Is atrazine carryover the same as HPPD carryover? No. Atrazine-related injury can cause chlorosis and necrosis through photosynthesis inhibition. Carryover, drift and direct application each leave a different field pattern. ### Does a plant-back interval authorize grazing? Planting, grazing and feeding restrictions address different activities. The current Callisto soybean section prohibits grazing or feeding soybean forage or hay to livestock.