The length of time a herbicide persists or remains active in the soil determines the period of weed control that can be expected through the current growing season and the potential for carryover to the following year. Although most herbicides break down within the same growing season in which they are applied, some herbicides persist longer than others and may be especially harmful to specific crops. The overall potential for carryover is a function of the herbicide used, the accuracy of application, the follow crop grown, and the environmental conditions following herbicide application.
Degradation of most herbicides in soil is the result of chemical and microbial breakdown processes. The rate of breakdown increases with soil temperature, and both processes generally require adequate soil moisture. Because a large portion of the herbicide is degraded in the summer and early fall following application, very dry conditions during this period will increase the potential for carryover of many herbicides. Carryover from one season to the next is less likely to occur after a mild, wet winter than after a cold, dry winter where the soil remains frozen for an extended period of time.
Soil moisture level and temperature also affect the follow crop's growth and thus its tolerance to herbicide injury. The crop's more likely to show injury symptoms from herbicide carryover if it is weakened by stress from adverse climate, disease, or nutritional deficiencies. Yield reduction from early herbicide injury is more likely to occur when adverse growing conditions continue throughout the growing season. The crop may easily overcome herbicide injury if favorable growing conditions occur throughout the season.
Herbicide carryover is also influenced by herbicide rate, distribution, soil type, and time. While most herbicides are safe in rotation at use rates, higher rates occurring where herbicide is not uniformly distributed may result in carryover problems. Poor distribution is generally the result of improper calibration or agitation, sprayer overlapping, or non-uniform incorporation.
Longer intervals between herbicide application and follow crop planting allow the herbicide more time to break down, reducing the risk of carryover. Delayed planting the year following application reduces the probability of carryover. Where double cropping or intercropping practices are used, carryover problems may increase due to the number of crops planted within a fairly short period of time.
Herbicides are more persistent in fine-textured, high organic matter soils than in coarse-textured, low organic matter soils. Soil's absorptive capacity for herbicide increases as organic matter and clay content increases. Because microbial and chemical degradation reactions occur mainly in the soil solution, absorption of herbicide on soil tends to "protect" the herbicide from breakdown. However, adsorption also results in a reduced availability of herbicide for plant uptake and increased persistence does not always result in an increased potential for carryover.
The persistence of some triazine (atrazine) and sulfonylurea (chlorimuron, prosulfuron) herbicides is greater at high soil pH than at low pH. Persistence of Scepter and Command is longest at low pH (<5.9). Follow label directions regarding the application of herbicides and soil pH.
The sensitivity of a crop and its genetic makeup affect the potential for carryover injury. Vegetable and ornamental crops are generally more sensitive to herbicide carryover than field crops. Within a specific crop, some varieties are more tolerant of a given herbicide than others. Herbicide labels contained restrictions regarding the interval that must occur between application of a herbicide and the planting of various crops.
Guidelines To Avoid Carryover Problems:
2) Calibrate the sprayer and apply herbicide accurately and uniformly.
3) If incorporating, make sure it is done thoroughly and uniformly.
4) Consider applying reduced rates of a persistent herbicide in combination with a less persistent herbicide.
5) Select herbicides based on rotation plans. Follow the recrop restrictions on herbicide labels.
6) Apply herbicide as early as possible and delay planting of the follow crop if carryover is suspected.
Testing for Herbicide Residues
In fields where a carryover problem is suspected, bioassays or soil tests may be performed to determine if unacceptable levels of herbicide residue are present. In a bioassay, one or more sensitive species are grown in the "suspect" soil and compared to the growth in "check" soil not treated with the herbicide in question. This comparison makes it possible to separate carryover injury from injury caused by plant disease, environmental stress, or failure to water plants at the right time. It may be most appropriate to bioassay soil with the plant species that is to be planted in the "suspect" field. For suspected triazine carryover, a bioassay using oat plants is often effective. For dinitroaniline (e.g. Treflan), sulfonylurea (e.g. Classic), and imidazolinone (e.g. Scepter), herbicides, corn can be an effective bioassay species. These three classes of herbicide chemistry inhibit root growth, so it is important to observe corn root growth in the bioassays.
Samples for bioassays should be taken from the field in early to mid-spring, leaving enough time to observe the effects before making a recrop decision. The method of sampling can be critical. A group of samples mixed together may not be accurate because the resulting average will not show whether "hot spots" of high herbicide concentration exist. Where soil has been moldboard plowed, sample to the depth of tillage (about 6 inches). In no-till or where soil has been chisel plowed, herbicide remains more concentrated in the upper few inches of the soil and samples should be taken from fairly shallow depths (about 3 inches).
Some laboratories will test soil samples for herbicide residues, but the procedure can be expensive. Interpretation of test results is difficult, since carryover potential is dependent not only on the actual herbicide concentration but also the availability of the herbicide to plants. Availability of herbicide varies with soil texture, organic matter content, and moisture. For the triazine herbicides and others that have been used for a number of years, it is possible to estimate carryover potential from test results. For some of the newer herbicides, a lack of information in general may preclude meaningful interpretation of test results.
The following table provides a rough guideline for planting various crops based on laboratory soil test results for triazine residues.
| Triazine Residue Level | "Safe" to plant | |
| 3 inch sample(no-till) | 6 inch sample(moldboard plow) | |
| less than 0.17 ppm | less than 0.08 ppm | oats, alfalfa |
| 0.17 to 0.35 ppm | 0.08 to 0.17 ppm | soybeans |
| greater than 0.35 ppm | greater than 0.17 | corn |
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