Herbicide carryover is a common question for specialty crop growers, particularly those who rent land for production. The following article, written by Kristen Callow, OMAFRA’s Weed Management Program Lead for Horticulture, with excerpts from Nebraska Cooperative Extension Publication G74-113A, outlines a general test that can help determine if atrazine remains in soil. Note that this is a general article applicable to most crops. For particularly sensitive specialty crops, you may want to consider including the crop you are interested in growing (e.g. tobacco) in the bioassay, in addition to the oats or beans suggested here.
How can you tell if you have atrazine carryover in your fields? Plants grown in soil samples can tell.
Residues of atrazine may remain in the soil and affect some susceptible crops the next year. Crops most often affected include most broadleaf horticultural crops, sugarbeets, soybeans, field beans, alfalfa, oats, and wheat. Attempts to predict the extent of carryover and damage to sensitive crops the year following atrazine use have been only partially successful. The rate of atrazine disappearance and, therefore, the amount remaining the next year, is affected by soil texture, pH and organic matter content, as well as atrazine application rate and timing, rainfall, and plant growth the previous year. Chemical analyses for atrazine are complicated, expensive and can be made only in specialized laboratories. In addition, results obtained from chemical analyses do not necessarily reveal whether your crop will be injured.
Plants Can Tell
Biological assays, using test crops similar to the field crop to be grown, are practical and can be done with simple equipment found in most homes or offices. The biological assay outlined as follows does not provide an exact measure of the amount of atrazine present in the soil, but it can show you whether sufficient atrazine remains to harm sensitive crops.
The Soil Sample
Get a representative sample of soil from the field suspected of having atrazine residue. Take samples from the top two to three inches in several locations. Atrazine residue often occurs in patches in a field. Sample enough places to avoid missing areas of high residue, such as turn-around areas and eroded knolls. Plants growing in such areas frequently show the most injury. You should take and analyze separately samples from areas where excessive residue is anticipated. Collect your soil sample from at least six places in each area sampled. Remember that the assay is only as reliable or representative as the sample you begin with. A total of 10 pounds of soil is required for each sample assayed. Assays should be run on the moist samples within two days after they are obtained from the field. Samples that cannot be assayed immediately should be stored in a freezer, if possible. Atrazine residue may decrease due to chemical and biological breakdown if samples are stored indoors under warm conditions. Wet soil should be spread out and allowed to partially dry to a workable condition. If the soil is cloddy, crush so the clods are no larger than wheat seed, but do not pulverize.
A “No Atrazine” Sample Needed
Potting soil can be used as a control sample with no atrazine residue.
Planting and Growing
Flower pots are satisfactory for planting and growing. Use the same kind of container for both treatments. Ensure there are holes in the bottoms for drainage. Fill one container with uncontaminated soil and another with soil as it came from the field. Properly identify each container. Atrazine injury symptoms on seedlings should become apparent between eight and 20 days after the plants emerge. Less time is required if high levels of atrazine are present. With temperatures lower than 70°F it will take longer. Plant six beans (soybeans, field beans or garden beans) or 10 oat seeds in each container. Press or punch the seeds no deeper than 1/4 inch into the soil. Thoroughly wet the soil. Germinate beans at 72°F. Lower temperatures are satisfactory for oats. Cover the containers with plastic food wrap until plants begin to emerge to help ensure favourable moisture conditions during germination. At emergence remove the plastic. Water as needed do not let the soil dry out.
Soon after emergence, thin the beans to three plants per container by clipping. Thinning should be done as soon as possible after emergence to reduce the amount of herbicide removed by discarded plants. It is not necessary to thin the oats. Plant and thin so there is uniform distance between the seed and plants. Keep the containers in a warm place (70 to 75°F) where they will get as much sunlight as possible. Sunlight is essential for the development of atrazine injury symptoms. Artificial light has much less intensity than sunlight and, therefore, is not entirely satisfactory.
The Symptoms and Crop Choices
When low atrazine levels are present, atrazine symptoms normally appear in two weeks, providing moisture, temperature and light have been favourable. Leaf tips or edges of the older, more mature leaves become chlorotic (yellow-brown). At about the same time a brown speckling may appear near the soybean leaf edges. Eventually chlorosis may spread through the whole leaf. Plants growing in uncontaminated soil should be normal. NOTE: If test plants die in both containers, disease could be the cause. If atrazine symptoms occur, plant the field from which the samples were obtained to an atrazine tolerant crop such as corn.
Why Limit Plant Population When Testing?
Research shows that with too many plants per container, low atrazine residues are not detected. It is most important that plant numbers be kept low. Three bean plants or 10 or less oat plants are all that should be used in five to 10 pounds of soil. These numbers more nearly approach field conditions, and provide a more accurate indication of low atrazine levels. Simply put, when large numbers of plants are present, each plant extracts a very small amount of atrazine from the soil. The amount is so small that it may not be toxic to the bean or oat plants. When fewer plants are grown in the same amount of soil, the roots have more room to spread out. With more soil per plant, each plant is able to extract more atrazine from the soil and will show greater injury symptoms, or even death, if the atrazine is present in harmful amounts.
Bioassay Not Limited to Atrazine
The bioassay outlined has been directed to atrazine (AAtrex). The same bioassay procedure could be used to determine soil residues of metribuzin (Sencor/Lexone), simazine (Princep), linuron (Lorox), cyanazine (Bladex) and other triazine or substituted urea herbicides. Similar procedures can be used to test for dinitroaniline herbicides (Prowl and Treflan), although injury symptoms and susceptible crops will differ from the triazine and substituted ureas.
Table 1. Soil Persistence of Some Common Herbicides Applied at Labelled Rates
|1 Month||1 To 3 Months||3 To 12 Months||More Than 12 Months|
|2,4-D||Basagran (bentazon)||Aatrex (atrazine)||Arsenal (imazapyr)|
|Liberty (glufosinate)||Broadstrike (flumetsulam)||Buctril (bromoxynil)||Hyvar X-L (bromacil)|
|Roundup, Touchdown, many others (glyphosate)||Callisto (mesotrione)||Classic (chlorimuron)||Telar (chlorsulfuron)|
|MCPA||Dacthal (DCPA)||Command (clomazone)||Tordon (picloram)|
|Dual II Magnum (s-metolachlor)||Converge (isoxaflutole)|
|Eptam, Eradicane (EPTC)||Karmex (diuron)|
|Frontier (dimethenamid)||Princep Nine-T (simazine)|
|Lorox (linuron)||Pursuit (imazethapyr)|
|Option (foramsulfuron)||Prowl (pendimethalin)|
|Sencor (metribuzin)||Reflex (formesafen)|
|Treflan (trifluralin, many)|
Note: timeframe may be wide due to variability of soil types and climatic conditions
Adapted from Table 2, http://ipm.illinois.edu/pubs/iapmh/15chapter.pdf
Table 2. Levels Needed to Detect Injury Symptomology of Group 5 Herbicides (atrazine, metribuzin, prometryn, simazine, etcetera)
Adapted from Triazine Residue Guide, Provincial Pesticide Residue Testing Laboratory, March 1982
|Soil Residues of Group 5 Herbicides||Injury Symptom|
|Less than 0.05 ppm or ug/g||Generally no injury except to very susceptible vegetables (e.g. cucumbers) and cash crops (e.g. sugarbeets, tobacco) planted in sandy soil during warm, dry weather.|
|0.05 to 0.10 ppm or ug/g||Slight injury or stunted growth may be expected to susceptible plants (see above) in sandy loam at normal weather conditions. Tomatoes, oats and alfalfa may be affected.|
|0.10 to 0.20 ppm or ug/g||Sugarbeets, tobacco, oats and vegetables are very sensitive to triazines and should not be planted. Beans (soya, kidney, white) and barley may be affected. Exception is siol with high organic matter content (>12%).|
|0.20 – 0.30 ppm or ug/g||Injury to most crops except corn, sorghum, sugarcane and turf grass.|
|0.30 and over||Severe injury can occur on all susceptible crops.|
- These residue levels are based on representative soil samples.
- Increased organic matter content in soil reduces injury.
- Dry and warm weather accentuates triazine activity.