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Chemical residue tests for herbicide carryover are useful, but are slow and expensive. Predicting crop injury potential from lab results is difficult because interpretations are affected by sampling procedure, soil type; and crop.
A biological assay (bioassay) can be carried out with simple equipment. Crop seeds are planted into soil samples suspected of containing carryover herbicide. Plants are then observed for possible herbicide injury symptoms.
Bioassay results do not indicate the actual amount of herbicide residue present in the soil. They can indicate the potential for crop injury, but are not infallible.
Bioassays must be set up correctly to obtain meaningful results. A good bioassay consists of the following:
Collect each soil sample from the surface 3 to 4 inches from at least six different locations in the field. Each Final sample should consist of 10 pounds (about 1 gallon) of soil.
It is useful to collect separate samples from moderate- to low-risk areas and from high-risk areas. High risk areas include field ends, turn- rows; eraded areas, and terrace channels. Collect a “check” sample from a similar, nearby area that has not been treated with the suspected herbicide.
If the bioassay will not be set up within two days; freeze soil samples to prevent herbicide breakdown, which could affect results.
If soils are excessively wet, spread them out on clean paper ina warm room and dry them to a workable condition. If the soil is cloddy,; crush so the clods are no larger than a wheat seed, but do not pulverize.
Mix each soil sample thoroughly with an equal volume of clean sand. This provides good drainage and prevents crusting, both of which could affect results.
Use four flower pots or milk cartons as containers for each sample. Put about two quarts of the soil-sand mixture in each container. Mark the four containers with the sample identification.
Plant two of the containers with seeds of the sensitive crop; two with seeds of the tolerant crop. This Provides an alternate sample in case one sample is damaged. Space seeds evenly and plant at the proper depth.
Water containers well; but not excessively, and cover with plastic to prevent drying out. Remove the covering from all containers once seedlings begin to emerge.
Using seeds of two different crops that are sensitive and tolerant to the suspected herbicide improves the accuracy of the bioassay results. For example, oats are sensitive to many triazine herbicides, while corn is generally tolerant. If a triazine is suspected, but both oats and corn show injury; then the problem may be due to something other than the suspected herbicide.
For example; oats, peas, and snap beans are considered sensitive to atrazine residues. Corn and sorghum are tolerant, soybeans are moderately sensitive to moderately tolerant to triazines.
Corn, wheat, and annual rye are considered sensitive to trifluralin residues. Beans and peas are tolerant.
Keeping plant numbers low is important. If plant density is too high, each plant extracts a small amount of herbicide residue from the soil. The individual amount may be so small that it may not be toxic to the sensitive plants.
When fewer plants are grown in the same amount of soil, the individual plants take up more herbicide from the soil and show greater injury symptoms - even death - if the residue is present in harmful amounts.
Plant 4 to 6 large-sized seeds (corn; beans, etc.) per container, then thin to 3 seedlings immediately at emergence.
Plant 10 to 12 small-sized seeds (oats, wheat, etc.) per container, then thin to 7 to 9 seedlings at emergence. Thin carefully to avoid damage to other seedlings.
Importance of the Check Sample
Plants grown in containers can be damaged by soil crusting; poor drainage, damping off; or diseases. These symptoms can be mistaken for herbicide injury if an untreated check sample is not included for comparison.
If plants die in the suspected samples, but not in the check samples; herbicide injury potential is high in the field. If plants die in both suspected and check samples, herbicide injury is likely not the cause.
The person interpreting the bioassay must know the proper appearance and the time when injury symptoms are expected to appear. Each herbicide family has its own characteristic symptoms, but other growth problems can also mimic herbicide injury symptoms.
Triazine injury symptoms require adequate sunlight to develop, so may not appear until two to three weeks after emergence. Grass-type plants show a browning or bronzing on the tips of more mature leaves; which then progresses toward the stem.
Broadleaf plants affected by triazines show chlorosis (yellowing) of the tips and edges of more mature leaves. Brown speckling may appear near leaf edges and chlorosis eventually spreads over the entire leaf.
Dinitroaniline herbicides (trifluralin, etc.) inhibit root development, so plants must be dug up and the roots examined at seedling emergence. Damaged plants will be stunted, have poor root development, and leaves may fail to unroll Properly.
Lateral roots will be short, thickened, and stubby. The hypocotyl (area below cotyledons) may be thickened in broadleaf plants affected by dinitroanilines.
Injury symptoms of other herbicides will differ; so must be identified properly.
Furrer, J.D. and T. Lavy. 1977. A quick test for atrazine carryover. Neb-Guide G74-113. Coop. Ext. Serv.; Univ. of Nebraska, Lincoln; Nebr.
Behrens, R. 1970. Quantitative determination of triazine residues in soils by bioassay. Residue Reviews. 32:355-349.
Below, F.E. 1979. Bioassay for herbicide carryover. Crops and Soils. Am. Soc. of Agron. 31:12- 14.