Agriculture Reference
In-Depth Information
shown), the estimated half-lives were 63 days and 92 days, respectively, demonstrating that
dry and cool conditions decrease the rate of herbicide dissipation and could result in enough
herbicide persisting in soil to cause carry-over injury to sensitive rotational crops.
120
100
pyroxasulfone
pyroxsulam
80
60
40
20
0
0
20
40
60
80
100
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Time (day)
Figure 5. Pyroxasulfone and pyroxsulam dissipation under laboratory conditions of 25°C and moisture
content of 80% field capacity determined by the 7-day sugar beet shoot length and the 4-day sugar beet
root length bioassay, respectively.
If active residues of herbicides remaining in soil from previous herbicide application are
suspected, growers may consider submitting field samples for testing. Soil testing can be
performed by chemical methods or bioassays. Because chemical methods determine total
amount of a herbicide in soil, they are poor predictors of residual phytotoxicity and potential
crop injury. Bioassays detect the presence of bioavailable residues and are generally
considered to be a better risk-management tool for making recropping decisions that is
available to growers [8, 9].
C
ONCLUSION
The examples of the application of sugar beet bioassays described in this chapter show
that sugar beet is a very useful bioindicator plant for detection of herbicides in soil.
Depending on the herbicide mode of action, different sugar beet growth parameter may
respond to a herbicide of interest. Root length and shoot length inhibition of sugar beet
proved to be suitable for detection of pyroxsulam (Group 2 herbicide) and pyroxasulfone
(Group 15 herbicide), respectively. Performing sugar beet bioassay in Whirl-Pak® bags is
simple and requires only 50 g of soil per bag. Plants are grown in Whirl-Pak® bags filled
with soil in a laboratory under fluorescent lights, and then after 4 days for pyroxsulam and 7
days for pyroxasulfone, intact sugar beet plants are removed from soil and are measured.
Bioassays using sugar beet plants are an effective method for evaluating phytotoxicity,
dissipation and residual activity of herbicides in soil environments.
