Agriculture Reference
In-Depth Information
options are limited and potential for crop injury is high [1]. However, introduction of
glyphosate-resistant sugar beet allowed the use of glyphosate to eradicate weeds without
harming the sugar beet crop [2]. Another issue related to sugar beet sensitivity to herbicides is
the potential carry-over injury to sugar beet from a herbicide that was applied in previous
crop and that persisted in soil particularly under conditions of low moisture and cool
temperature [3, 4]. Because of the sugar beet sensitivity to herbicides, sugar beet is a very
useful bioindicator plant for bioassay detection of herbicides in soil.
Herbicides in soil are determined either by plant bioassays or by chemical methods [5].
Bioassays detect the phytotoxic portion of herbicide that is available to susceptible plants,
while chemical methods determine total quantity of herbicide in soil. Because the
bioavailable herbicide is detected by bioassays, this methodology is suitable for assessment of
the effect of soil properties on herbicide behavior in soil [6, 7]. Also, because bioassays are
very sensitive, they are frequently used in soil testing for detection of residual herbicides that
may cause injury to rotational crops [8].
In the development of a bioassay, once it is established that sugar beet is susceptible to a
herbicide under investigation, the next step is the selection of the sensitive plant parameter to
be measured in response to a herbicide. Parameters that are easy to assess in plant bioassays
are root or shoot length [9], and root or shoot fresh or dry weight [10], but many other
parameters have been used [8, 11]. Next, the duration of sugar beet growth required to obtain
a significant response is selected. Shorter growth period is preferred as the effect of soil
nutrient status and other soil properties is minimized and plant response is primarily due to
the herbicide. Generally plant response is evaluated in a concentration range from zero to the
field application rate. Plant bioassays may be performed in the field and in the green-house
where a herbicide is applied by spraying [7, 10, 12], or in the laboratory with a herbicide
usually being soil-incorporated [6, 13, 14]. For laboratory experiments in which a soil-
incorporated herbicide is used, field application rate can be converted to the herbicide
concentration in soil by assuming that the applied herbicide remains either in the top 5 cm
[13], 10 cm [6] or 15 cm of soil [14] and a soil bulk density of approximately 1.3 g/cm
3
.
This chapter describes the sugar beet bioassay technique for detection of pyroxsulam and
pyroxasulfone in soil, and provides examples of the use of this bioassay. Pyroxsulam, a
Group 2 (acetolactate synthase inhibitor) herbicide and pyroxasulfone, a Group 15 (long
chain fatty acid elongase inhibitor) herbicide have different mode of action and both are
efficacious against certain grass and broadleaf weeds in various crops.
S
UGAR
B
EET
B
IOASSAY
T
ECHNIQUE
The laboratory bioassay using sugar beet (
Beta vulgaris
L. „Beta 1385‟) was performed in
2-oz Whirl-Pak® plastic bags that are 10 cm high and 6 cm long [9]. A quantity of 50 g of
soil was wetted to 100% moisture content at field capacity, hand mixed and transferred to a
Whirl-Pak® bag. The soil in the bag was lightly packed to form a layer that was
approximately 8 cm high, 6 cm long and 1 cm wide. Six sugar beet seeds were planted in soil
and the soil surface was covered with plastic beads to reduce soil drying (Figure 1). Sugar
beet plants were grown in the laboratory under fluorescent lights and plants were watered
