A handbbok on Weed Control in Rice.pdf

Chapter 5
Principal rice herbicides
Herbicides play an important role in
integrated weed management in rice.
Early-season weed competition
significantly reduces rice grain yield,
and preemergence herbicide treat-
ments are widely used. But most weed
seeds germinate over a long time, and
preemergence herbicides, with their
relatively short residual life, may not
control weeds long enough to opti-
mize rice yields. Then, postemergence
herbicides may be needed along with
other control measures. Moreover, any
one herbicide may not control all the
weeds present in a ricefield. Herbicide
mixtures are used to obtain a wider
range of weed control.
Herbicide mixtures,
rotations, and
sequences
Mixing herbicides and spraying them
simultaneously increases the range of
weed control. Using herbicide mix-
tures can also save time and reduce
application costs. A broadleaf herbi-
cide and a grass herbicide are often
mixed together (e.g., bensulfuron +
butachlor). Residual and foliar contact
herbicides may be combined (e.g.,
thiobencarb + 2,4-D).
Herbicides that may be combined
often are sold as formulated products.
When these are not available, two or
more herbicides may be mixed in the
spray tank at the time of application-
a tank-mix combination. Combina-
tions must be selected carefully and
comply with manufacturers’ recom-
mendations to avoid product
incompatibility.
Herbicide classification
and uses
The herbicides commonly used for
weed control in rice are described
here. Table 5.1 lists the weeds con-
trolled by the herbicides. Details on
their use are discussed in Chapters 6-9.
Every effort has been made to ensure
that the information presented is
correct (Roberts 1982, Swarbrick 1984,
Attwood 1985, Chemical and Pharma-
ceutical Press 1986, Thomson 1986,
Worthing 1987). But because herbicide
activity varies from locality to locality,
only general recommendations are
given. Specific recommendations
should be obtained from weed special-
ists in the reader’s locality.
Most herbicides are organic com-
pounds. Herbicides are considered
ideal if they are toxicologically safe,
selective to rice, cost-effective, effective
on weeds, and have no lasting adverse
effects on the environment. Herbicides
may be classified, for convenience, by
method and timing of application. Or
they may be classified by chemical
group, which also gives an indication
about how the herbicide may be used.
Anilides
Anilides are used to control germinat-
ing annual weeds, especially grasses.
They often are most active as surface
preemergence treatments. The primary
mechanism of action is through inter-
ference with nucleic acid and protein
synthesis. Butachlor, pretilachlor, and
propanil are examples of this group.
Butuchlor. Butachlor is absorbed
primarily through germinating shoots
and secondarily through roots. Its
mode of action is inhibition of protein
synthesis. It is used at 2-3 kg ai/ha for
preemergence control of most annual
grasses at the 1- to 2-leaf stages and of
certain broadleaf weeds, and can be
applied postemergence. For trans-
planted rice, it is applied 3-7 d after
transplanting. For direct seeded rice, it
is applied 10-12 d after emergence.
Principal herbicides 65