A handbbok on Weed Control in Rice.pdf

Bensulfuron controls many broadleaf
weeds and sedges. It can be
applied as preemergence or early
postemergence treatment. When used
in combination with a grass herbicide
such as thiobencarb, molinate, or
butachlor, bensulfuron provides
excellent weed control. Application
can be made up to the 3-leaf stage of
the weeds. Bensulfuron provides good
control of S. maritimus when applied
6-12 d after emergence of the weed
(Bernasor and De Datta 1986).
Water solubility at 25 °C is
120 mg/liter. LD 50 in rats is
75,000 mg/kg. LC 50 (96 h) for rainbow
trout is >150 mg/liter.
Polycyclic alkanoic acids
Polycyclic alkanoic acids control grass
weeds. In general, polycyclic alkanoic
acids are lost from the soil through
biodegradation, rather than through
leaching or volatilization. Rainfall after
foliar application can greatly reduce
herbicide absorption because of a
reduced amount of herbicide in
contact with the plant. One example of
the group is fenoxaprop. Herbicides
such as MCPA, 2,4-D, and bentazon
antagonize polycyclic alkanoic acids.
Fenoxaprop. Fenoxaprop
(0.03-0.05 kg ai/ha) is a postemergence,
selective herbicide for control
of annual and perennial grassy weeds.
It is a contact herbicide that is partly
systemic. Rice is tolerant from the
3-leaf stage to early tillering.
Fenoxaprop has no soil activity. It
should not be applied with phenoxy
compounds.
Water solubility at 25 °C is
0.8 mg/liter. LD 50 is 2,357 mg/kg.
Miscellaneous herbicides
Several herbicides do not clearly fall
into any of the groupings. These
herbicides differ in chemistry, selectivity,
and mode of action. Bentazon,
chlomethoxynil, cinmethylin,
oxadiazon, and quinclorac are
examples.
Bentazon. Bentazon (1-2 kg ai/ha)
selectively controls a number of broadleaf
weeds and sedges, primarily by
contact action. Broadleaf weeds at the
2- to 10-leaf stage are controlled most
readily. Delay in application will result
in inadequate control. In irrigated rice,
bentazon should be applied only to
weeds that have emerged above the
water level. Bentazon does not control
grasses and has no known preemergence
activity. Available commercial
mixtures include bentazon + MCPA
and bentazon + propanil.
Water solubility is 500 mg/liter.
LD 50 for rats is about 1,100 mg/kg. It
has low fish toxicity. LC 50 (96 h) for
rainbow trout is 510 mg/liter.
Chlomethoxynil (chlometoxyfen).
Chlomethoxynil is used at
1.5-2.5 kg ai/ha in transplanted and
upland rice. It is applied 3-8 d after
transplanting rice.
Water solubility is 0.3 mg/liter at
15 °C. LD 50 for mice is 3,300 mg/kg.
LC 50 (40 h) for carp is 237 mg/liter.
Cinmethylin. Cinmethylin provides
excellent control of grasses and
moderate suppression of broadleaf
weeds and sedges. It has been tested at
100-200 g ai/ha for transplanted rice
4-9 d after transplanting and at
100 g ai/ha for direct seeded rice
7-9 d after seeding.
Water solubility is 61 mg/liter.
LD 50 is 3,900 mg/kg.
Oxadiazon. Oxadiazon is a preemergence
herbicide applied at
0.5-0.75 kg ai/ha. Uptake by weed
seedling shoots causes plant death
because root uptake is low. Its
postemergence activity against grasses
is limited. Because it has low water
solubility and high adsorption,
oxadiazon is not leached and has good
persistence. It can be used in transplanted
and direct seeded rice.
Commercial mixed formulations
include oxadiazon + 2,4-D and
oxadiazon + propanil.
Oxadiazon can be specifically
formulated to be applied directly onto
the surface of the ricefield floodwater.
A shaker bottle with a calibrated
stopper is used to spread oxadiazon
directly onto the water without any
dilution. The field should be flooded
3-5 cm at application and water should
not be disturbed for 2-5 d after
application.
Water solubility is 0.7 mg/liter at
20 °C. LD 50 for rats is more than
8,000 mg/kg. LC 50 (96 h) for rainbow
trout is 1-9 mg/liter; for catfish,
³15.4 mg/liter.
Piperophos. Piperophos is a
piperidine compound used as a
preemergence and postemergence
herbicide. It controls annual grasses
and sedges. The commercial product
containing piperophos + dimethame-
tryn in the ratio 4:l is used in rice to
control most annual weeds. It is
applied at 1-2 kg ai/ha at the 2- to
4-leaf stage of weed. It has good selec-
tivity in transplanted and upland rice.
Principal herbicides 71