A handbbok on Weed Control in Rice.pdf

Propanil can be used at 3-4 kg ai/ha
in irrigated and rainfed rice. It should
be applied when most weeds have
emerged. The water level in a flooded
ricefield should be lowered to expose
weeds about 24 h before propanil
application. Raise the water level again
1-3 d after treatment, before any new
weeds emerge. A slight leaf burn may
occur on rice after application, but the
rice plant normally recovers quickly.
Propanil should not be applied if
rain threatens to fall within 5-6 h after
application. It should not be applied to
rice at the late tillering stage, about
60 d after planting. Propanil-treated
rice crops should not be treated with
organophosphorus or carbamate in-
secticides within 14 d before or after
treatment because those insecticides
inhibit the detoxification of propanil
by rice. Commercial mixtures include
propanil + molinate, propanil +
bentazon, and propanil + bifenox.
Water solubility is 0.2 g/liter at
25 °C. LD 50 is 1,400 mg/kg. It has high
fish toxicity-LC
50 (96 h) is
13 mg/liter, indicating that streams
and lakes should be protected from
contamination.
Bipyridyliums
Herbicides from this group are
primarily postemergence, foliar-acting
compounds with no soil activity.
Paraquat and diquat are generally
nonselective. On treated plants,
toxicity symptoms are a characteristic
rapid scorch and desiccation. Light,
oxygen, and chlorophyll are required
for maximum manifestation of
phytotoxicity.
68 Weed control handbook
Paraquat. Paraquat is nonselective,
with fast contact action when applied
postemergence. It kills most annual
weeds and grasses, including rice. It
can be used in zero tillage or mini-
mum tillage systems and in stale
seedbed land preparation, at applica-
tion rates of 140 to 840 g ai/ha. It is
rapidly inactivated on contact with soil
by strong adsorption to clay.
Water solubility is 70 g/liter at
20 °C. LD 50 for rats is 150 mg
paraquat/kg. It has medium fish
toxicity, depending on the formulation
used-LC
50 (96 h) for rainbow trout is
32 mg/liter.
Dinitroanilines
Butralin and pendimethalin are
examples of dinitroanilines. Members
of this family are active when applied
to the soil and must be applied before
weed seed germination. In general,
dinitroaniline herbicides do not
control established weeds. Their mode
of action is inhibition of both root and
shoot development (mitotic poison).
Butralin. Butralin is a preemergence
herbicide. It is selective to upland rice
when applied pre-emergence and to
pregerminated rice at the 1- to 4-leaf
stages (4-6 d after seeding). It is active
against many broadleaf weeds.
Water solubility is 1 mg/liter at
24 °C. LD 50 for albino rats is 12,600
mg/kg. It has high fish toxicity-LC
50
(48 h) for rainbow trout is 3.4 mg/liter.
Pendimethalin. Pendimethalin used
as a preemergence herbicide inhibits
germination and seedling develop-
ment of susceptible weeds. Where
grasses are expected to be a problem, it
should be applied after planting and
before emergence of rice and weeds. It
can also be applied as a postemer-
gence treatment with propanil, which
combines the direct contact action of
propanil and the residual activity of
pendimethalin. Because soil and
weeds must be completely exposed to
spray coverage, no floodwater should
be on the field at the time of applica-
tion. The residual activity of
pendimethalin is activated by mois-
ture. It is most effective when ade-
quate rainfall or irrigation is received
within 7 d after application.
Water solubility is 0.3 mg/liter at
20 ºC. LD 50 for albino rats is 1,050-1,250
mg/kg. It has high fish toxicity-
LC 50 (96 h) for channel catfish is
0.42 mg/liter.
Diphenyl ethers
This group includes bifenox,
fluorodifen, oxyfluorfen, and
chlomethoxynil. Diphenyl ether
herbicides are classified as contact
herbicides. When applied preemer-
gence, they inhibit seed germination
and early seedling growth. They are
relatively insoluble in water, and do
not readily leach. They are used
principally preemergence or early pos-
temergence to control broadleaf weeds
and grassy weed seedlings. In general,
these herbicides more effectively
control broadleaf seedlings than
grassy seedlings.
Bifenox. Bifenox is primarily a
broadleaf herbicide. It can be applied
at 2 kg ai/ha as preemergence or
postemergence up to the 2-leaf stage of
rice. Bifenox has been found to be
highly toxic to direct seeded flooded
rice, resulting in low grain yields (IRRI
1974). Mixtures include bifenox +
2,4-D and bifenox + propanil.