A handbbok on Weed Control in Rice.pdf

Wetland tillage is common in
most tropical Asian countries. Tradi-
tionally, it involves plowing and pud-
dling the soil. The processes involved
are
flooding the field for about 7 d.
plowing the soil to 10- to 20-cm
depth, to bury the weed seeds and
weed and rice stubble.
harrowing to break up big clods in
the soil. Two to three harrowings
are done at 7- to 10-d intervals.
leveling the field.
Puddling hastens crop establish-
ment and tillering of transplanted
rice seedlings. This results in vigor-
ous rice growth and increases the
crop’s competitive ability against
weeds. During transplanting, weed
seedlings also are trampled and
incorporated into the soil.
Dryland tillage is the basis of
many rice culture systems. Almost all
rice-growing areas in the United
States and southern Australia, most
of Latin America and West Africa,
and parts of tropical Asia and Europe
use dryland tillage. In dryland prepa-
ration, soil clods are broken up so
they do not interfere with seeding
and seedling emergence. The seedbed
is left rough because germination of
weed seeds, which are usually much
smaller than rice, is encouraged by
fine tilth. Rice is seeded immediately
following the last tillage operation, to
give rice an even start with weeds.
Limited tiIlage covers a range of
land preparation techniques, from
zero tillage to elimination of one
preseeding cultivation. Herbicides
replace the omitted tillage operations.
Limited tillage has soil conservation
advantages.
Smallholders who use slash-and-
burn land preparation are practicing
limited tillage. Rice is planted in a
dead mulch with only enough soil
disturbance to cover the seed.
Minimum and zero tillage techniques
can result in timely land preparation
and savings in labor, water, power,
and capital.
42 Weed control handbook
Under limited tillage, perennial
weed problems increase over time.
Where perennial weeds are controlled,
limited tillage can be used for
both upland and lowland rice culture.
Herbicide use is an integral part of
a limited tillage system. However, if
inappropriate herbicides are used,
perennial broadleaf weeds and
grasses common in fallow in the
tropics are not controlled and significant
yield losses can occur. Plowing
and harrowing more often does not
eliminate the need for direct weed
control. Farmers still must follow
land preparation with other weed
control methods. It is more costeffective
to reduce preplanting
harrowings and combine those with
direct weed control methods.
Crop rotation. Weeds adapt to the
growing conditions of rice. A ricefield
that is tilled regularly has more
annual weeds than many undisturbed
habitats, which have more
perennial weeds. Continuous cropping
encourages the buildup of
difficult weeds.
Each rice culture is associated with
a characteristic weed problem,
influenced by the cultural practices
used. Lowland rice has predomi-
nantly water-tolerant weeds, upland
rice has mostly dryland weeds.
Rotation of lowland rice with an
upland crop reduces infestation by
water-tolerant weeds in the rice and
by upland weeds in the upland crop.
Growing a broadleaf crop in rotation
with rice allows the use of herbicides
effective against grassy weeds, which
are difficult to control in rice.
Varietal selection. Improved rice
cultivars resistant to diseases and
insects are more competitive against
weeds than are traditional rices.
Traditional, tall varieties with droopy
leaves are thought to be more com-
petitive against weeds, but research
has failed to measure competitive
differences that are due to cultivar.
The height advantage of traditional
varieties is offset by the high-tillering
capacity of modern high-yielding
cultivars. Traditional varieties also
have other attributes, such as susceptibility
to lodging, that make them
undesirable.
Modern rice cultivars, although
competitive against weeds, can have
more weed problems than traditional
rice cultivars. The erect leaves and
short stature of modern rices allow
light to reach the soil and stimulate
weed seed germination. Within the
crop canopy, small differences in
crop height affect the quantity and
quality of light received by weeds.
The high nitrogen rates used on
modern cultivars also aggravate
weed problems. This makes competitiveness
with weeds a breeding
objective for modern rices. When
available, such cultivars should be
used.
Time of seeding. When soil moisture
is not limiting, allowing weeds to
germinate before tilling and seeding
rice reduces weed populations. Rice
seeding, however, should not be
delayed beyond the optimum time of
planting.
In rainfed rice crops, time of
seeding is critical. Rice plants affected
by drought become stunted and
cannot compete effectively with
weeds that are more tolerant of
drought. Dry weather after sowing
can reduce the number of weeds that
germinate near the soil surface, but
rice seed germination will also be
delayed. Perennial and large weed
seeds from deeper soil depths are still
able to germinate and compete
strongly with the rice crop.
Planting method. Planting method
affects the ability of rice to compete
with weeds. Rice may be transplanted
or drill seeded in straight
rows, or broadcast seeded. Straightrow
planting is necessary for hand
weeding and for using mechanical
weeders. In a broadcast crop,
mechanical and hand weeding
damage the rice plants.