A handbbok on Weed Control in Rice.pdf

leaf size), with the resultant shade
helping to suppress late-germinating
weeds. Phosphorus encourages rice
root development and increases
tillering. Vigorous rice root growth is
advantageous in below-ground
competition with weeds for moisture
and nutrients.
Weeds take up nutrients in large
quantities, however, and sometimes
absorb fertilizer faster than rice.
When weeds are not controlled
effectively, however, fertilization is of
little significance. There is little or no
response to N by rice in shade, and P
and N left on the soil surface will
stimulate growth of shallow weed
seeds. At high fertilization levels, it is
not possible to produce high rice
yields without weeding.
Early- to midseason N applications
are beneficial to rice, but weeds must
be controlled to maximize the effect
of N on grain yield. Figure 3.1 shows
the best time to apply N in various
rice cultures.
Water management. Since ancient
times, water has been used to manage
weeds in ricefields. Many nonaquatic
weeds do not survive in submerged
environments and many aquatic
weeds do not survive in upland
environments.
Water control during the early rice
growth stages has a major effect on
weed control. As weeds become
established, controlling them through
water management is more difficult.
Grassy weeds can be largely elimi-
nated by continuous flooding to
15-cm depth maintained throughout
crop growth. The response of broad-
leaf weeds and sedges to different
water depths varies. After continuous
flooding, C3 weeds (see page 3) will
be dominant.
44 Weed control handbook
Weed problems intensify in
irrigated and rainfed lowland rice
when water supply is inadequate.
Increased weed control benefits are
obtained with improved water
supply and control, high levels of
fertilizer, and improved cultivars
used in concert.
Increasing water depth to control
weeds as part of integrated weed
management is cost effective. When
combined with other direct weed
control methods, increased water
depth can give considerable savings
in production costs.
Timely and thorough drainage of
flooded fields reduces aquatic weed
problems.
Manual methods
Manual weed control includes
burning, hand pulling, and mechani-
cal hand weeding. These labor-
intensive methods are the oldest and,
in many cases, the farmer’s only
means of controlling weeds in rice,
and are highly effective.
Several hand tools are still the
principal means of rice weed control
in many developing countries. But
manual methods are slow,
unattractive, and tedious, and are
often carried out after the rice crop
has been severely damaged by
weeds. Hand weeding is often
ineffective on weeds with special
survival mechanisms such as the rice
mimics (e.g., I. rugosum ) and deep-
rooted perennial weeds. Rice mimics
are difficult to distinguish from rice
at the early growth stage. Repeated
hand weedings are necessary to
effectively control all weeds.
Burning. Burning, common under
the slash-and-burn system of land
preparation, kills weed seeds and
weed seedlings, gets rid of unwanted
vegetation, and reduces the amount
of weed seeds returned to the soil. It
also encourages germination of weed
seeds near the soil surface. Thorough
burning can keep the ricefield free of
weeds for the first 2-3 wk.
Burning saves labor, is low cost,
and adds neutralizing ash to low-pH
soils. On the other hand, widespread
uncontrolled burning leaves the soil
bare, increasing soil erosion and loss
of N and other nutrients. Careful
planning and rational use can
minimize the adverse effects of
burning. Local regulations on
burning should be followed.
Hand pulling. Hand pulling
controls weed seedlings growing
near and between rice plants where
implements are difficult to use. Hand
pulling is not effective in dry soil,
where weed seedlings break and
resprout easily. Frequent hand
pulling is necessary for effective
weed control because very small
weed seedlings that are not removed
grow quickly to reinfest the ricefield.
This method is the most labor intensive
of all weed control measures,
and is best suited to small farms.
In some places, lowland rice
farmers trample weeds instead of
hand pulling.
Mechanical weeding. Weeding using
hand tools is common in almost all
tropical rice-growing areas. But the
degree of weeding and the problems
associated with it largely depend on
the type of rice culture. In many
countries, hand tools such as the hoe,
narrow spade, Swiss hoe, knife,
machete, and pointed sticks are
primarily used to remove weeds in
upland rice. Weeds within rows must
be removed by hand. The amount of
labor required to weed 1 ha ranges
from 10 to 30 d.