A handbbok on Weed Control in Rice.pdf

An upland field should be
harrowed to break up soil clods, but
field leveling is not critical. Rice
should be planted as soon as possible
following the last harrowing to pro-
vide rice an even start with the weeds.
The stale seedbed technique can
reduce weed problems in upland rice.
After land preparation, weeds that
grow are killed at the 2- to 5-leaf stage
by herbicides or using mechanical
methods. Germination of most of the
viable weed seeds is essential to the
success of the stale seedbed technique.
Under unfavorable conditions for
weed seed germination (e.g., dry
soils), no weed control advantage may
be achieved.
Use of the stale seedbed technique
should not delay rice seeding beyond
the optimum time. Planting at the
optimum time assures more rainfall
from seeding through spikelet filling;
late-planted rice is likely to suffer
drought and reduced solar radiation
during the reproductive stage, which
can substantially reduce yields.
Zero tillage can be used to establish
an upland rice crop where no difficult-
to-control perennial weeds are found
in the fallow vegetation or when
appropriate herbicides are available.
But zero tillage may not be successful
under all soil conditions; restricted
root development and reduced grain
yield have been observed (Stone et a1
1980).
Planting methods
Broadcasting, dibbling, and drilling
are the common seeding practices for
upland rice. Broadcasting is common
in many countries in Asia, Africa, and
92 Weed control handbook
Latin America. Dibbling, or hilling, is
practiced in Africa and Asia by farmers
using slash-and-burn systems. A
pointed stick is used to make holes in
the soils; 4-8 unsprouted seeds are
dropped in and covered with soil. In
Latin America, mechanized drilling is
becoming increasingly popular.
Row drilling or dibbling in rows
makes weeding and other management
practices easier. Timely sowing
and rapid canopy closure minimize
weed growth and ensure good stand
establishment.
Weed problems in broadcast seeded
fields are higher because mechanical
hand weeding cannot be done, and
weeding must be delayed because it is
difficult to tell grassy weeds from rice
at early growth stages.
Cultivar
Upland rice cultivars with drought
avoidance (through deep root sys-
tems) and drought recovery abilities
are preferred. Intermediate-statured
cultivars with moderate tillering, big
panicles, blast resistance, and tolerance
for iron deficiency and aluminum
toxicity are also desirable.
Plant population
Seeding rate and spacing for upland
rice vary with planting method and
the rice cultivar used. A high plant
population is important for upland
rice to quickly develop a canopy that
will suppress weed growth. Seeding
rates vary from 80 to 150 kg/ha,
depending on the seeding method.
Row spacing also varies between 20
and 30 cm. Broadcast seeding requires
more seeds than drilling or dibbling.
Tall leafy cultivars should be planted
at wider spacing than semidwarf culti-
vars. Growing tall cultivars at narrow
spacing increases lodging.
Crop rotation is practiced to pre-
vent the buildup of weeds adapted to
upland ricefields, but easy to control in
other crops. Herbicides that control
problem weeds but are toxic to rice
also can be used on a tolerant crop in
the rotation. That will reduce the rice
weed problem.
Fertilizer
Nitrogen response is high for modern
short- to medium-statured upland rice
cultivars that are resistant to lodging.
In most soils, split application of N
gives higher grain yield than does
single basal application.
Applying fertilizer to upland rice,
however, will reduce grain yield if the
field is not weeded. The first applica-
tion should be delayed until after
weeding. Subsequent topdressings of
N after weeding will maximize fertil-
izer-use efficiency. The N application
rate should be reduced in drought-
prone areas.
Phosphorus deficiency is common
in upland rice, especially in Oxisols
and Ultisols in Brazil, West Africa, and
some parts of South and Southeast
Asia (Gupta and O'Toole 1986). In the
Philippines, 18 kg P/ha is recom-
mended for upland rice (PCARR
1977). Some coarse-textured soils in
high-rainfall areas are affected by
potassium deficiency. Applying
33 kg K/ha has been adequate in some
African countries (IRCN 1982). Zinc,
iron, and sulfur deficiencies also occur
in upland rice. Deficiency of any of
these nutrients will reduce the vigor of
rice and, hence, its competitiveness
against weeds.