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V. Land Preparation and Crop Establishment Techniques
FIG.5. Effect of previous land usage on grain yield of upland rice. (Adapted from Castin
et al., 1983b.)
that planting maize (Zea mays L.) or mungbean in the dry season reduced
weed growth and weeding time and increased herbicide performance and rice
yields in the following season as compared with plots kept weed-free by using
paraquat or being maintained as weedy fallow (Fig. 5). In plots where maize
was planted during the preceding dry season, yields were 1.6 tons/ha
compared to 1.3 and 0.7 ton/ha, respectively, in the mungbean and weedy
OF SEEDBED PREPARATION IN DIFFERENT
In Asia, little mechanization is used to prepare seedbeds. Fields are plowed
and harrowed with animal-drawn implements when rain falls (De Datta and
Ross, 1975). In Africa, upland rice fields are tilled manually.
Deep plowing is also a weed control method in upland rice. Curfs (1975)
found fewer weeds on soils that were deep plowed and rototilled with a
powerful tractor than on those that were shallow plowed or zero tilled. Pande
and Bhan (1966a) also showed the effectiveness of deep (28 cm) tillage in
reducing weed biomass and increasing panicle number and grain yield as
compared to tillage at 7, 14, and 21 cm (Fig. 6). At IRRI, Lopez et al. (1980)
and De Datta and Llagas (1984) reported that increasing the number of
rototillings reduced weed population, weed biomass, and weeding time
(Lopez et al., 1980) and increased grain yields.
c . REDUCEDTILLAGE
AND WEED GROWTH
Crops can be grown with minimum soil disturbance and reduced energy
inputs for cultural operations if herbicides are used to control weeds.
WEEDS AND WEED MANAGEMENT
5 - 30
. , ac
Tillage depth (cm)
FIG.6. Effect of varying depth of tillage on panicle number and yield of upland rice and on
dry matter production of weeds. (Adapted from Pande and Bhan, 1966a.)
Minimum tillage and no tillage of riceland have produced inconsistent results
in the tropics. Lack of success has been attributed to the rapid regeneration of
perennial weeds and the failure of herbicide controls (Seth et al., 1971; IRRI,
1977; Moody and De Datta, 1977; Olofintoye, 1982; Castin et a/., 1983a,b;
De Datta, 1983).
Lacsina et a/. (1980) observed that consistent satisfactory performance of
herbicides is imperative if minimum or zero tillage is to be successful. Failure
to kill weeds by applying herbicides can be more detrimental to crop yield
than failure to control subsequent regrowth of weeds or to suppress later
weed emergence. Olofintoye and Mabbayad (1981a) reported a stand reduction with no tillage versus conventional tillage and that mulching without
tilling did not influence crop establishment.
Another land preparation method of weed control is the stale-seedbed
technique. After land preparation, weeds that emerge following rain are
destroyed by paraquat or tillage. The crop is then planted with minimum
disturbance of the soil. This method may reduce the need for later weedings
(Renaut, 1972). At IRRI (IRRI, 1979, 1980), the stale-seedbed technique
reduced weed weight but rice yields did not differ between weeded and
unweeded plots. However, in a subsequent study, Castin et a/. (1983a) found
that the stale-seedbed technique did not control weeds as well as conventional land preparation. Eliminating a particular weed species or weed
group encourages the buildup of other weed species.
D. THEEFFECTOF TIMEOF LANDPREPARATION
In rain-fed rice-growing regions, land is commonly fallow during the dry
season, during which weeds grow, complete their life cycle, and shed their
S. SANKARAN A N D S. K. DE DATTA
seeds or multiply vegetatively. A heavy weed cover at the start of the rainy
season hampers land preparation. In rain-fed agriculture, the time of land
preparation is vital to crop establishment and weed control. Soil cultivation
at different times of the year can influence the emergence, number, and species
of a weed community. Studies in IRRI uplands during the 1979 wet season
(Castin and Moody, 1980) showed that weed population in land prepared in
May was always higher than in land tilled in March or April, irrespective of
weed control treatments within the first 2 weeks after seeding.
Curfs (1975), at the International Institute of Tropical Agriculture (IITA)
in Nigeria, found no difference in the weed species or density and rice yield
due to different times of tillage, whether the main tillage operation was
carried out at the beginning of the wet season, just before seeding, at the end
of the wet season, or at the end of the dry season.
VI. FERTILIZER APPLICATION AND WEED MANAGEMENT
Of the major plant nutrients, nitrogen is usually the first to limit growth
(Blackman and Templeman, 1938). Pande and Bhan (1966b) and Chakraborty (1973) showed that weed species absorbed more nitrogen and phosphorus than upland rice at all rice growth stages.
The extent of nutrient competition differs with the time and method of
fertilizer application, even if the same quantity is applied at different times.
Fertilizer application should be timed to prevent weed proliferation and
maximize benefit to the crop. Where effective weed control is impossible,
nitrogen application should be delayed until weed nitrogen uptake has
slowed so more will be taken up by the competing rice crop (Matsunaka,
1970). However, the effectiveness of this recommendation depends on crop
Noguchi and Nakayama (1978) found that weeds grew better in the crop
row when fertilizer was band placed. Weeds between rows grew better with
broadcast fertilizer (Table VIII). Soundararajan and Mahapatra (1978) also
reported that the highest rice yield was obtained when nitrogen was applied
in split doses and fields were repeatedly hand weeded.
Riyanto (1977) observed that weed weight did not respond to the rate or
time of nitrogen application. Similarly, yield reduction caused by weeds was
not influenced by basal or split application of fertilizer. Singh et al. (1975)
reported no difference in weed population due to nitrogen placement in
upland rice. The same workers also observed that broadcasting nitrogen
promoted weed growth and rice yield.
WEEDS AND WEED MANAGEMENT
The Effect of Weed Location and Fertilizer Placement Method on
Weed Dry Weight"
Weed weight (g/m2)
Adapted From Noguchi and Nakayama (1978).
The amount of nitrogen applied to rice influences competition for nutrients. De Datta (1977a,b) suggests that applying less fertilizer than is needed
to produce maximum yield is better when weed control is inadequate. This
recommendation emanated from the observation that yield loss caused by
weeds is greater at low and high rates of applied nitrogen than at intermediate levels. However, Pande and Bhan (1966b) reported that rice yields
almost doubled when nitrogen application was increased from 60 to 80 kg/
ha, indicating that rice was more competitive with more available nitrogen.
Without weeds, rice yield increases with fertilizer application. However,
Okafor and De Datta (1976b) obtained greater yield increments in nonweeded controls than in weeded plots when nitrogen application increased.
As the nitrogen rate increased, the percentage of yield loss due to weeds
decreased. De Datta (1977a,b) concluded that, in many cases, the yield of the
nonweeded fertilized plot was considerably lower than that of the weeded
plot without fertilizer. The first half of the growing season is the most
important in the competition for nitrogen (Chakraborty, 1973).
Seedbed preparation for upland rice significantly has a great bearing on the
time and method of nitrogen application (Olofintoye, 1980). Under conventional tillage, weed weight was greater when nitrogen was applied in split
doses (Table IX). With zero tillage, basal nitrogen application yielded
maximum weed biomass. There was no difference in weed weight in the staleseedbed technique, irrespective of time of nitrogen application. Pande and
Bhan (1966b) and Singh and Sharma (1981) reported that weed density and
biomass and soil nitrogen depletion increased as row spacing increased.
Okafor and De Datta (1976a,b) found that rice yield losses caused by
competing C. rotundus in uplands were greater for IR442-2-58than for IR5.