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VI. Fertilizer Application and Weed Management
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.
S. SANKARAN AND S. K. DE DATTA
The Effect of Land Preparation Method and Time and Method of
Fertilizer Application on Weed Weight at Harvest of C-22 Rice"
Weed weight (g/m*)
Time of N applicationb
'Adapted from Olofintoye (1980).
* Within a time or method of nitrogen application in a column or row, means followed
by a common letter are not significantly different at the 5 % level.
' All nitrogen was applied at seeding.
dOne-third of the nitrogen was applied at seeding, one-third 15 days after seeding
(DAS), and one-third 50 DAS.
Applying nitrogen fertilizer to upland rice influenced the composition of
the weed flora in the field. For example, the broadleaf weed population
increased from 62% before nitrogen application to 82% with 90 kg N/ha
applied (CRIA, 1976).
In the early wet season, Okafor and De Datta (1976b) found that yield
reduction caused by C. rotundus was greater with fertilizer than without. In
the late wet season, yield decline caused by C. rotundus was greater with
fertilizer application, but there was no difference between nitrogen levels. In
the dry season, yield loss increased with nitrogen level.
Noguchi and Nakayama (1978) reported that the response of D. ciliuris,
Chenopodium album L., P. oleracea, Cyperus microiria Steud., and A . lividus to
fertilizer was greater than that of upland rice. They responded more in the
early and middle growth stages than at later stages.
Studies in the Philippines and Thailand emphasized the close relationship
between fertilizer inputs and weed control. The benefits of added fertilizer
increased markedly with better weed control (IRRI, 1973). De Datta and
Malabuyoc (1976) wrote that weed infestation and control affected the
nitrogen response of modern varieties of rice. This contention was further
substantiated by experiments at IRRI and in farmers' fields. Data averaged
from two locations showed that the nitrogen responses of IR20 during the
1973 wet season and the 1974 dry season were greater with than without
weed control. Fagade (1979) also reported that upland rice grown with low
WEEDS AND WEED MANAGEMENT
Effects of Nitrogen Application and Weeding on Grain Yield
of Upland RiceaFb
Grain yield (tonha)
Nitrogen applied (kg/ha)
0 kg N/ha
30 kg N/ha
60 kg N/ha
Propanil at 14 DAS
Hoeweeding at 14 and 28 DAS
Hoeweeding at 14,28, and 40 DAS
Propanil at 14 DAS + hoeweeding at
Adapted from Fagade (1979).
C.V. = 12%.
weeding levels did not respond to nitrogen application, but yield increased
between 1 1 and 15% with nitrogen application under high weeding levels
With an efficient weed control program it is possible to increase the
fertilizer use efficiency of the crop. When limited fertilizer is available, the
productivity per unit of applied fertilizer can be maximized by good weeding
The economics of upland rice farms limits herbicide use. Herbicides are
expensive and can (not all do) create toxic residues. Several herbicide-fertilizer mixtures have successfully reduced the herbicide rate and maintained
efficiency and selectivity.
Sankaran et al. (1974) reported that crop yield increased significantly when
butachlor and urea were applied compared with applying butachlor alone.
However, weed control efficiency was not significantly increased by adding
2,4-D and urea or ammonium sulfate effectively controlled weeds in upland
rice (Mukhopadhyay et al., 1971). Mani et al. (1973) observed improved rice
growth after a low rate of 2,4-D and 4-(chloro-o-tolyloxy)butyric acid
(MCPB) was applied with urea. Kaushik and Mani (1980) reported that the
herbicide rate could be reduced 50% when MCPB was applied with a 3%
urea solution. However, Sankaran et al. (1974) found that adding urea to 2,4D did not affect the efficiency and selectivity of the herbicide.
S. SANKARAN AND S. K. DE DATTA
A propanil-urea mixture was more efficient and selective on upland rice
than propanil alone (Patro and Tosh, 1975),despite a slight phytotoxic effect
on the crop. Mukhopadhyay et al. (1971) and Kaushik and Mani (1980)
showed that the herbicide rate could be reduced without affecting efficiency
by using a propanil-3% urea mixture. Sankaran et al. (1974), on the other
hand, reported that a propanil-urea mixture controlled weeds better but did
not influence crop yield.
V I I. S 0 IL M0 IST U R E- H E R B I CI D E R E LATI0 NS H IPS IN
UPLAND R I C E
AND WEED EMERGENCE
Competition for soil moisture may begin early, particularly in low-rainfall
areas (Sharma et al., 1977). More weeds emerge earlier at high soil moisture
than at low moisture level (Yamamoto and Ohba, 1977). Janiya et al. (1983)
reported that soil moisture from 0 to 15 cm had pronounced effects on weed
species emergence 4 weeks after land preparation. The pattern of weed
emergence varies with the time of land preparation and is largely influenced
by the frequency and amount of rainfall.
CONTENT A N D HERBICIDE
Soil moisture status in upland rice influences not only weed emergence but
also the amount of herbicide in solution. Applying herbicides to upland
seedbeds with dry soil or when dry conditions follow immediately afterward
reduces the herbicide effectiveness (Indhaphun et al., 1979; Schiller and
Indhaphun, 1979, 1980). Jikihara and Kimura (1979) observed that herbicidal activity was higher in wet than in dry soil. The soil moisture content at
and after herbicide application (Sahu, 1978) and the solubility of herbicides in
water (Chen and Chen, 1979) determine the efficiency of soil-applied herbicides.
Olofintoye and Mabbayad (1981b) reported that frequent watering after
postemergence application of butachlor caused the herbicide to leach, in
toxic amounts, into soil zones where rice seeds were germinating. In India,
Sahu (1978) observed that soil moisture content exceeding 30% increased the
WEEDS A N D WEED MANAGEMENT
depth of butachlor movement and the susceptibility of emerging crop
seedlings. In the Philippines, Sankaran and De Datta (1984) found that a soil
moisture content of 35% or more increased the herbicidal activity of
butachlor, oxyfluorfen, and oxadiazon in upland rice. Nako (1977) observed
that increased soil moisture content after applying thiobencarb decreased
establishment and inhibited rice growth at the seedling stage. In very dry soils
in Bolivia, Tollervey et al. (1980) observed that pendimethalin did not
effectively control R.exaltata.
Rao and Dubey (1977) reported that high soil moisture caused severe
toxicity with dinitramine, piperophos plus dimethametryn, and butachlor;
moderate toxicity with nitrofen; and low toxicity with thiobencarb. Jikihara
and Kimura (1979) reported that thiobencarb 50% plus prometryn 5 % at 8
liter/ha gave excellent weed control in wet soil but not in dry soil.
Sankaran and De Datta (1984) reported that soil moisture status determined the success of chemical weed control in upland rice. During the 1983
dry season, they used a line-source sprinkler system to regulate soil moisture
while evaluating the effectiveness of pendimethalin and oxadiazon. With both
herbicides, IR36 yields were similar to those of the hand weeded check at
high moisture levels (81 1 and 691 mm). When moisture levels dropped below
cumulative pan evaporation (688 mm), yields with chemical and hand
weeding were similar to those in the unweeded check (Fig. 7). Below 525 mm
there was no grain yield, although use of herbicides and hand weeding
effectively controlled weeds.
FIG.7. Effect of herbicides on grain yield of upland rice IR36 at different moisture regimes.
(From Sankaran and De Datta, 1984.)