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VII. Soil Moisture–Herbicide Relationships in Upland Rice

VII. Soil Moisture–Herbicide Relationships in Upland Rice

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WEEDS A N D WEED MANAGEMENT



311



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.)



S. SANKARAN AND S . K. DE DATTA



312



c. LEAFWATER POTENTIAL OF UPLAND RICE AND WEEDS

Limited soil moisture in upland rice areas with inadequate rainfall lowers

the leaf water potential of rice compared with associated weed species (IRRI,

1979). Sankaran and De Datta (1984) found that mid-day leaf water potential

of upland rice was much lower than that of R.exaltata and C. rotundus (Fig.

8). The leaf water potential of the associated weeds did not fall below - 7 bar,

even under severe stress. The leaf water potential of rice plants went down to

-30 bar. Iwata and Takayanagi (1974a,b) reported that upland weeds are

more adaptable to low soil moisture than rice. The leaf water potential of rice

did not vary significantly at high (811 mm) and intermediate (525 mm) soil

moisture levels, with or without weeding. However, at low soil moisture levels

(214 mm), the leaf water potential of rice in the untreated control was below

that in the pendimethalin-treated and hand weeded check plots.



-O

4



h



I



n



.-" -8'



--



I



Roffboelha exalfalo



m



Cyperus rotundus



'



I I



I



0

1

-



-



V Pendirnethalin



-351



I



811



I



525

Water applied (rnrn)



I



214



FIG. 8. Changes in the leaf water potential of weeds and upland rice at three moisture

regimes. IRRI, 1983 dry season. (From Sankaran and De Datta, 1984.)



WEEDS A N D WEED MANAGEMENT



313



VIII. WEED CONTROL METHODS IN UPLAND RICE

De Datta (1980) divided weed control and management into four categories: substitutive, preventive, complementary, and direct. He suggested combining these practices to fit farmer resources and to minimize the possible

buildup of a single noxious weed or group of weeds.

A. CULTURAL

CONTROL

Hand weeding is the most widely used weed control method for upland

rice. Normally, two weedings are done, the first 3 weeks after sowing and the

second 2 weeks later. For upland rice, Indonesian farmers still use only hand

weeding with simple tools (Ronoprawiro, 1975). Schiller and Indhaphun

(1979) wrote that upland rice fields in northern Thailand could produce well

with two manual weedings-one before 20 days and the other at 50 DAS. In

India, Upadhyay and Choudhary (1979) reported that weeding 3 and 6 weeks

after sowing upland rice produced maximum yield. However, hand weeding

within the crop row was still necessary. Hand and hoe weedings after upland

rice sowing currently are the best methods of weed control in Nigeria (IITA,

1972). Several workers in India have confirmed the efficiency of hand weeding

as compared with other weed control methods (Ghosh, 1976; Ghosh et al.,

1977; Dixit and Singh, 1981).

Although hand weeding is effective, it is tedious, time-consuming (Aryeetey, 1970; De Datta and Beachell, 1972), and labor intensive (Muller and

Bilderling, 1953; De Datta and Beachell, 1972). Labor is expensive (Muller

and Bilderling, 1953) and sometimes scarce (Jan, 1973), either because of

short periods of high requirement or because of continual shortages (Jan,

1973). Additionally, by the time weeds can be removed by hand, they have

already competed with the crop and reduced yields (Cates, 1969).

Akhanda (1966) reported that hand weeding 25 DAS required 321 laborhr/ha but resulted in high upland rice yields. Ray (1973) estimated that a

single hand weeding needed 300 to 700 labor-hr/ha. Several hand weedings

are needed for successful upland rice production. Delaying hand weeding

beyond 25 DAS sharply increased labor requirements and significantly

decreased yield.

In Nigeria, Curfs (1975) reported that hand and hoe weeding performed

better than other mechanical weed control methods, but that their time

requirements were high. At IITA in Nigeria, in an upland field heavily

infested with weeds, one hand weeding at 14 DAS required 244 labor-hr. A 1week delay increased the labor required for weeding by more than 40 hr



314



S. SANKARAN AND S. K. DE DATTA



(Moody, 1975). Between 25 and 45 DAS each day of delay in weeding

reduced grain yield by 43 kg/ha and increased weeding by 23 labor-hr/ha (De

Datta, 1979). Three weedings produced the highest grain yield but required

530 to 838 labor-hr/ha. In terms of kilograms of rough rice per labor-hour,

one weeding at 25 DAS was most efficient.

Cultural practices such as land preparation, timely weeding (Moody,

1975), planting appropriate rices, using proper seeding rates, and fertilizer

management may partially substitute for chemical weed control in upland

rice.

Plant density and row spacing influence weed incidence. Bhan (1968)

found that narrow (15cm) spacing was superior to wide (30 and 45cm)

spacings in minimizing weed competition and increasing productive tillers

and yield in upland rice. However, Clarete and Mabbayad (1978) found that

15-, 30-, or 45-cm row spacings did not appreciably affect total weed count.

Reduced plant stand and wider row spacing left more area for weed growth

and caused low grain yields. Tosh et al. (1981) showed that row seeding

produced higher plant population, lower weed dry matter accumulation and

nutrient uptake by weeds, and higher grain yield than broadcast sowing

(Table XI). Merlier (1978) observed that maintaining optimum plant density

with a high seeding rate helped suppress vigorous weeds during early rice

growth.

Table XI

Effect of Seeding Methods on Weed Dry Matter Accumulation,

Nutrient Uptake, and Grain Yield'**



Seeding

method

Broadcast

Row

a



Plant

population

10 DAS

(No./m*)



Weed dry matter

accumulation

60 DAS

(gin2)



108 a

144 b



171 a

157 b



Nutrient uptake by weeds

60 DAS (kg/ha)

N



P



K



Grain

yield

(toniha)



10

9



2.7

2.4



28

22



1.5 a

1.8 b



Adapted from Tosh et al. (1981).

In a column, means followed by the same letter are not diifferent at the 5 % level by DMRT.



In Sierra Leone, Jones and Tucker (1978) tested rice straw and rice husk

mulches as weed suppressors and obtained 48 and 32% yield increases. Both

mulches controlled weeds as effectively as hand weeding. Results of IRRI

research showed very low soil moisture tension values (85 cbar and 1.75 bar)

in dry soil mulch plots and highest values (25-30 bar) in weedy fallow plots.

Dry soil mulch conserves moisture for the following upland rice crop. In



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VII. Soil Moisture–Herbicide Relationships in Upland Rice

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