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III. Ecology of Upland Rice Weeds

III. Ecology of Upland Rice Weeds

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WEEDS AND WEED MANAGEMENT



295



reported that continued application of butachlor caused a shift from monocots to dicots. Sankaran and De Datta (1984) reported that C. benghalensis,

an annual broadleaf weed, became dominant in upland rice when R. exaltata

was completely controlled by preemergence applications of pendimethalin. A

shift favoring slow-growing broadleaf weeds over fast-growing annual grasses

is desirable because it significantly reduces weeding time. Unfortunately, the

shift sometimes is from a moderately easy-to-control weed to one that is

difficult to control (Mercado, 1983).



c. ADAPTATIONAND GROWTHOF WEEDS IN UPLAND RICE

Iwata and Takayanagi (1974a) reported that D. ciliaris seeds were more

adaptable to low soil moisture content (40% of field capacity) than upland

rice and other weeds such as P . oleracea, Amaranthus retro$exus L., and E.

indica. E. indica was more adaptable to high soil moisture content, even

waterlogging, than other weeds.

Iwata and Takayanagi (1974b) studied the growth rate of D. ciliaris and

upland rice sown at the same time in monoculture and mixed culture. The

optimum growth period of Digitaria was shorter, but the growth increments

during the period were larger than those of rice. In mixed cultures, Digitaria

reduced the number of tillers, the plant height, and the dry weight of rice.

In West Africa, perennial weeds of cleared, drier forest and derived savanna

zones include Cyperus spp. and I. cylindrica. Their rhizomes make them more

difficult and expensive to control than annual weeds. Infested areas are

generally allowed to lay fallow for several years before rice is planted again

(Akobundu and Fagade, 1978).



IV. WEED COMPETITION

A. THEUPLAND

RICE ECOSYSTEM

Weed competition largely governs the development of upland rice. The

usual low yield of upland rice has been attributed mainly to inadequate and

irregular moisture supply, heavy weed infestations, lack of suitable cultivars,

nutritional imbalance, and inadequate cultural practices, including inefficient

control of disease and insect pests (De Datta and Beachell, 1972). Among

these limiting factors (except poor water supply), inadequate weed control is,

perhaps, the most difficult constraint to increasing upland rice production

(De Datta, 1972; Madrid et al., 1972).



296



S. SANKARAN AND S. K. DE DATTA



..



ENVIRONMENT

Rainfall

Solar radiation

EdaDhic conditions



m



.



YIELD



+I



MANAGEMENT



WEED MANAGEMENT



..



Primary

Herbicide

Hand hoeing

and weeding



..

..

...



Secondary

Cultivation

Cropcompetition

Seeding method

Variety

Seeding rate

Fertilizer

Preventive



-



I

1-



1



k*

WEED PRESSURE



FIG.1. Schematic diagram of an upland rice ecosystem showing the relationship between

weed management, weed pressure, and crop yield. (Adapted from OBrien, 1981.)



Figure 1 depicts the crop-weed balance in the upland rice ecosystem and

the various factors that influence it. Primary weed control methods affect rice

yield by reducing weed pressure. Secondary weed control methods, such as

seedbed preparation, moisture conservation, and crop rotation, directly

reduce weed pressure and increase rice yields. Other secondary methods, like

seeding method and density, increase the competitive ability of rice. Fertilizer

application increases weed and rice growth.

In the upland rice ecosystem, seed is sown when soil moisture is adequate

for germination. Rice and weed seedlings compete for moisture, nutrients,

and light during germination and growth (Akobundu and Fagade, 1978;

Borgohain and Upadhyay, 1980).

The rice crop and weeds use the same environmental factors for their

growth needs. Competition begins as soon as a factor cannot support the

normal growth needs of the crop and the weeds (Utomo, 1981).

Weeds in upland rice can withstand drought better than rice because they

have deeper roots and high root-length density to tap moisture from deeper

soil layers (Sankaran and De Datta, 1984).

High-cost inputs such as seeds and fertilizers are useless under upland

conditions without efficient weed control. In shifting cultivation, the land is

abandoned when weeds seriously reduce yields. After reviewing current

practices in shifting cultivation, Moody (1975) reported that existing weed

control methods are unsuitable for continuous or large-scale farming. He

emphasized the need for research to find economical, effective control

measures.



297



WEEDS AND WEED MANAGEMENT



B. CRITICAL

PERIOD

OF WEEDCOMPETITION

The critical period of weed-crop competition is between early growth,

during which weeds can grow without affecting crop yield, and the point after

which weed growth does not affect yield (Zimdahl, 1980). Establishing the

critical period of competition is essential to develop effective, economical

weed control measures (Sharma et al., 1977). In upland rice, researchers

(Kawatei et al., 1966; Park and Kim, 1971; Mercado, 1979; Schiller and

Indhaphun, 1979; De Datta, 1980; Kolhe and Mittra, 1981) pinpointed the

optimum weed-free period (Table V).

Table V

The Critical Period of Weed-Crop Competition in Upland Rice



Country



No. of days weeds

can be left in

competition after

rice emerges



Days after which

emerging weeds can

be left to compete

with the crop



Asia

India



10



20



21



20

15



42

30

45

42

50

25

40

63

50



Ghosh et al. (1977);

Sharma et al. (1977)

Upadhyay and Choudhary (1979)

Kolhe and Mittra (1981)

Sahai et al. (1983)

Kawatei et al. (1966)

Park and Kim (1971)

Akhanda (1966)

Mercado (1979)

Wells and Cabradilla (1981)

Schiller and Indhaphun (1979)



15

42



45

56



Morales and Vargas (1976)

Carson (1975)



India

India

India

Japan

Korea

Philippines

Philippines

Philippines

Thailand

Africa and

Latin America

Colombia

Ghana



~



15

-



-



Source



The first 15 days after seeding (DAS) rice seem to be the maximum period

during which weeds can be tolerated without affecting the final crop yield

(Morales and Vargas, 1976; Ghosh et al., 1977; Sharma et al., 1977; Schiller

and Indhaphun, 1979; Upadhyay and Choudhary, 1979; De Datta, 1980;

Wells and Cabradilla, 1981).The weed-free requirement for upland rice varies

from the first 30 to 60 DAS, depending on edaphic and climatic conditions

and weed flora.

In their studies on weed-crop competition, Wells and Cabradilla (1981)

found that competition began during the first 3 weeks (20 days) of the crop.



298



S. SANKARAN AND S. K. DE DATTA



Grain yield decreased as weed growth duration increased. Weed growth

increased exponentially during the first 60 days, reaching a maximum dry

weight of 6.6 ton/ha. Removing weeds at this stage gave little yield increase.

The critical period of weed competition was between 2 and 9 weeks after

sowing.

Moody (1982) wrote that if weeds mature rapidly, shorter-duration rice

cultivars would compete for a proportionately longer time than longerduration cultivars. Because the late-maturing cultivar grows longer, it has

more ability to compensate for weed competition.

Yields from plots not weeded for 10 days after germination were equal to

those of the weed-free check (Ghosh et al., 1977). Weed presence for 20 days

or more reduced yield significantly. O n the other hand, keeping the plots

weed-free for the first 20 days was as good as keeping them weed-free for 50

days or throughout the growing period. No weed competition occurred and

yield was unaffected if weeds grew at 10 DAS. George et al. (1968), Morales

and Vargas (1976), Sharma et al. (1977), Schiller and Indhaphun (1979), and

Wells and Cabradilla (1981) had similar findings that weeds do not appreciably reduce yield if they begin to grow 15 DAS rice.

Merlier (1978) observed that weed dry weight and yield of the rice cultivar

Iguape cateto were negatively correlated (Fig. 2) and that increasing the

duration of grass weed competition progressively reduced panicle weight and

number.

I



L



- - - Dry weight of weeds

10-



o

A



Yield

1974



'



,'



1975



0c



I



I



I



I

I



/'



?'



I



I



P'



0

\

1



P



'P



/'

I



I

I



I



FIG.2. The effect of dry weight of weeds after different periods of infestation and yield of

variety Iguupe cateto. (Adapted from Merlier, 1978.)



299



WEEDS AND WEED MANAGEMENT



In Ghana, Carson (1975) reported that weeding upland rice could be

delayed until 6 weeks after seeding without adversely affecting yield. Beyond

6 weeks, however, yield losses increased with a corresponding delay in weed

removal. Eight weeks of early weed-free conditions prevented substantial

yield reduction. The critical stage of weed competition was between 6 and 8

weeks after seeding.

The weed-free requirement of upland rice emphasizes the need for selecting

preemergence herbicides that are residually active and effective for the first 50

DAS to ensure optimum grain yield. Weed control after the critical period

could prove wasteful.



c. NATURE

AND EFFECTOF COMPETITION

Blackman and Templeman (1938) reported that, in a normal rainfall year,

competition is principally for nitrogen. Chakraborty (1973) reported higher

nitrogen content in weed species at the vegetative, flowering, and postflowering stages, indicating severe competition for nitrogen throughout the upland

rice growing season. At IRRI,De Datta (1974a,b) and Okafor and De Datta

(1976b) studied the effect of nitrogen level on varying densities of C. rotundus

in upland rice. De Datta (1974b) reported a significant negative correlation

between C. rotundus dry weight and rice grain yield as influenced by nitrogen

application (Fig. 3).



C



0



120 kg N/ha

318138-000369X

r = -09822**



5-



r = -0



n- L., '



200



I



9339*

I



I



400

500

Dry weight of perennial nutsedge (g/rn21



300



600



FIG.3. Correlation and linear regression between weed weight and grain yield of upland rice

as influenced by nitrogen level. IRRI, 1972 wet season. *,Significant at 5% level; **, significant at

1% level. (From De Datta, 1974b.)



300



S. SANKARAN A N D S. K. DE DATTA



Severe weed infestation in upland rice depressed the height (Wells and

Cabradilla, 198 l), dry matter (Chakraborty, 1973), tiller and panicle numbers

(Okafor and De Datta, 1974; Sharma et a!., 1977; Kolhe and Mittra, 1981),

leaf area index, and light transmission ratio (Okafor and De Datta, 1974) of

rice.

Utomo (198 1) observed that rice plants and weeds compete in two ways.

The tops of the plants compete for light and the root zone competes for

nutrition (nitrogen and humus). In these competitions, rice plants develop

less chlorophyll and contain less nitrogen, which causes fewer panicles and

spikelets per plant.



D. FACTORS

INFLUENCING

COMPETITION

Many factors influence competition. Weeds and rice modify the environment by influencing the growth of constituent plants. Plant species, plant

density, and distribution and duration of plants in the ecosystem directly

affect competition (Bleasdale, 1960, cited by Zimdahl, 1980). Climatic and

edaphic conditions modify rice-weed competition (Fig. 4).

Upland rice yield is influenced by weed density per unit area, especially

during early growth. Weed density and rice yield have a sigmoid relationship

(Utomo, 1981). Eussen (1981) reported that A . conyzoides densities from 32

to 1024 plants/m2 reduced grain yield of an upland rice cultivar in Bicol,



- species

- densities

-distribution



- duration



Weeds



-7



(weeding)



edaphic and climati



- densities

- othercrops

- distribution



- duration



Upland r i c e 1

production



(crop)



FIG.4. Schematic outline of the factors influencing the upland rice-weed complex. (Adapted

from Bleasdale, 1960.)



301



WEEDS AND WEED MANAGEMENT



Philippines, by 19 to 72%. Grain yields declined progressively as weed

density increased. Eussen and Hadi (1981) observed a similar effect of D.

ciliaris on grain yield. Takayanagi and Iwata (1978) reported that D. ciliaris

had a higher dry matter production at early growth stages.

C. rotundus, generally considered the world's worst weed (Holm et al.,

1977), reduced the rice yield 33 and 93% at 32 and 1024 plants/m2,

respectively. Grain yield reduction was mainly caused by reduced spikelet

and panicle number.

Eussen and Martoyo (1981) found that Porophyllum ruderale (Jacq.) Cass.

at 64 plants/m2 present for 60 DAS retarded tillering and reduced grain yield

by 50%. At 1024 plants/m2, the effect appeared 30 DAS and reduced yield by

91%. D. aegyptium populations reduced yield similarly (Utomo, 1981).

The level of competition varies with weed species. The extent of competition can be determined by calculating the relative space occupied by each

species following the De Witt (1960) method. Utomo (1981) reported that D.

ciliaris occupied more space than Digitaria fuscencens (Presl) Henr. and

Digitaria ternata (A. Rich.) Stapf.

In a greenhouse experiment, Soetrisno et al. (1981) showed that D. ciliaris

retarded rice growth and decreased yield. They found the relative yield value

of rice in mixed culture to be smaller than that of D. ciliaris. Weeds occupied

more space than rice.

Okafor and De Datta (1974) compared the effects of three groups of weeds

(annuals, C. rotundus, and annuals plus C. rotundus) on the growth and yield

of drilled and broadcast IR5 (Table VI). A combination of annual weeds and

Table VI

Effect of Weed Composition on Leaf Area Index (LAI), Light Transmission

Ratio (LTR), and Grain Yield of IR5 Rice under Two Methods of Seeding"

Drilled



Weed community

Weed free

Annuals

Perennial sedge

(C. rotundus)

Annuals plus perennial

sedge



Broadcast



LTR at

panicle

initiation

stage



LA1 at

flowering

stage



Grain

yield

(tonha)



LTR at

panicle

initiation

stage



LA1 at

flowering

stage



Grain

yield

(tonha)



7.1 a

3.4 b

3.4 b



4.9 a

2.7 b

2.8 b



4.7 a

1.2 c

2.7 b



5.6 a

3.1 ab

4.1 a



5.6 a

2.2 b

3.2 b



4.9 a

1.6 c

2.4 b



3.1 b



1.3 b



0.8 c



2.5 b



1.4 bc



0.9 d



IRRI, 1972 wet season. In a column, values followed by the same letter are not significantly

different at the 5 % level as determined by DMRT. From Okafor and De Datta (1974).



302



S. SANKARAN AND S. K. DE DATTA



C. rotundus caused the greatest reduction in leaf area index (LAI), light

transmission ratio (LTR), and grain yield.

Many soil and environmental factors modify weed competition. Soil

nutrient level influences the nature and duration of competition in upland

rice ecosystems (De Datta and Malabuyoc, 1976). Increasing nitrogen

fertilizer application increased nitrogen uptake of C. rotundus (Okafor, 1973).

Weeds grow better under adequate levels of nutrients, thus making them

Table VII

Yield Reduction in Upland Rice Due to Uncontrolled Weed Growth in

Asia, Africa, and Latin America

Country

Asia

Bangladesh



India



Indonesia

Philippines



Sri Lanka

Thailand

Africa

Ghana



Liberia

Nigeria

Senegal

Sierra Leone

Gambia

Upper Volta

Latin America

Brazil

Panama



Yield reduction (%)



90

(80- 100)”

59

(32-86)



75

(70-80)

68

(35-100)

50

62

(38-86)



71

(58-84)

63

(39-87)

90

(80-100)

48

58

(25-90)

100

62

95

100



Source



BRRI (1981)

Mani et al. (1968); Manna et al. (1971);

Mukhopadhyay et al. (1971); Spratt and

Chowdhury (1978); Singh and Sharma

(1981); Moorthy and Dubey (1981)

Syam and Effendi (1977)

IRRI (1967); Vega et al. (1967); De Datta

(1972); Okafor and De Datta (1974);

Mercado (1979); Sarkar and Moody (1981)*

Jayasekera and Velmurugu (1964)

Schiller and Indhaphun (1979); Teerawatsakul

(1981)

Aryeetey (1970); Carson (1975)

WARDA (1981)

Moody (1973); Fagade (1976)

WARDA (1976)

Jones and Tucker (1978)

WARDA (1976)

WARDA (1976)

Burga and Tozani (1980)

Smith (1973)



‘Numbers in parentheses denote the range of grain yield reductions.

Mean of 10 wet seasons (1971-1980) yield data at IRRI and mean range of loss in yield at 9

locations in the Philippines.



WEEDS A N D WEED MANAGEMENT



303



more competitive. Eussen and Zulfaldi (1981) reported that at 100-300 kg N/

ha, weeds reduced grain yields by 25 %. Grain yield reduction was independent of nitrogen level. Therefore, weed control becomes more imperative as

fertilizer applications increase.



E. YIELD LOSSESDUETO WEEDS

Weeds cause two types of crop losses. The most important is the direct

yield loss resulting from competition, followed by indirect loss from reduced

crop quality (De Datta, 1980). All cereal crops yield less under severe weed

stress (Table VII).

Data from the upland rice experiments from 1971 to 1980 at the International Rice Research Institute (IRRI) show weeds reduce grain yield by 2 ton/

ha with a mean of 89% (Sarkar and Moody, 1981). Yield reduction fluctuated

from 0.9 to 3.3 ton/ha, with a mean loss of 2.3 tons/ha.

When weed pressure is extremely severe at early rice growth, the yield loss

can be 100% (IRRI, 1967, 1968; Vega et al., 1967; Manna et al., 1971; De

Datta, 1972; Madrid et al., 1972; Moody, 1973; Smith, 1973; Williams, 1975;

Fagade, 1976; WARDA, 1976). Dubey and Thomas (1977) reported 4 5 8 0 %

yield loss in upland rice, 42% of which occurred in the first 3 weeks of crop

growth.



V. LAND PREPARATION AND CROP ESTABLISHMENT

TECH NlQU ES

Land preparation reduces the weed problem in the following crop. The

rain-fed environment is so heterogenous that there is not one set of land

preparation methods for all situations (Pillai, 1981).



A. LANDMANAGEMENT

BEFORE



AND AFTER



UPLANDRICE



Most countries of Asia, Africa, and Latin America produce upland rice

during the rainy season, which normally lasts for 3 to 7 months. Generally, a

single rice crop is grown in uplands, after which the field remains fallow. This

practice facilitates weed growth and weeds complete their life cycle with

adequate soil moisture, which increases weed problems for the next crop.

Weekly harrowing in the dry season gives good soil tilth, which favors rice

seed emergence and weed growth. For example, Castin et al. (1983b) found



304

2.0



-



b



1.5 b



Jz



\

C



c



0 1.0



.-



)

I



._C



-



a

a



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

fallow plots.

OF SEEDBED PREPARATION IN DIFFERENT

REGIONS

B. METHODS



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.



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