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X. Phytotoxicity of Herbicides and Residues

X. Phytotoxicity of Herbicides and Residues

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Table XVI

Effect of Single Herbicide and Herbicide Combinations on Upland Rice Yield versus Effect of Hand Weeded and Untreated Controls

Grain yield (tonha)

W



w

P



Efficient herbicide

(kg a.i./ha) and its

combination"



Efficient

herbicide/

combination

A



Asia

Alachlor (2.5)

Butachlor (2.0)

Butachlor (1.0)

Butachlor (2.0)

Butachlor (2.0) fb one HW

Piperophos-dimethametryn (2.0)

Linuron (1.0) fb one HW

Pendimethalin (2.0)

Pendimethalin (2.0) fb one HW

Penoxalin (2.0)



2.6

3.6

1.1

3.2

3.5

3.6

2.5

2.7

2.9

3.8



Hand

weeded

B



Untreated

control

C



Yield

increase

over control

(A - C)



Reference



3.5

3.6

1.6

2.6

3.5

3.2

1.9

3.3

2.9



0.8

1.o

0.4

0

0.8

0

0.2

0.6

1.0

1.6



1.8

2.6

0.7

3.2

2.7

3.6

2.3

2.1

1.o

2.2



Ghosh et al. (1977)

Ahmed and Islam (1983)

Cadag and Mercado (1980)

De Datta (1972)

Munroe et al. (1981)

De Datta (1979)

Sabio and Pastores (1981)

Pillai (1981)

Sankaran and De Datta (1984)

Moorthy and Dubey (1981)



4.0



Country



India

Bangladesh

Philippines

Philippines

Philippines

Philippines

Philippines

India

Philippines

India



Phenmedipham (2.0)

Propanil (4.0)

Dymrone (10.0) fb butachlor

(2.0) fb bentazon (2.0)

Propanil (2.24)

Propanil (3.0) in 2 splits

Propanil (2.8) + 2,4-D BE (0.6)



1.8

6.3

4.2



1.7

6.4

4.0



0.7

0.4



0.9

4.8

3.8



Chattejee et al. (1971)

Bhan et al. (1972)

Okafor and De Datta (1976b)



India

India

Philippines



1.4

2.8

2.6



1.2

2.1

2.2



0.7

0.6

0.9



0.7

2.0

1.7



India

India

Indonesia



3.2

4.4



2.0

4.5



0.5

2.0



2.7

1.8



Patro and Misra (1969)

Tosh et al. (1981)

Mangoensoekardjo and

Kadnan (1971b)

Patro and Tosh (1974)

Dixit and Sin& (1981)



Propanil (2.0) + 2,4-D Na (1.5)

Propanil (2.0) fb one HW



1.5



India

India



Ajkica

Piperophos-dimethametryn (0.6)

Propanil (3.0) + one HW

Propanil (2.0) + thiobencarb

(2.0)

Propanil (3.0) bentazon (2.0)

Thiobencarb (3.0)



2.4

2.5

5.7



2.2

2.2

4.6



1.6

1.9

2.6



0.8

0.6

3.1



Jones and Tucker (1978)

Fagade (1976)

Carson (1975)



Sierra Leone

Nigeria

Ghana



3.4

4.8



4.5

-



0.6

3.8



2.8

1.0



Ghosh (1976)

Kennard (1973)



Tanzania

Guyana



Latin America

Propanil (8.0) 2,4,5-T (1.0)

Oxadiazon (1.0)



4.8

2.1



3.9

1.8



0.4

0.4



4.4



Morales and Vargas (1976)

Filho and Carvalho (1981)



Colombia

Colombia



+



+



a



Po, followed by; HW, hand weeding.



1.7



326



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



rice seed germination. Surface-applied herbicide came in contact with the

germinating seed and caused toxicity, characterized by reduced plant stand,

chlorotic leaves, and stunted growth. Thiobencarb (Mamun and Shimizu,

1976) and butralin, at 2.0 kg a.i./ha, were less toxic, and diclofop-methyl was

not toxic.

Pande et al. (1981) summarized weed control research in rice in India and

observed that butachlor and oxadiazon caused 50% seedling mortality

without affecting the final yield. Dichiormate caused bleached, papery white

leaves. Lopez and Mercado (1978) reported that preeemergence application

of dinitramine at 1.5 kg a.i./ha gave excellent weed control and crop safety

compared to preplanting incorporation, which injured roots and reduced dry

weight by 70%.

Nako (1977) observed that shallow planting and high soil moisture after

applying thiobencarb decreased establishment and inhibited growth of

seedlings at about the one-leaf stage.

The presence of a short mesocotyl in varieties like IR40 was cited as one of

the reasons for the tolerance of certain rices to herbicides even at high doses

(Manipon et al., 1981).

Nangju (1973) and Nangju et al. (1976) tested several methods of overcoming herbicide phytotoxicity in upland rice. Seed pelleting with activated

carbon and band application of an activated carbon slurry over the rice seed

did not reduce the toxicity of chloramben and atrazine. In contrast, a 2.5-cm

layer of activated carbon and vermiculite (1:l) covering the rice seed

overcame the herbicide toxicity. Nako (1977) found that applying thiobencarb at 10 kg a.i./ha (double the normal rate) did not damage rice seedlings,

even with high soil moisture, when seeds were planted 3 cm deep.

One way to increase selectivity, lengthen the period of weed control, and

provide greater reliability under varying environmental conditions is to use

an antidote or protectant to counteract the herbicide (Blair et al., 1976).

Ruscoe and Moody (1981) reported that a 1% seed coating of the herbicide

1,8-naphthalic anhydride (NA) protected the seed from the toxic effects of

soil-incorporated butachlor and thiobencarb and increased yield. However,

NA had little effect on the reaction of rice to preemergence treatments of

these herbicides.

Chakraborty and Majumdar (1973) reported that applying 3 kg a.i.

propanil/ha, 2,4-D, or 1 kg a.i. MCPA/ha at 21 DAS did not leave any

phytotoxic residue in upland soils and did not injure the following mustard

[Brassica nigra (L.) Koch.] crop.

Carson (1975) reported that sorghum [Sorghum bicolor (L.)Moench cv.

Naga White] responded to 1 ppm of thiobencarb, butachlor, and propanil

with a good linear relationship between the growth response and herbicide

concentration. There were no detectable residues in the bioassay test 9



327



WEEDS AND WEED MANAGEMENT



months after applying the herbicides. Deleterious effects of butachlor, propanil, and thiobencarbon soil pH and organic matter at 6.0,4.0, and 3.0 kg

a.i./ha, respectively, were not observed.



XI. ECONOMICS OF WEED CONTROL IN UPLAND RICE

In the Philippines, Singh et al. (1981) reported that the rolling weeder,

high-wheel cultivator, and hand hoe were time-saving compared with hand

weeding. Hoe weeding produced the highest net income and hand weeding

the lowest net return. In India, the sweep hoe gave excellent weed control and

a high net return compared with conventional hand weeding (Singh et al.,

1976). Although repeated weedings gave effective control and increased grain

yield, there was a net loss in income compared with hoeing plus hand weeding

at 3 and 6 weeks (Upadhyay and Choudhary, 1979). Sabio et al. (1980) found

the farmers' practice of two cultivations followed by one hand weeding was

more remunerative than two timely hand weedings.

A preemergence spray of 2.0 kg a.i. butachlor/ha (Tasic et al., 1980; Sabio

and Pastores, 1981) followed by one hand weeding (Singh and Chauhan,

1978; Fisher et al., 1980; Ahmed and Islam, 1983) yielded a higher income

than conventional hand weeding. Ghosh et al. (1977) found that although

chemical weed control (preemergence butachlor at 2.5 kg a.i./ha) was more

expensive than mechanical weeding, the net income from butachlor-treated

plots was higher than for other weed control treatments (Table XVII). In Fiji,

Mandal (1977) reported that even butachlor at 3.6 kg a.i./ha was more

efficient and economical than traditional mechanical weeding.

Table XVII

Grain Yield with Different Methods of Weed Control in Upland Rice and Their Relative

Economics",*



Treatment



Grain yield

(ton/ha)



Control (unweeded)

Mechanical weeding (four times)

Postemergence propanil (2.5 kgfia)

Preemergence butachlor (2.5 kg/ha)



0.8 a

2.1 b

2.0 b

2.2 b



a



Adapted from Ghosh et al. (1977).



cost of

weed control

(US%/ha)



8

5

14



Net income

(US%/ha)

-



30

23

34



* In a column, means followed by the same letter are not significantly different at the

5 % level by DMRT.



328



S . SANKARAN AND S. K. DE DATTA



A postemergence application of propanil at 2.0 or 4.0 kg a.i./ha (Doming0

and Palis, 1966; Raghavulu and Sree Ramamurthy, 1973) followed by either

2,4-D (Akhanda, 1966; Chakraborty and Majumdar, 1973) or one hoeing

and weeding (Kolhe and Mittra, 1981) gave higher net returns than conventional hand weeding.

A combination of 1.0 kg a.i. 2,4-D sodium salt/ha and MCPA followed by

one hoeing and weeding (IRRI, 1967; Misra and Roy, 1970; Singh and Khan,

1972) gave a higher net return than the weed-free check in upland rice.

The choice of alternative weed control methods can be based on the

relative cost of labor versus chemicals. The daily wage rate in a given area and

the approximate time required for one hand weeding determine the cost per

hectare of hand weeding. This cost can be compared to the cost of herbicides

to determine the economical choice.

Hand weeding is still the major form of weed control in upland rice in

Indonesia, Bangladesh, and India, where wage rates are low (De Datta and

Barker, 1977). Considering the labor-hours saved and the returns from

herbicide use, herbicides may soon be adopted in many areas.



XII. CRITICAL RESEARCH NEEDS

Many research papers are available on upland rice weeds and their

management in different countries and under different ecological conditions.

However, there are still questions the answers to which will provide greater

understanding of the ecophysiology of upland weeds and permit formulation

of appropriate control measures. Certain areas should be studied.

1. Weed competition will be greatly minimized by methods to improve

crop establishment of upland rice by refining seeding methods and identifying

other limiting 'factors. Preplant weed control should be developed to aid

stand establishment and moisture utilization.

2. Studies should evaluate the biology, ecology, and control of weeds in

specific upland rice regions. Survival mechanisms of weeds, including adaptation to soil disturbance, and crop competition must be examined to develop

cultural practices to minimize weed competition. Vegetation analyses are

necessary to understand why changes in weed population occur with time

and different management practices.

3. Understanding the effect on the plant environment of continuous use of

a single weed control method and the formulation of effective controls for

resistant weed ecotypes that will develop because of such continuous

practices.



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