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VI. Choice of Pesticides and Inocula

VI. Choice of Pesticides and Inocula

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MARTIN ALEXANDER



selective advantage of the toxicant is not present. Whether the fixation is

markedly or only modestly enhanced, a savings in cost can be achieved if the

pesticide used to promote nitrogen fixation is also part of the farmer’s system

of pest control. From the viewpoint of the marketing of pesticides, a chemical

that has two functions would have special attraction.

Many of the compounds now widely sold for pest control can be the basis

for the selective enhancement of nitrogen fixation. For example, legume seeds

are often treated with antifungal agents to prevent or minimize such fungal

diseases as damping-off and seed and seedling rots, and these protective

compounds applied to the seeds are often ideal choices for the inhibition of

protozoa preying on rhizobia and bacteria competing with the root-nodule

microsymbionts. The very fact that many fungicides at recommended rates

are deleterious to rhizobia (Fisher, 1976; Fisher and Hayes, 1981; Tu, 1980)

indicates they are good antibacterial as well as antifungal agents, and the data

cited above also show that widely used fungicides have antiprotozoan

activity. Similarly, the insecticide used to control ostracods, cladocerans, or

other invertebrate grazers could be one already being applied to control pests

of rice. Thus, compounds such as lindane or carbofuran would have a dual

purpose. Not only would they control grazing on the algae, but they would

also have the already desired function, lindane being important for protection

of rice against stem borers and carbofuran being widely used to control a

variety of insects, mites, and nematodes. Similarly, in regions where herbicides are already used to control weeds in rice fields, the herbicide might also

be selected for its toxicity to the indigenous algae that otherwise would

reduce nitrogen fixation. Such dual functions might also make pesticide use

more attractive to farmers not already applying them for the control of

particular groups of pests.

The pesticide-resistant inoculum must be carefully chosen, however. Not

only must it be active in nitrogen fixation but it should tolerate stresses of the

habitat, and it should survive well when the crop is no longer being grown or,

for lowland rice, during drying of the soil. Furthermore, the mechanism of its

resistance to the pesticide must be checked to be sure the organism does not

owe its tolerance to its ability to degrade the toxicant, because then the

desired control of both the pests and the species affecting the nitrogen-fixing

inoculum would be lost.



VII. LIMITATIONS

One of the major limitations of the proposed approach is the cost of the

pesticide. This expense, although real, can be absorbed as part of the cost of



ENHANCING NITROGEN FIXATION



279



pest control if the chemical is already used for the suppression of pathogens,

insects, or weeds.

A potential problem is the lack of movement of many chemicals from the

site of their first introduction into the soil. A fungicide applied to the seed

may inhibit the predatory protozoa and competing bacteria and thus

promote colonization by the inoculum strain around the germinating seed

and the nearby roots, but the pesticide will have no such effects on roots at

some distance away unless the chemical moves through or with the developing roots or is translocated through the soil. Even where such movement is

not appreciable, nitrogen fixation may be stimulated, as reported above,

possibly because of the initial benefits to the plant and the continuing activity

of the organisms so favored. Nevertheless, the availability of chemicals that

are translocated would provide a more widespread influence in soil, and thus

presumably a greater benefit, than those compounds that have a restricted

zone of influence. Chemicals that move from seeds to roots or that move

downward following foliar application would overcome such problems.

Evidence that basipetally translocated pesticides may be effective in favoring

nitrogen fixation comes from studies of oxamyl. This compound is translocated downward through leguminous plants (Martin and Edgington, 1981),

and after its application to the foliage of soybeans, it increased the yield,

nitrogen content, and percentage of nitrogen in soybeans inoculated with an

oxamyl-resistant strain of R.japonicum (Hossain and Alexander, unpublished

data). Several other fungicides applied to the above-ground portions of

plants alter the composition of the rhizosphere microflora (Halleck and

Cochrane, 1950; Rao and Sharma, 1978), although such changes may also

arise from alterations in the composition of the root excretions.

A significant limitation is the poor mobility of bacteria along roots and

through soil. This is true of many and possibly of all bacteria, and only a

small percentage of rhizobial cells have been found to be transported for

distances as short as 2.7cm (Madsen and Alexander, 1982), although

movement of even a few cells could be a prelude to their establishment if

many of the organisms grazing or competing with them are suppressed by the

pesticide. Possibly the best means for overcoming this limitation is to use, as

inocula, bacteria that persist in soil; in this way, the pesticide-resistant

bacterium that was inoculated onto seeds in earlier years would endure and

become better dispersed in soil with root growth, cultivation, and water

movement, and although present at distant sites from newly planted seeds, it

would be available to colonize roots that grow to microsites that contain the

persistent microorganism. The persistence of Rhizobium in soil is thoroughly

documented (Lowendorf, 1980).

As with other organisms whose control is sought by pesticides, the

predators and competitors may become resistant to the chemicals. Acquired



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resistance because of frequent exposure to insecticides, for example, may

explain the tolerance of the ostracod Heterocypris luzonensis to carbofuran

and lindane (Grant et al., 1983a). The solution to the problem of acquired

resistance may be the introduction of new pesticides or the rotation of

chemicals to prevent the acquired resistance from eliminating the benefit

from enhanced nitrogen fixation.

Chemicals that are readily degraded also may pose difficulties. The

compound must persist long enough to control the predators and competitors. For example, the effectiveness of carbofuran in controlling ostracods

diminished as the insecticide was degraded, although lindane, which was

more persistent under the test conditions, continued to suppress invertebrates

(Grant et al., 1983a). This is not a limitation unique to the control of species

harmful to nitrogen fixers, however.



VIII. SUMMARY AND CONCLUSIONS

For appreciable fixation of nitrogen, Rhizohium must colonize the rhizosphere of its host legume; free-living nitrogen-fixing bacteria must grow

around the roots of nonlegumes; and blue-green algae (cyanobacteria) must

multiply in fields of lowland rice. Inoculation may lead to little or no nitrogen

fixation, because the introduced microorganism may not become established

or may not reach large numbers. Rhizobium populations may not become

large or may be reduced in size because of predation by protozoa or

competition with bacteria in the rhizosphere. Similar biological controls may

affect free-living nitrogen-fixing bacteria in the root zone of nonlegumes.

Grazing by ostracods and cladocerans reduces the extent of colonization of

flooded soils by blue-green algae. The effects of predation by protozoa and

competition with other bacteria in soil and grazing by invertebrates and

competition with algae in paddy fields may be overcome by use of pesticides

that suppress the predators and competitors together with inocula of

nitrogen-fixing species resistant to these pesticides.

The proposed method represents a novel approach to enhancing nitrogen

fixation. Some of the initial studies needed to establish the feasibility of the

approach have been conducted, although several issues still need to be

resolved. Additional investigations are required to find better chemicals, to

obtain resistant bacteria active on various legumes as well as cereals and

other nonlegumes, to develop resistant algae, and to test these procedures

under various field conditions.

It is likely that future study will show other organisms to be important in

suppressing or reducing the rate of growth and colonization of rhizobia,



ENHANCING NITROGEN FIXATION



28 1



bacteria around the roots of nonlegumes, and blue-green algae in rice fields.

Moreover, no attention has yet been given to other nitrogen-fixing microorganisms or symbiotic associations, for example, the Azolla-Anabaena

association. Nevertheless, the approach of using pesticides to control species

that are harmful to the nitrogen fixers and nitrogen fixers resistant to these

compounds should serve as a new means to bring about greater nitrogen

gains in agriculture.

REFERENCES

Alexander, M. 1981. Annu. Rev. Microbiol. 35, 113-133.

App, A. A., Watanabe, I., Alexander, M., Ventura, W., Daez, C., Santiago, T., and DeDatta, S. K.

1980. Soil Sci. 130, 283-289.

Barber, L. E., Russell, S. A., and Evans, H. J. 1979. Plant Soil 52,49-57.

Balandreau, J., and Villemin, G. 1973. Reo. Ecol. Biol. Sol 10, 25-33.

Bohlool, B. B., and Schmidt, E. L. 1973. Soil Sci. Soc. Am. J. 37, 561-564.

Chao, W.-L., and Alexander, M. 1981. Soil Sci. SOC.Am. J. 45, 48-50.

Chowdhury, M. S. 1977. In “Exploiting the Legume-Rhizobium Symbiosis in Tropical Agriculture” (J. M. Vincent, ed.). Misc. Publ. (145), College of Agriculture, University of Hawaii,

Honolulu.

Danso, S. K. A., and Alexander, M. 1975. Appl. Microbiol. 29, 515-521.

Dobereiner, J. 1978. In “Limitations and Potentials for Biological Nitrogen Fixation in the

Tropics” (J. Dobereiner, R. H. Burris, and A. Hollaender, eds.), pp. 13-24. Plenum, New

York.

Fernando, C. H. 1977. Geo-Eco-Trop. 3, 169-188.

Fisher, D. J. 1976. Pestic. Sci. 7, 10-18.

Fisher, D. J., and Hayes, A. L. 1981. Ann. Appl. Biol. 98, 101-107.

Grant, I. F., and Alexander, M. 1981. Soil Sci. SOC.Am. J. 45, 773-777.

Grant, I. F., Egan, E. A., and Alexander, M. 1983a. Soil Biol. Biochem. 15, 193-197.

Grant, I. F., Tirol, A. C., Aziz, T., and Watanabe, I. 1983b. Soil Sci. SOC.Am. J. 47, 669-675.

Habte, M., and Alexander, M. 1977. Arch. Microbiol. 113, 181-183.

Halleck, F. E., and Cochrane, V. W. 1950. Phyroputhology 40,715-718.

Hirano, T., Shiraishi, K., and Nakano, K. 1955. Shikoku Nogyo Shikenjo Hokoku 2, 121-137.

Holland, A. A. 1970. Plant Soil 32, 293-302.

Kapusta, G., and Rouwenhorst, D. L. 1973. Agron. J. 65,916-919.

Katznelson, H. 1946. Soil Sci. 62, 343-354.

Keya, S. O., and Alexander, M. 1975. Soil Biol. Biochem. 7, 231-237.

Kuykendall, L. D., and Weber, D. F. 1978. Appl. Environ. Microbiol. 36,915-919.

Lennox, L. B., and Alexander, M. 1981. Appl. Environ. Microbiol. 41, 404-411.

Lim, G. 1963. Ann. Bot. (London) 27,55-67.

Lowendorf, H. J. 1980. Adv. Microb. Ecol. 4, 87-124.

McCann, A. E., and Cullimore, D. R. 1979. Residue Rev. 72, 1-31.

Madsen, E. L., and Alexander, M. 1982. Soil Sci. SOC.Am. J. 46,557-560.

Maity, A. K., and Saxena, J. 1979. Nutl. Acud. Sci. Lett. (India) 2, 113-114.

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1073-1079.

Martin, R. A., and Edgington, L. V. 1981. Pestic. Biochem. Physiol. 16,87-96.

Mendez-Castro, F. A., and Alexander, M. 1983. Appl. Emiron. Microbiol. 45, 248-254.



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Obaton, M. 1977. In “Biological Nitrogen Fixation in Farming Systems of the Tropics” (A.

Ayanaba, and P. J. Dart, eds.), pp. 127-133. Wiley, New York.

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Purchase, H. F., and Nutman, P. S. 1957. Ann. Bot. (London) 21,439-454.

Raghu, K., and MacRae, I. C. 1967. Can. J. Microbiol. 13, 173-180.

Ramirez, C., and Alexander, M. 1980. Appl. Environ. Microbiol. 40, 492-499.

Rao, A. V., and Sharma, R. L. 1978. Acra Bor. Indica 6, 71-74.

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1977. IRRI Res. Pap. Ser. 3, 1-16.

Wilson, J. T., Greene, S., and Alexander, M. 1979. Appl. Environ. Microbiol. 38, 916-921.

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pp. 535-602. Academic Press, New York.



ADVANCES IN AGRONOMY . VOL . 38



WEEDS AND WEED MANAGEMENT

IN UPLAND R I C E

S . Sankaran and S . K. De Datta

Department of Agronomy

International Rice Research Institute

Manila. Philippines



I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I1. Weed Flora of Upland Rice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A . Weeds in Upland Rice in Seven Asian Countries . . . . . . . . . . . . . . . .

B. Weed Composition in Upland Rice in Africa. . . . . . . . . . . . . . . . . . .

C. Weed Composition in Upland Rice in Latin America . . . . . . . . . . . . .

D. Distribution Pattern of Weeds in Upland Rice . . . . . . . . . . . . . . . . .

111. Ecology of Upland Rice Weeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A . Factors Influencing the Weed Flora in Upland Rice Fields . . . . . . . . . .

B. Shifts in Weed Flora Due to Weed Control Methods . . . . . . . . . . . . . .

C. Adaptation and Growth of Weeds in Upland Rice . . . . . . . . . . . . . . .

IV . Weed Competition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A . The Upland Rice Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Critical Period of Weed Competition . . . . . . . . . . . . . . . . . . . . . . .

C. Nature and Effect of Competition . . . . . . . . . . . . . . . . . . . . . . . . .

D. Factors Influencing Competition . . . . . . . . . . . . . . . . . . . . . . . . . .

E. Yield Losses Due to Weeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V . Land Preparation and Crop Establishment Techniques . . . . . . . . . . . . . . .

A . Land Management before and after Upland Rice . . . . . . . . . . . . . . . .

B. Methods of Seedbed Preparation in Different Regions . . . . . . . . . . . . .

C. Reduced Tillage and Weed Growth . . . . . . . . . . . . . . . . . . . . . . . .

D. The Effect of Time of Land Preparation on Weed Emergence. . . . . . . . .

VI . Fertilizer Application and Weed Management . . . . . . . . . . . . . . . . . . . .

VII . Soil Moisture-Herbicide Relationships in Upland Rice . . . . . . . . . . . . . . .

A . Rainfall Distribution and Weed Emergence. . . . . . . . . . . . . . . . . . . .

B. Soil Moisture Content and Herbicide Activity . . . . . . . . . . . . . . . . . .

C. Leaf Water Potential of Upland Rice and Weeds . . . . . . . . . . . . . . . .

VIII . Weed Control Methods in Upland Rice . . . . . . . . . . . . . . . . . . . . . . . .

A . Cultural Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. Mechanical Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Chemical Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D. Weed Control in Upland Rice-Based Cropping Systems . . . . . . . . . . . .

E. Biological Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Integrated Weed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IX. Yield Response of Rice to Herbicides and Herbicide Combinations . . . . . . . .

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Copyright 0 I985 by Academic Press,Inc.



All rights of reproduction in my form reserved.



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X. Phytotoxicity of Herbicides and Residues . . . . . . . . . . . . . . . . . . . . . . .

XI. Economics of Weed Control in Upland Rice . . . . . . . . . . . . . . . . . . . . .

XII. Critical Research Needs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix: Common Names and Chemical Formulas of Herbicides . . . . . .

References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



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I. INTRODUCTION

Upland rice (Oryza satiua L.) is grown in Asia, Africa, and Latin America,

mostly by subsistence farmers. In Africa and Latin America, the predominant

cultural practice used for rice production is upland culture, in which rice is

produced in very much the same way as other cereals. In Asia most rice is

produced under lowland conditions, in which the crop is flooded for most of

the growth period. Nevertheless, 11 million ha of upland rice is planted each

year in Asia. With 2 million ha in Africa and 6 million in Latin America, this

makes about 19 million ha used for upland rice worldwide each year, or

about 12% of the total rice area.

Upland rice is grown under a wide range of management intensities,

varying from shifting cultivation, as in Malaysia, the Philippines, West Africa,

and Peru, to highly mechanized systems, as in Brazil (De Datta, 1981). In

Asia, upland rice is important in India, Bangladesh, Sri Lanka, Indonesia, the

Philippines, Thailand, and Vietnam.

In West Africa, 75% of the rice-growing area (1.87 million ha) is upland

rice. Key upland rice-growing countries are Sierra Leone, Guinea, Nigeria,

Ivory Coast, and Liberia.

Latin America has 6.5 million ha of rice, 6 5 7 0 % of which is upland. Brazil

has 5.4 million ha of rice, of which 4.2 million are upland. Most upland rice in

Brazil is grown on small- to medium-sized farms with rolling topography.

Colombia, Guyana, Panama, Ecuador, Peru, Venezuela, and several Central

American countries also grow upland rice.

Rice yields under upland conditions are as low as 1 ton/ha (De Datta and

Ross, 1975) because of poor cultivars, irregular and inadequate rainfall, and

weed competition. Weeds and soil moisture are the greatest limitations to

higher yields. Weeds can cause complete crop failure.

Weed competition varies with culture type, seeding method, cultivar, and

production practices. This variation presents opportunities to develop combinations of practices to minimize weed problems.



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