Introduction

The continuous application of new postemergence herbicides for graminae weed control in leguminous covers, has provoked a considerable modification of the weed complex, and a significant proliferation of broadleaf weeds, which were not considered in the past to be an element of importance in oil palm plantation management. For this reason, a great variety of broadleaf weeds is commonly found in oil palm plantations, such as: Fleurya aestuans , Photomorphe peltata , Blechum pyramidatum , Amaranthus spinosus , Melampodium perfoliatum , Commelina spp., Ipomoea spp., Lantana camara , Sida rombifolia , etc. after the purification of harmful graminaceous in legume covers like Paspalum fasciculatum , Rottboelia exaltata , Panicum maximum and Pennisetum purpureum .

Currently there are no products with a selectivity for cover legumes that can control broadleaf weeds in late postemergence; this is the reason why it is becoming an acute problem in several latin american oil palm plantations which employ this type of control strategy.

In the past, the only product that presented adequate selectivity for different types of weed control was 2,4 DB. Nevertheless, this product is very difficult to obtain in Latin America, and very little practical research has been done on kudzu covers ( Pueraria phaseoloides ) regarding this type of herbicide.

The purpose of this paper is to describe the main features of two products which show a great potential in the purification of cover crops.

I. Flex (Fomasafen)

A. General characteristics

Flex is a novel herbicide belonging to the family of diphenyls; it has proven effective for handling broadleaf weed control in legumes, and it is being researched by Imperial Chemical Industries (ICI) in several parts of the world.

This product alters the photosynthetic apparatus of the weeds, which provokes foliar necrosis and a rapid desiccation of the tissues. Using the normal doses of 0.25-0.375 a.i.*ha^-1 , the compound may be used safely in any growth stage of legumes like soybean ( Glycine max ).

This product shows low acute toxicity in mammals, birds and fish; apparently only insignificant residues are accumulate in crops.

B. Chemical & Physical Properties

Chemical denomination: 5-(2 chloro-4-(trifluoromethyl) phenoxy) -N-methylsulfonyl-2-nitrobenzamide.
Technical denomination: fomasafen
Commercial denomination: Flex (ICI)
Molecular formula: C₁₅H₁₀CIF₃N₂O₆S
Appearance: white crystalline solid

C. Formulation

Flex is available as an aqueous solution: 250 grams of Flex (as a sodium salt) per liter of commercial product.

D. Biological Action

This product is not well translocated within the phloem. Hence, to ensure its efficiency it should be applied on both sides of the leaves, on the petioles and apexes. A non-ionic surfactant must always be added to improve its action.

Soil absorption occurs when the product is translocated to the radicular area after rainfall. Translocation through the xylem will depend largely on the species of weeds and their growth stage. Soil applications are more effective at germination time.

After foliar application in the more susceptible species, necrosis rapidly sets in, provoking the diseccation and death of the plant. Legumes will temporarily present deformations and wrinkling of the leaves shortly after application, and this condition will sometimes be accompanied by a slight chlorosis. These symptoms have been observed with the use of overdoses and when used in combination with elevated quantities of surfactants.

This herbicide exerts its main action on the photosynthetic mechanism. Photosynthesis is altered by the generation of superoxide radicals within the chloroplasts. Legumes owe their tolerance to their capacity to rapidly break the herbicide's diphenylic ether bond to form inactive compounds.

The properties of Flex indicate that it is most effective at early postemergence; this way, lower doses than the ones required at strict preemergence may be used.

This product is particularly active against broadleaf weeds. The sensitivity of the weeds decreases as the biomass increases; it also varies in accordance with the type of species present in the weed complex. The widest control spectrum is obtained when postemergence applications are carried out on weeds at an initial growth stage of 2-3 leaves, with doses that vary from 0.25-0.375 Kg. a.i.*ha^-1 . An increase in the doses (0.5 Kg. a.i.*ha^-1 ) allows the herbicide to control some of the less susceptible species and somewhat lengthens the periods between applications.

If rainfall occurs four hours after application, no effect on the activity of the herbicide is observed. The product fixes itself on the soil's organic matter, where it is gradually inactivated.

Some weeds frequently controlled by Flex are: Abutilon teofrasti, Acalypha australis, Amaranthus retroflexus, A. spinosus, A. viridis, Ambrosia spp ., Chenopodium album, Ipomoea spp. , Datura stramonium, Xanthium pennsylvanicum, Euphorbia heterophylla, Bidens pilosa, Borreria alata, Brassica spp. , Commelina spp., Emilia sonchifolia, Galisonga spp., etc.

E. Application

An increase in the volume of the applications has allowed an improvement in the activity of the product, which is probably due to a better distribution of the droplets. Volumes of 250 to 350 l*ha^-1 are the most commonly used with this type of product, and are closely related to weed population, growth stage and weed density.

The use of high pressures in the spraying equipment improves the effectiveness of the herbicide, possibly due to a greater penetration in the foliage and a better distribution of the product. The most commonly used pressures vary from 0.41 to 0.68 Pa.

Fan nozzles like the Teejet 80015, 8002, 110015, 11002 (Spraying Systems Co.) produce adequate and enough droplets at the recommended pressures.

As mentioned before, this herbicide demands the addition of a non-ionic surfactant to develop its action. Generally, Agral 90 at 0.01% is used. Increasing the surfactant concentration to 0.2% improves the herbicide's action, especially against some particularly resistant types of broadleaf weeds like Chenopodium spp., Anoda cristata , and Acanthospermun australe . However, the higher concentrations may also increase the legumes phytotoxicity. The degree of toxicity will depend on the environmental factors, plant genus, growth stage and physiological status of the legume.

F. Effects of Environmental Factors

The results obtained with Flex will vary, depending on meteorological conditions and the physiological status of the cover. Environmental factors have not yet been investigated in detail to allow a complete analysis; however, the following tendencies have been noted:

1. This herbicide is most effective against weeds inactive growth, and when the temperature, soil humidity and air humidity are high. Such conditions, along with abundant light, may also cause greater phytotoxicity in legumes.
2. With a low relative humidity (less than 45%), or with water deficit, the product's action is reduced, thus exerting a deficient control of weeds and their sprouts.
3. The action of Flex is not affected by rainfall, if it does not occur within four hours after application.
4. Precipitation occurring within the first week after application, will transport the product to the soil and will facilitate the control of some species of weeds that absorb the product though their radicular system.

G. Product Handling: Precautions

Laboratory studies indicate that this herbicide is slowly degraded in the soil under aerobic conditions, generally with a mean lifespan of 6 months; however, under anaerobic conditions, it is more rapidly degraded (one month).

It has been demonstrated that photodegradation occurs at relatively low solar light intensities, and it presents a mean lifespan of 40 days; this characteristics could be one of the main causes of the initial loss of action of the product in field conditions.

Some field studies indicate that Flex does not accumulate at deep soil levels. The greater part of the residues have been found in the first 15 cms. of soil. The information obtained indicates that product residues are rapidly reduced within the first weeks after application; afterwards, the process slows down.

H. Additional Information

It may be mixed with fluazifop-butyl (Fusilade) to widen the spectrum against grasses. Up to now, its use in the purification of cover legumes in oil palm plantations is still only experimental. In Costa Rica, this product has not yet been registered for its use with Elaeis guineensis nor with P. phaseoloides .

II. COBRA (LACTOFEN)

A. General Characteristics

Cobra is a herbicide from the family of diphenyls which presents a high degree of selectivity when applied in postemergence to soybean and peanut crops. This product has been developed by PPG Industries Inc. and shows great potential for the control of broadleaf weeds associated with leguminous crops. Taylor (1985) mentions that during the 1985 soybean agricultural year, more than 100 soybean cultivators informed having obtained excellent results with this product in controlling broadleaf weeds.

B. Chemical and Physical Properties

Chemical denomination:
(1-carboethoxy) ethyl 5- (2- chloro - 4 - (trifluoromethyl)phenoxy) -2-nitrobenzoate
Technical denomination: Lactofen
Commercial denomination: COBRA (PPG)
Molecular formula: C₁₉H₁₅CIF₃NO₇
Appearance: dark brown

C. Formulation

It is available as a concentrate (2EC) emulsifiable to 240 g a.i. *l^-1 of commercial product.

D. Biological Action

This herbicide is used in early postemergence; however, it has some preemergent action. COBRA is a herbicide which is absorbed by different soil particles. Strictly in preemergence, the effectiveness of its absorption by weeds depends on the texture, structure and organic matter content of the soil. When it is applied in postemergence, a non-ionic surfactant must be included (Agral, Triton, etc.) to improve its effectiveness. Some studies indicate that this product is resistant to lixiviation, but tends to dissipate rapidly after application. It has also been noted that the biological action of some of the microorganisms in the soil, lead to the degradation of the product. COBRA has a mean lifespan of 1-2 months. Preemergent action is not evident until the sixth week. When applied in postemergence on soybean or peanut crops, the developed leaves present some decoloration and the younger leaves, some folding. However, their posterior development is normal and the crop will overcome these conditions rapidly. In susceptible weeds, toxicity is characterized by chlorosis and necrosis of the photosynthetic tissues.

COBRA is a product of the same family of Flex, and probably their effects are similar, since they interfere with the photosynthesis of the susceptible weeds.

This product is applied in postemergence, when weeds have 2 to 6 leaves (2-3 weeks after the sowing of legumes). Some residual preemergent effect has been observed in postemergent applications.

The doses commonly used in postemergence vary from 0.1 to 0.2 kg. a.i.*ha^-1 and in preemergent applications from 0.2 to 0.4 kg. a.i.*ha^-1 .

Some of the species generally controlled by COBRA are: Bidens spp., Solanum rostratum, Ambrosia artemisifolia, Richardia scabra, Physalis wrightii, Sesbania exaltata, Datura stramonium, Amaranthus spp., Mollugo verticilata, Portulaca oleracea, Xanthium spp., Acalypha spp., Commelina spp., Ipomoea spp., Cucumis spp., Sida spp., Abutilon theophrasti, Euphorbia spp., Hibiscus spp., Asclepias spp., Helianthus annus, Anoda cristata .

E. Application

In postemergent applications, a volume of 200 to 280 l*ha^-1 is recommended. The operating pressure for the equipment must be around 0.21 - 0.41 Pa, using conic or fan nozzles.

The addition of a non-ionic surfactant in a concentration varying from 0.1 to 0.25% (v/v) improves the herbicidal action. The use of agricultural oils mixed with this particular product is not adequately documented.

F. Product Handling: Precautions

COBRA must be very carefully handled, as it may cause irreversible damage to eyes and provoke skin irritation.

The manufacturing company does not recommend applying this product during plant stress periods, such as excessive humidity, water deficit, or low temperatures. Under these conditions, weeds may present a hardening of the cuticle, which will impede the absorption of the product. Susceptible crops have also been observed to present less tolerance to the product and will therefore have greater possibilities of suffering phytotoxicity.

G. Additional Information

This herbicide must be mixed with other pesticides. Its use in the purification of leguminous covers in oil palm plantation is still only experimental. In Costa Rica, this product has not yet been registered for use with Elaeis guineensis nor with Pueraria phaseoloides .

References

CARMONA, A. 1988. Nuevos productos potencialmente útiles para el combate de malas hierbas en palma aceitera. Bol. Téc. OPO-CB 2(2): 57-63.

IMPERIAL CHEMICAL INDUSTRIES. sf. Fomasafén (PP021). Boletín de datos. Plant Protection Division. sp.

PPG INDUSTRIES INC. 1985/86. Cobra. A new experimental, selective herbicide for controlling a broad spectrum of broadleaf weeds. Technical Bulletin. 15 p.

__________. sf. Cobra. Specimen Label soybeans. Technical Bulletin. 13 p.

ROSE, R.P. and RIABOV, J. 1985. Today's Herbicide: Reflex 2LC herbicide - A new selective postemergence broadleaf weed. Weeds Today 16(3): 5.

TAYLOR, R. 1985. Today's Herbicide: Cobra postemergence herbicide shows promise for producers and PPG. Weeds Today 16(4): 3.

THOMPSON, W.T.1987.Agricultural Chemicals. Book II-Herbicides. Thompson Publications, California. 301p.