EP0000260A1

EP0000260A1 - Dry compactible, swellable herbicidal compositions and pellets produced therefrom, and use thereof

Info

Publication number: EP0000260A1
Application number: EP78300059A
Authority: EP
Inventors: Robert Eyer Albert
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1977-06-21
Filing date: 1978-06-20
Publication date: 1979-01-10
Also published as: AR223464A1; AU523551B2; EP0000260B1; ES470975A1; BR7803894A; CA1099940A; DE2860711D1; NZ187624A; GR64539B; IT7824749A0; IT1096591B; AU3722378A; SE7803658L

Abstract

Dry compactible, swellable herbicidal compositions and. pellets produced therefrom and use thereof.

The herbicidal composition consists essentially of 5-25% by weight of a herbicide which is bromacil, diuron, bromacil-diuron complex, 3-cyclohexyl-6-(dimethylamino)-1- methyl-s-triazine -2,H(1H,3H)- dione, karbutilate, tebuthiuron, N-[4,6-dimethoxy- 1,3,5 triazine 2-yl) aminocarbonyl]benzene sulfonamide or a mixture of two or more thereof, 20 to 60% by weight of swelling bentonite, 5 to 15% by weight of anhydrous sodium sulfate, 10 to 25% by weight of urea, 2 to 10% by weight of polyethylene glycol having a weight-average molecular weight of from 200 t6 600 and 1 to 12% by weight of water, and optionally a die release agent and/or an inert diluent. The composition can be dry compacted into a herbicidal pellet which is resistant to impact and crushing and which swells and disintegrates when contacted with small amounts of liquid water to release the herbicidal compound. The pellet can provide selective control of undesirable brush in rangeland and woodland areas.

Description

This invention relates to dry compactible, swellable herbicidal compositions and pellets produced therefrom, and use thereof. Pellets produced from the compositions are primarily of use for controlling undesirable brush in rangelands and woodlands by aerial application in low-rainfall areas.
Undesirable weeds and thorny brush growth such as mesquite, huisache, yaupon, _Mc_Cartney rose and the like infest millions of hectares of rangeland. Once heavily infested, such rangelands are no longer suited for livestock grazing.
Broad-spectrum herbicides, which are. normally applied to infested areas_'with aerial or ground sprays, or by a dry granule broadcast method, destroy not only the undesirable brush, but also much needed grasses and other ground cover. Although selective herbicides can be used to avoid injury to ground cover, the high concentrations of active ingredient which are needed to destroy the hardy brush varieties can make the cost of selective herbicides prohibitive.
In woodlands, particularly pine woodlands which are "farmed" to produce trees for the pulp and paper industry, brush growth can retard the development of young trees and may increase the planting-to- bervesting cycle by as much as eight years. Broadcast applications of conventional herbicide formulations at :ates high enough to control the undesirable brush can hill or severely damage the young trees.
The compositions of this invention are aimed typically at woodland and brush areas which are arid and have low rainfall. Any herbicide which is'to be useful for this type of area must require very little water to make the material active in the soil. Furthermore, economy of application, where large areas and difficult terrain are involved, favors the use of airplanes.
With certain herbicides -- such as the ones used in the compositions of this invention -- selection between the large brush that must be controlled vs, the seedling trees or grass for grazing whose growth the treatment should promote, can be obtained by applying the herbicide as large pellets or bri- quettes on a grid pattern, for example on a 1-3 meter grid. As the herbicide is moving downward from the relatively few loci of the grid, the large root systems of the brush can pick up a lethal dose of herbicide while in the remaining areas the grass or the seedling trees grow without damage.
This combination of application requirements can only be met with large pellets or balls which have very special physical properties. The handling, shipment, storage and use of such material requires that these pellets be hard, crush and impact resis- tant, and unaffected by high relative humidity, while, at the same time, after application, the pellets must swell and disintegrate after only small. amounts of rainfall to release the active herbicide.
Thus, there is need for a composition having a critical set of properties which are in part contradictory and are difficult to attain. Furthermore, it is important to produce these compositions, i.e. these pellets and balls, economically. Hence, expensive wet extrusion and drying processes should be avoided in favor of dry-compaction or briquetting.
The present invention provides a dry-compactible, swellable herbicidal composition consisting essentially of about 5 to 25% by weight of a herbicide which is 5-bromo-3-sec-butyl-6-methyluracil (hereinafter bromacil), [3-(3,4-dichlorophenyl)-1,I-dimethylurea] (hereinafter diuron), bromacil-diuron complex, 3-cyclohexyl-6-(dimethyl- amino)-1-methyl-s-triazine-2,4(1H,3H)-dione (hereinafter Velpar weed killer), karbutilate, 1-(5-t-butyl-1,2,4-thia- diazol-2-yl)-1,3-dimethylurea (hereinafter tebuthiuron), N-[(4,6-dimethoxy-1,3,5-triazin-2-yl)aminocarbonyl] benzenesulfonamide or a mixture of two or more thereof, 20 to 60% by weight of swelling bentonite, 5 to 15% by weight of anhydrous sodium sulfate, 10 to 25% by weight of urea, 2 to 10% by weight of a polyethylene glycol with a weight-average molecular weight ranging from 200 to 600 and 1 to 12% by weight of water. The composition optionally includes up to 5% by weight of a die release agent which is stearic acid, calcium stearate or magnesium stearate, and/or up to 30% by weight of an inert diluent.
The term "bromacil-diuron complex" as used herein means the 1:1 molar complex of the two components as described in U.S. Patent 3,914,230, the teachings of which are incorporated herein by reference.
Points prepared by dry compacting these compositions are particularly useful for aerial application to woodlands and rangelands for controlling undesirable brush. The pellets of this invention combine the advantages that they can be produced by dry compaction (e.g. the composition does not require a wet extrusion step followed by heating and/or drying), are impact- and crush-resistant while dry, and are highly resistant to disintegration in high-humidity atmospheres. Once applied, these pellets will swell and disintegrate rapidly and hence will release the active herbicide when exposed to only a small amount of liquid water and will not re-harden when dried.

DETAILED DESCRIPTION OF THE INVENTION

Improved physical properties can be achieved in herbicide compositions used for controlling brush on rangelands and woodland by dry-compacting a composition consisting essentially of about 5 to 25% by weight of active ingredient, about 20 to 60% by weight of a swelling bentonite, about 5 to 15% by weight of anhydrous sodium sulfate, about 10 to 25% by weight of urea, about 2 to 10% by weight of a polyethylene glycol with a weight-average molecular weight ranging from 200 to 600, about 0 to 5% by weight of a die release agent, about 1 to 12% by weight of water and from 0 to 30% by weight of an inert diluent into pellets or balls having a density in the range of about 0.5 to 1.75 grams/milliliter and a volume in the range of about 0.75 to 2.0 cubic centimeters. The term"pellet" is used herein to mean a shaped body such as a briquette, bolus, ball or any other suitable shape.
The phrase "consisting essentially of" is not intended to limit the claims so as to exclude other ingredients when the specification clearly indicates other constituents may be present. The phrase does exclude ingredients which would affect the basic and novel characteristics of the invention defined by the claims.
The pellets of this composition have a critical balance of physical properties which is important in the control of undesirable brush in arid regions used for grazing or forestation. The process and composition combine to yield a pellet which is hard, crush- and impact- resistant, and unaffected by high relative humidity in storage. Yet, after application, the pellet can swell and disintegrate upon contact with only a small amount of liquid water (rainfall) to activate and release the herbicidal active ingredient.
Generally, the pellets of this invention are applied by aerial means in a low-density pattern over the acreage to be treated. Spacing the pellets on the average about 1 to 3 meters apart provides for optimum distribution of active ingredient. Other grid patterns and pellet spacing may also be used.
The compositions of this invention will typically contain the following essential ingredients:

(1) an active ingredient having the desired biological activity; (2) a swelling bentonite to accomplish structural disintegration of the pellet on contact with liquid water; (3) anhydrous sodium sulfate to prevent the pellet from rebonding if drying should occur after the pellet has been initially exposed to liquid water;
(4) an inert diluent to adjust the weight and composition of the pellet and a combination of (5) urea; (6) polyethylene glycol having a weight-average molecular weight of 200-600; and (7) water below about 12% by weight of the total composition to serve as plasticizer, binder and lubricant during dry pressing.

As used herein the term "swelling bentonite" means absorptive clays generally described as montmor- illonites and capable of expanding their lattice structure upon the absorption of water. Such clays are described by R. K. Iler; The Colloidal Chemistry of Silica and Silicates, pp. 191-198, Cornell University Press, Ithica, New York, 1955. Examples of commercially available swelling bentonites are: VolclayO, Wy-O-Gel®, and other Wyoming bentonites.
The active compounds useful in this invention are herbicides selected from the group consisting of bromacil, diuron, bromacil-diuron complex, mixtures of bromacil-diuron complex with bromacil or diuron, Velpar® weed killer, karbutilate, tebuthiuron, and
triazine 2-yl) aminocarbonyl]ben- zens sulfonamlde, and combinations of these.
The ingredients are dry blended in any suitable blending apparatus in the desired proportions. The blended ingredients may also be ground in a hammer-mill, ball mill, rod mill or other suitable grinding equipment. The composition is reblended after milling to ensure uniformity. Dry compacting is accom-' plished by conventional methods to form the water swellable pellets. Briquetting and pill pressing are particularly suitable methods for dry compacting these compositions. In the case of some mixtures of ingredients (depending upon the properties of the active herbicidal chemical, the particle size range and the proportions of the ingredients) deaerating the ground and blended composition may be necessary prior to the dry compacting operation in order to prepare a.pellet with the required crushing strength and impact resistance. Deaerating can be accomplished economically by precompacting the composition by passing the mixture through: (1) "squeeze" rolls; or (2) a deaerating (compacting) screw feeder immediately prior to dry compacting. The bulk density of deaerated mixtures can be increased up to two to three times that of the aerated mixture. The deaerated mixtures can be compacted by roll briquetting at much higher rates than the corresponding aerated mixtures. In order to prevent sticking of the compacted material in the dry- forming die a small amount of a die release agent may be added to the composition. Suitable die release agents are stearic acid, calcium stearate or magnesium stearate. The die release agent is blended into and ground with the other ingredients.
Dry processing prevents premature swelling of the bentonite and obviates the need for any drying step in preparing the pellets. It should be noted that bentonites swell when initially exposed to small amounts of liquid water for about 5 to 120 minutes. If dried under mild conditions such as might exist in rangelands, only excess water is removed by evaporation. The water that produces the swelling by absorption into the lattice of the bentonite is not evaporated. Therefore, swelling generally occurs only once. However, wet bentonite will dry in time and become hard, resulting in a reduced surface area from which the active ingredient may only be slowly leached by successive wetting. To prevent the bentonite from hardening, anhydrous sodium sulfate is incorporated into the composition. Anhydrous sodium sulfate is used because it is capable of forming stable hydrates from 0.5 moles of water up to 10 moles of water per mole of anhydrous salt.
The binder material for the composition of this invention includes urea because of its water solubility and biological considerations. The presence of small amounts of water in the composition, preferably from about 1 to 12% by weight, more preferably from about 4-8% by weight, acts as a plasticizer for the urea and aids the bonding process during dry compaction. Water-insoluble and slowly soluble binders, such as sodium silicate, retain their binding properties for extended periods in the presence of liquid water and would inhibit the swelling of the bentonite. Other water soluble binders such as sugar, polyvinyl alcohol, and polyvinyl pyrollidone generally require more than 12% by weight of water for effective binding. Their rates of solubility are also much less than urea so that they do not cooperate as efficiently with the other ingredients of the compacted composition to allow for swelling and as rapid a release of the active ingredient.
The polyethylene glycol operates within the composition as a liquid reservoir together with the water. Polyethylene glycol also protects against
the ingredients of the composi- then which could result from temperature changes during storage. Thus, premature swelling and crumbling of the pellets is prevented. The pellets of this invention may be stored in sealed containers for up to about two years without any significant or deleterious change in chemical or physical properties.
The polyethylen glycol should have from about 200 to 600 weight average molecular weight. Lower molecular weigh glycols, for example ethylene glycol and diethylene glycol, exhibit relatively high vapor pressures add tend to volatilize at high ambient temperatures. High molecular weight glycols, normally liquid at ambient temperatures, are generally viscous and difficult to distribute uniformly during preparation of the composition.
Inert diluents may be incorporated into the composition up to about 30% by weight of the dry compacted product to adjust the weight of the pellet. Preferred diluents include, but are not limited to, inert clays such as non-swelling bentonites, sub-bentonites (Panther Creek clay), attapulgites (Attaclay®), kaolinites (Barden clay) and diatomaceous earth.
Ground or aerial application techniques can be used to distribute the pellets of this invention in the desired pattern over the area to be treated. where large or remote surface areas are to be treated, aerial application is the most economical and preferred method. The pellets are released from an aircraft at a rate which will achieve the desired grid spacing and, hence, distribution of active ingredient, when the pellets come to rest pn the ground.
By the term grid spacing or grid pattern is meant an average spacing of pellets over a hectare of treated area. For example, to apply 1.12 kg of active ingredient in a grid pattern of one hectare using pellets containing 15% by weight of active herbicidal chemical requires 2990 pellets (7.5 kg) applied in a square grid pattern with an average spacing between adjacent pellets of 1.8 meters. Precise spacing cannot be absolutely maintained-between each pair of pellets due to effects of external forces such as air turbulence, contact with canopy foliage, deflection by branches and rebounding or bouncing of some pellets which occasionally strike exposed hard ground or other firm objects on the ground surface. However, these slight disturbances in the precise grid pattern desired do not affect the overall biological performance of the pellets under field conditions.
At the ground location of each pellet any destruction of ground cover by released herbicide is generally confined to a circular area no larger than about 30 centimeters in diameter. The area of the denuded spots is determined by the soil conditions controlling the ground penetration rate of the active ingredient which is being released from the pellet, the amount and rate of rainfall, and the particular active ingredient and its release rate from the pellet.
, A grid distribution of the pellets creates generally conical "columns" of herbicide within the soil with the apex at the-surface location of the pellets. When released from the pellet by small amounts of liquid water, such as from dew or rain, the active ingredient penetrates downwardly through the soil. This type of distribution and penetration allows the active ingredient to reach the roots of hardy brush species and avoid contact with the shallow rooted, more desirable grasses and ground cover. Only a small portion of a plant or tree root system need be exposed to the herbicide to retard or eliminate it.
The swelling and crumbling of the pellets of this invention when contacted with small amounts of liquid water not only permits the active ingredient to be released quickly from the formulation, but also destroys the physical structure of the pellet. Thus, small animals such as rabbits and mice are much less likely to find a pellet intact and be exposed to the active ingredient. The active herbicide is quickly released to the soil from the pellet when swelling occurs, and small animal population is protected.
Usual application techniques employ either mechanical "slingers" or pneumatic devices which distribute the pellets over swaths or bands up to about 15 meters wide. The term "slinger" is used to define a rapidly rotating disk having radial spaced baffles. The pellets are dropped onto the "slinger" at a controlled rate, are struck by the baffles and impelled a given distance from the "slinger" to achieve the desired swath width.
Of the pneumatic applicators, one is a "fish tail" type used for aerial seeding and fertilizer applications from fixed wing aircraft. The herbicide pellets are dropped into a ram air section having an air stream velocity of about 40 to 55 meters/second and forced by the air stream against baffles in the "fish tail" discharging area. A second type of pneumatic applicator is adapted for use with helicopters. It employs a blower to provide a high velocity air stream which ejects the pellets in a swath pattern from a duct system into which the pellets are fed.
It will be appreciated by those familiar with application techniques that the pellets are subject to a multiciplicity of forces during application. As the pellets fall to the ground they strike the leaf canopies and branches of taller trees and brush. They are also affected by the nature and velocity of any crosswinds as they fall.
The pellets of this invention provide the desired combination of physical properties and exhibit high crushing and impact strength to resist damage from shipping, handling and the forces associated with application. Because of their improved physical properties, the pellets of this invention can be better controlled during aerial application to achieve the desired grid pattern. The pellets are dense enough to resist deflection by crosswinds and the leaves, of brush and tree canopies.
Crushing strength is determined by first dry-compacting the desired formulation, after blending and grinding, to obtain a cylindrical sample pellet 16 mm in diameter by about 9 to 11 mm long. Compacting is accomplished by applying 1400-1450 kg/cm² pressure to a piston which compacts the loose formulation in a cylindrical die. After compaction, the pellet is placed horizontally between two horizontal parallel steel plates and the plates are loaded until crushing occurs. Pellets of this invention resist crushing with loads less than about 75₀₀ grams or 3.7 kg/cm².
Impact resistance is measured by dropping a sample pellet as compacted above onto a smooth, hard horizontal surface such as concrete or steel from a height of 3 meters. Pellets of this invention normally resist breakage for 5 drops.
The reaction.of a sample pellet to wetting with liquid water and subsequent drying is determined by placing the pellet into a covered Petri dish and applying 0.5 ml (5 drops) of water thereto. Swelling and crumbling is observed during the first 2 to 5 hours. The pellets of this invention will crack immediately after wetting and swell to about 2 to 3 times their original volume during the observation period. The cover is then removed and the pellets are allowed to dry at about 23°C and between about 25 to 60% R.H. When dried the pellets of this invention do not rebond but remain crumbled.
Apparent density of the pellets is measured
by displacement. The density of the pellets of whis invention is such that mercury will not penetrate the pores thereof. Pellets of this invention exhibit an apparent density of about 1 g/ml or greater.
Composition ranges of the pellets of this invention are given in Table 1:
More preferred composition ranges are shown in Table 2:
Optionally, small quantities of dispersant and/or wetting agent can be included in the composition.
Particular compositions for grid applications are selected on the basis of the following considerations: (1) amount of active herbicidal chemical to be applied per unit area: (2) grid spacing; and (3) total weight of composition to be applied per unit area. The weight of the pellets is generally determined by the metering accuracy of the aerial application equipment. Usually about 10-12 kg/hectare is the minimum weight of total composition which can accurately be applied to a given area. In some instances, however, total weight can be as low as 6 kg/hectare where well constructed, accurately calibrated equipment is used. For example, to apply 1.12 kg active chemical per hectare in a total of 11.2 kg of composition in a generally square grid pattern of 1.8 m x 1.8 m average spacing requires that the composition contain about 10% by weight of the active chemical and be uniformly distributed as 2990 compacted pellets per hectare. Each pellet will contain about 0.375 g of herbicidal chemical. Other compositions and grid spacings are given in Table 3.
This invention is further described by the following examples. Velpare 90 Weed Killer is a composition which contains about 89-91% by weight of 3-cyclohexyl-6-dimethylamino)-l-methyl-s-triazine-2,4-(1H, 3H)dione and about 11-9% by weight of inert diluents and process impurities.

EXAMPLE 1

The following ingredients are blended together:
After thorough blending the composition is ground in a bantam hammer mill (Mikropul Corp., Summit, New Jersey) using a screen having 1 mm dia. holes. The particle size range of the ground formulation indicated 5.9 wt. % larger than 150 µm 30.2 wt. % larger than 74 µm and 47.6 wt % smaller than 44 µm.
The ground composition is dry compacted using a model 50 KHD briquetting press (Klockner-Humboldt-Deutz Ag., Köln, West Germany) with rolls having 1. cc pockets to form briquettes of 1. cc volume weighing about 2.1 g each. The briquettes are physically stable when exposed to 95% RH atmosphere for 100 hours and exhibit a crushing load of 11,000 g. The pellets swell and crumble after being contacted with 1 ml of liquid water.

EXAMPLE 2 (Comparative)

This example demonstrates the effect of removing the polyethylene glycol from the composition.
The composition of Example 1, without the polyethylene glycol 200, is prepared by blending and grinding. In this instance, however, cylindrical shapes, 1.27 cm in diameter are formed by pressing the composition in a cylinder and ram die using a laboratory press and a pressure of about 450 kg/cm gauge.
The cylindrical shapes are exposed to 85% and 100% RH atmospheres for 16 hours. The shapes exposed to 85% RH swell about 10% volumetrically, exhibit surface tackiness and are cracked. Those exposed to 100% RH swell about 25% volumetrically, exhibit surface tackiness and are so severely cracked they cannot be handled without crumbling.

EXAMPLE 3 (Comparative)

This example demonstrates the effect of removing the sodium sulfat3 from the composition.
11.4 g of Velpar® 90 weed killer, 44.4 g of the Volclay 90, 33.3 g of Attaclay and 11.2 g of crystal urea are thoroughly blended and ground as in Example 1. The ground composition is pressed as in Example 2 to form dense, strong cylindrical shapes.
The shapes are contacted with about 0.2 ml of liquid water which is equivalent to about 0.5 cm of rain. The compacted shapes swell and crack initially. When allowed to dry at 23°C and 40% RH the cylinders rebond into strong, coherent structures. Rewetting does not cause swelling and recracking.

EXAMPLE 4

The following ingredients are blended together:
After thorough blending the mixture is ground in a bammer-mill (No. 1 Mikro-polverizer, Mikropul Corp., Summit, N.J.) using a screen with lmm dia. holes.
The ground composition is dry-compacted using a Model 220 Komarek Roll Briquetter (K.R. Komarek Inc., Elk Grove Village, ILL) using rolls having 2.2cc volume to form 2.2cc bolus shaped briquettes weighing 3.75 g each and containing 10 wt % of active herbicide.
The briquettes are physically stable to 100 hrs. exposure at 90% RH, exhibit a crushing load ^>11.0 kg and exhibit an impact resistance ^>10 drops. When werted with 1 ml of water per briquette, the briquettes swell and crumble and do not rebond upon drying.

EXAMPLE 5

The briquettes of Example 4 were hand distributed on a 1.8 meter x 1.8 meter (6 ft. x 6 ft.) square grid pattern over four separate 0.1 hectare (0.25 acre) plots at an over application rate of 11.2 kg/ha (10 1b/acre) of briquettes and providing 1.12 kg/ha (1 1b/a) of active herbicide. The plots contained seedling loblolly and short-leaf pines, the desired spscies being "farmed" to provide a source of pulpwood. The plots also contained the undesirable hardwoods: dogwood; hickory; hornbean; maple; oaks; sweetgum; and sourwood, which compete with the pines for food, water and sunlight and seriously retard their growth.
Five months after the treatments the undesired species were defoliated and dying with no apparent injury to the pine species. One year after the treatments the hardwood species were eliminated. The pine seedings in the treated plots exhibited larger growth and vigor compared to similar adjacent but untreated plots.

EXAMPLE 6

The briquettes of Example 4 were applied from a helicopter using a Simplexe Pneumatic Seed applicator to distribute 22.4 kg/ha (20 lb/A) of briquettes, in a square grid pattern, 1.3 meters x 1.3 meters (4.25 feet x 4.25 feet) on 1 ha (2.47A) loblolly and short-leaf pine seedling plots. The seedling plots were infested with dogwood, blackjack oak, redoak, postoak, sourwood, hickory, sweetgum and yellow poplar.
Five months after treatment the undesired hardwood were ccmpletely eliminated with no damage to either loblolly or short leaf pine. The pine species showed much greater growth and vigor as compared to adjacent untreated plots.

EXAMPLE 7

The following ingredients are blended together and ground in a laboratory hammer-mill:
10 g portions of Mixture "A" were blended with 2.3 g of each of several wettable powder herbicides to prepare 12.3 g of each of the following compositions:
3.75 g portions of each of the five compositions are dry compacted into cylindrical pellets as in Example 2. The pellets exhibit the following properties.

EXAMPLE 8

The following compositions are thoroughly blended and ground using a laboratory hammer-mill:
The ground compositions are dry compacted as in Example 7, and the pellets exhibit the following properties.
Sample 4 contains insufficient binder to form a strong impact and crush resistant pellet.

Claims

1. A dry compactible, swellable herbicidal composition consisting essentially of 5-25% by weight of a herbicide which is bromacil, diuron, bromacil-diuron complex, 3-cyclohexyl-6-(dimethylamino)-1-methyl-s-triazine-2,H(lH,3H)-dione, karbutilate, tebuthiuron, N-[4,6-di- methoxy-1,3,5 triazine 2-yl)aminocarbonyl]benzene sulfonamide or a mixture of two or more thereof, 20 to 60% by weight of swelling bentonite, 5 to 15% by weight of anhydrous sodium sulfate, 10 to 25% by weight of urea, 2 to 10% by weight of polyethylene glycol having a weight-average molecular weight of from 200 to 600 and 1 to 12% by weight of water.

2. The herbicidal composition of claim 1 which further includes up to 5% by weight of a die release agent which is stearic acid, calcium stearate or magnesium stearate.

3. The herbicidal composition of claim 1 or 2 which further includes up to 30% by weight of an inert diluent.

4. The herbicidal composition of claim 1 consisting of 8 to 15% by weight of said herbicide, 30 to 50% by weight of swelling bentonite, 5 to 15% by weight of anhydrous sodium sulfate, 10 to 20% by weight of urea, 2 to 10% by weight of polyethylene glycol having a weight-average molecular weight of from 200 to 600, 1-4% of a die release agent which is stearic acid, calcium stearate or magnesium stearate, 4 to.8% by weight of water and 10 to 25% by weight of an inert diluent.

5. The herbicidal composition of claim 4 wherein the herbicide is 3-cyclohexyl-6-(dimethylamino)-1-methyl-s-triazine-2,H(lH,3H)-dione and the polyethylene glycol is polyethylene glycol 200.

6. A dry compacted, water swellable herbicidal pellet consisting essentially of the composition of any of claims 1-5, said pellet having a crushing strength greater than 3.7 kg/cm² and an impact resistance of at least 5.

7. The pellet of claim 6 having a density in the range of 0.5 to 1.75 g/ml and a volume in the range of 0.75 to ₂.₀ cm³.

8. A method for the control of undesirable brush in woodlands or rangelands by aerial application of a herbicidal composition characterised in that the pellet of claim 6 or 7 is used as said herbicidal composition.

9. The method of claim 8 wherein the pellets are applied at an average spacing in the range 1-3 m.