HERBICIDAL COMBINATION The present invention is directed towards an improved herbicidal combination comprising a 2-(substituted benzoyl)-l,3-cyclohexanedione, salt or metal chelate thereof and l,r-dimethyl-4,4'-bipyridinium (paraquat).The invention is further directed towards a method of controlling the growth of undesirable vegetation by applying a herbicidally effective amount of such combination to the locus of such vegetation and to the use of such a combination for the control of unwanted vegetation. The protection of crops from weeds and other vegetation which inhibits crop growth is a constantly recurring problem in agriculture. To help combat this problem, researchers in the field of synthetic chemistry have produced an extensive variety of chemicals and chemical formulations effective in the control of such unwanted growth. Chemical herbicides of many types have been disclosed in the literature and a large number are in commercial use. m some cases, active herbicides have been shown to be more effective in combination than when applied individually. The result is often termed "synergism", since the combination demonstrate a potency or activity level exceeding that which it would be expected to have, based on a knowledge of the individual potencies of the components. The present invention resides in the discovery that a 2-(substituted benzoyl)-l,3-cyclohexanedione, salt or metal chelate thereof and l,l'-dimethyl-454'- bipyridinium, already known individually for their herbicidal properties, display a synergistic effect when applied in combination. The compounds forming the combination which is the subject of the present invention are independently known in the art for their effects on plant growth. For example, the herbicidal cyclohexanedione compound 2-(2'-nitro-4'- methylsulphonylbenzoyl)-l,3-cyclohexanedione (mesotrione) is a selective herbicide disclosed in U.S. Patent No. 5,049,046 along with a number of other cyclohexanedione compounds. It is also commercially available under the tradename Callisto®. 1,1'- Dimethyl-4,4'-bipyridinium, known as paraquat, had its herbicidal properties discovered in 1955 and was first marketed in 1962. Accordingly, the present invention provides a herbicidal combination comprising a 2-(substituted benzoyl)-l,3-cyclohexanedione, salt or metal chelate thereof and 1,1'- dimethyl-4,4'-bipyridinium or a salt thereof. In a further aspect, the invention provides a method of controlling undesirable vegetation, said method comprising applying to the locus of the vegetation a herbicidally effective combination comprising a 2-(substituted benzoyl)-l,3-cyclohexanedione, salt or metal chelate thereof and l,r-dimethyl-4,4'-bipyridinium or a salt thereof. In a still further aspect of the invention, there is provided the use of a herbicidal combination comprising a 2-(substituted benzoyl)-l,3-cyclohexanedione, salt or metal chelate thereof and l,r-dimethyl-4,4'-bipyridinium or a salt thereof for the control of undesirable vegetation. Suitably, the 2-(substituted benzoyl)-l,3-cyclohexanedione for use in the present invention is a compound of formula (I)

wherein X represents a halogen atom; a straight- or branched-chain alkyl or alkoxy group containing up to six carbon atoms which is optionally substituted by one or more groups -OR1 or one or more halogen atoms; or a group selected from nitro, cyano, - CO2R2, -S(O)1nR1, -0(CH2XOR1, -COR2, -NR2R3, -SO2NR2R3, -CONR2R3, -CSNR2R3 and -OSO2R4; R1 represents a straight- or branched-chain alkyl group containing up to six carbon atoms which is optionally substituted by one or more halogen atoms; R2 and R3 each independently represents a hydrogen atom; or a straight- or branched-chain alkyl group containing up to six carbon atoms which is optionally substituted by one or more halogen atoms; R4 represents a straight-or branched-chain alkyl, alkenyl or alkynyl group containing up to six carbon atoms optionally substituted by one or more halogen atoms; or a cycloalkyl group containing from three to six carbon atoms; each Z independently represents halo, nitro, cyano, S(O)JR5, OS(O)nJR5, C1-6 alkyl, C1-6 alkoxy, C1-6 haloalkyl, C1-6haloalkoxy, carboxy, C1-6 alkylcarbonyloxy, C1-6 alkoxycarbonyl, C1-6 alkylcarbonyl, amino, C1-6 alkylamino, C1-6 dialkylamino having independently the stated number of carbon atoms in each alkyl group, C1-6 alkylcarbonylamino, C1-6 alkoxycarbonylamino, C1-6 alkylaminocarbonylamino, C1-6 dialkylaminocarbonylamino having independently the stated number of carbon atoms in each alkyl group, C1-6 alkoxycarbonyloxy, C1-6 alkylaminocarbonyloxy, C1-6 dialkylcarbonyloxy, phenylcarbonyl, substituted phenylcarbonyl, phenylcarbonyloxy, substituted phenylcarbonyloxy, phenylcarbonylamino, substituted phenylcarbonylamino, phenoxy or substituted phenoxy; R5 represents a straight or branched chain alkyl group containing up to six carbon atoms; each Q independently represents C1-4 alkyl or -CO2R6 wherein R is C1-4 alkyl; m is zero, one or two; n is zero or an integer from one to four; r is one, two or three; and p is zero or an integer from one to six and any agriculturally acceptable salt or metal chelate thereof. Suitably, X is chloro, bromo, nitro, cyano, C1-C4 alkyl, -CF3, -S(O)nJR1, or -OR1; suitably each Z is independently chloro, bromo, nitro, cyano, C1-C4 alkyl, -CF3, -OR1, - OS(O)mR5 or -S(O)mR5; suitably n is one or two; and suitably p is zero, one or two. Preferably, the 2-(substituted benzoyl)- 1, 3 -cyclohexanedione of formula (I) is selected from the group consisting of 2-(2'-nitro-4'-methylsulphonylbenzoyl)-l,3- cyclohexanedione, 2-(2 ' -nitro-4' -methylsulphonyloxybenzoyl)- 1 ,3 -cyclohexanedione, 2- (2'-chloro-4'-methylsulphonylbenzoyl)-l,3-cyclohexanedione, 4,4-dimethyl-2-(4- methanesulphonyl-2-nitrobenzoyl)- 1,3 -cyclohexanedione, 2-(2-chloro-3-ethoxy-4- methanesulphonylbenzoyl)-5-methyl-l,3-eyclohexanedione and 2-(2-chloro-3-ethoxy-4- ethanesulphonylbenzoyl)-5-methyl-l,3-cyclohexanedione or salt or metal chelate thereof; most preferably is 2-(2'-nitro-4'-methylsulphonyl benzoyl)-l,3-cyclohexanedione or salt or metal chelate thereof. The 2-(substituted benzoyl)-l,3-cyclohexanedione of formula (I) may exist in enolic tautomeric forms that may give rise to geometric isomers. Furthermore, in certain cases, the various substituents may contribute to optical isomerism and/or stereoisomerism. All such tautomeric forms, racemic mixtures and isomers are included within the scope of the present invention. Agriculturally acceptable metal chelates of compounds of formula (I) are described in more detail in EP 0800317. In particular, metal ions which may be useful in forming the metal chelate compounds include di- and tri-valent transition metal ions such as Cu , Zn , Co , Fe , Ni and Fe . The selection of a particular metal ion to form the metal chelate compound will depend upon the dione compound to be chelated. Those skilled in the art will readily be able to determine the appropriate metal ion for use with a specific dione compound, without undue experimentation. The preferred metal ions are divalent metal ions, particularly Cu2+, Zn2+, Co2+, with Cu2+ being especially preferred. l,r-Dimethyl-4,4'-bipyridinium may also exist either as the acid or as a mono- or di-salt, for example l,r-dimethyl-4,4'-bipyridinium dichloride. AU salts are within the scope of the present invention. In one preferred embodiment of the invention, the herbicidal combination comprises mesotrione acid and paraquat dichloride. The rate at which the herbicidal components are applied will depend upon the particular type of weed to be controlled, the degree of control required, and the timing and method of application, hi general, the components can be applied at an application rate of between about 1O g a.i./hectare (g/ha) and about 1800 g a.i./ha, based on the total amount of active ingredient. An application rate of between about 150 g a.i/ha andlOOOg a.i./ha is preferred. Suitably, the 2-(substituted benzoyl)-l,3-cyclohexanedione is applied at a rate of 20-300 g a.i./ha, preferably 40-250 g a.i./ha; and paraquat is applied at a rate of 50-1500 g a.i./ha, preferably 100-750 g a.i./ha. In an especially preferred embodiment of this invention, the components are administered in relative amounts sufficient to provide an application rate of at least 500 g a.i./ha, of which the 2-(substituted benzoyl)- 1,3-cyclohexanedione provides at least 100 g/ha. A still further aspect of the invention provides a herbicidal combination for the control of unwanted vegetation as hereinbefore described, wherein said herbicidal combination further comprises one or more additional active ingredients. The additional active ingredient is suitably a pesticide, such as a herbicide, a fungicide, an insecticide, a nematocide or the like; preferably, the additional active ingredient is a herbicide. Examples of suitable herbicides include a triazine, such as atrazine, terbuthylazine, simazine, diuron etc. or S-metolachlor, acetochlor, dimethanamid or p-dimethanamid, flufenacet. Further examples of herbicides which maybe of use in the invention will be known to those skilled in the art. The components used in the method of the invention can be applied in a variety of ways known to those skilled in the art, at various concentrations. The combination of the invention is useful in controlling the growth of undesirable vegetation by pre-emergence or post-emergence application to the locus where control is desired, depending on the crop over which the combination is applied. The combination of the invention is particularly effective when applied post-emergence, preferably early post-emergence over crops which are tolerant to the active components. The combinations of the invention can be used over a wide range of crops (or applied using directed applications in non-tolerant crops), such as plantation crops (bananas, fruit trees, rubber, trees, tree nurseries), vines, sugarcane, cotton, corn (maize), wheat, rice, potato or sugarbeet. Suitable crops include those which are tolerant to one or more of the components in the composition. The tolerance may be natural tolerance produced by selective breeding or can be artificially introduced by genetic modification of the crop. Tolerance means a reduced susceptibility to damage caused by a particular herbicide compared to the conventional crop breeds. Crops can be modified or bred so as to be tolerant, for example to HPPD inhibitors like mesotrione, or EPSPS inhibitors like glyphosate. Corn (maize) is inherently tolerant to mesotrione. The components used in the method of the invention may be administered simultaneously or sequentially. If administered sequentially, the components may be administered in any order in a suitable timescale, for example with no longer than 24 hours between the time of administering the first component and the time of administering the last component. Suitably, all the components are administered within a timescale of a few hours, such as one hour. If the components are administered simultaneously, they may be administered separately or as a tank mix or as a pre-formulated mixture of all the components or as a pre-formulated mixture of some of the components tank mixed with the remaining components. Therefore, a yet further aspect of the invention provides a herbicidal composition comprising a 2-(substituted benzoyl)- 1,3-cyclohexanedione, salt or metal chelate thereof and 1 , 1 l-dimethyl-4,4l-bipyridinium or a salt thereof. l The compositions of the invention are useful as herbicides, demonstrating broad- spectrum control of the unwanted vegetation. The composition of the invention can be used over a wide range of crops (or applied using directed applications in non-tolerant crops), such as plantation crops (bananas, fruit trees, rubber, trees, tree nurseries), vines, sugarcane, cotton, corn (maize), wheat, rice, potato or sugarbeet. Suitable crops include those which are tolerant to one or more of the components in the composition. The tolerance may be natural tolerance produced by selective breeding or can be artificially introduced by genetic modification of the crop. Tolerance means a reduced susceptibility to damage caused by a particular herbicide compared to the conventional crop breeds. Crops can be modified or bred so as to be tolerant, for example to HPPD inhibitors like mesotrione, or EPSPS inhibitors like glyphosate. Corn (maize) is inherently tolerant to mesotrione. The herbicidal compositions of this invention also preferably comprise an agriculturally acceptable carrier therefore. In practice, the composition is applied as a formulation containing the various adjuvants and carriers known to or used in the industry for facilitating dispersion. The choice of formulation and mode of application for any given compound may affect its activity, and selection will be made accordingly. For example, the herbicidal composition of this invention may be a dustable powder, gel, a wettable powder, a water dispersible granule, a water-dispersable or water-foaming tablet, a briquette, an emulsifiable concentrate, a microemulsifiable concentrate, an oil- in-water emulsion, a water-in-oil emulsion, a dispersion in water, a dispersion in oil, a suspoemulsion, a soluble liquid (with either water or an organic solvent as the carrier), an impregnated polymer film, or other forms known in the art. These formulations may be suitable for direct application or may be suitable for dilution prior to application, said dilution being made either with water, liquid fertilizer, micronutrients, biological organisms, oil or solvent. The compositions are prepared by admixing the active ingredients with adjuvants including diluents, extenders, carriers, and conditioning agents to provide compositions in the form of finely-divided particulate solids, granules, pellets, solutions, dispersions or emulsions. Thus, it is believed that the active ingredients could be used with an adjuvant such as a finely-divided solid, a mineral oil, a liquid of organic origin, water, various surface active agents or any suitable combination of these. The active may also be contained in very fine microcapsules in polymeric substances. Microcapsules typically contain the active material enclosed in an inert porous shell which allows escape of the enclosed material to the surrounds at controlled rates. Encapsulated droplets are typically about 0.1 to 500 microns in diameter. The enclosed material typically constitutes about 25 to 95% of the weight of the capsule. The active ingredient may be present as a monolithic solid, as finely dispersed solid particles in either a solid or a liquid, or it may be present as a solution in a suitable solvent. Shell membrane materials include natural and synthetic rubbers, cellulosic materials, styrene- butadiene copolymers, polyacrylonitriles, polyacrylates, polyesters, polyamides, polyureas, polyurethanes, other polymers familiar to one skilled in the art, chemically- modified polymers and starch xanthates. Alternative very fine microcapsules maybe formed wherein the active ingredient is dispersed as finely divided particles within a matrix of solid material, but no shell wall surrounds the microcapsule. Suitable agricultural adjuvants and carriers that are useful in preparing the compositions of the invention are well known to those skilled in the art. Liquid carriers that can be employed include water, toluene, xylene, petroleum naphtha, crop oil, acetone, methyl ethyl ketone, cyclohexanone, acetic anhydride, acetonitrile, acetophenone, amyl acetate, 2-butanone, chlorobenzene, cyclohexane, cyclohexanol, alkyl acetates, diacetonalcohol, 1,2-dichloropropane, diethanolamine, p- diethylbenzene, diethylene glycol, diethylene glycol abietate, diethylene glycol butyl ether, diethylene glycol ethyl ether, diethylene glycol methyl ether, N,N-dimethyl formamide, dimethyl sulfoxide, 1,4-dioxane, dipropylene glycol, dipropylene glycol methyl ether, dipropyleneglycol dibenzoate, diproxitol, alkyl pyrrolidinone, ethyl acetate, 2-ethyl hexanol, ethylene carbonate, 1,1,1-trichloroethane, 2-heptanone, alpha pinene, d- limonene, ethylene glycol, ethylene glycol butyl ether, ethylene glycol methyl ether, gamma-butyrolactone, glycerol, glycerol diacetate, glycerol monoacetate, glycerol triacetate, glycerol triacetate, hexadecane, hexylene glycol, isoamyl acetate, isobornyl acetate, isooctane, isophorone, isopropyl benzene, isopropyl myristate, lactic acid, laurylamine, mesityl oxide, methoxy-propanol, methyl isoamyl ketone, methyl isobutyl ketone, methyl laurate, methyl octanoate, methyl oleate, methylene chloride, m-xylene, n-hexane, n-octylamine, octadecanoic acid, octyl amine acetate, oleic acid, oleylamine, o-xylene, phenol, polyethylene glycol (PEG400), propionic acid, propylene glycol, propylene glycol monomethyl ether, propylene glycol mono-methyl ether, p-xylene, toluene, triethyl phosphate, Methylene glycol, xylene sulfonic acid, paraffin, mineral oil, trichloroethylene, perchloroethylene, ethyl acetate, amyl acetate, butyl acetate, propylene glycol monomethyl ether and diethylene glycol monomethyl ether, methanol, ethanol, isopropanol, and higher molecular weight alcoholds such as amyl alcohol, tetrahydrofurfuryl alcohol, hexanol, octanol, etc., ethylene glycol, propylene glycol, glycerine, N-methyl-2-pyrrolidinone, and the like. Water is generally the carrier of choice for the dilution of concentrates. Suitable solid carriers include talc, titanium dioxide, pyrophyllite clay, silica, attapulgite clay, kieselguhr, chalk, diatomaceous earth, lime, calcium carbonate, bentonite clay, Fuller's earth, cotton seed hulls, wheat flour, soybean flour, pumice, wood flour, walnut shell flour, lignin, and the like. A broad range of surface-active agents are advantageously employed in both solid and liquid compositions, especially those designed to be diluted with carrier before application. The surface-active agents can be anionic, cationic, nonionic or polymeric in character and can be employed as emulsifying agents, wetting agents, suspending agents, or for other purposes. Typical surface active agents include salts of alkyl sulfates, such as diethanolammonium lauryl sulfate; alkylarylsulfonate salts, such as calcium dodecylbenzenesulfonate; alkylphenol-alkylene oxide addition products, such as nonylphenol-C.sub.18 ethoxylate; alcohol-alkylene oxide addition products, such as tridecyl alcohol-C.sub.16 ethoxylate; soaps, such as sodium stearate; alkylnaphthalenesulfonate salts, such as sodium dibutylnaphthalenesulfonate; dialkyl esters of sulfosuccinate salts, such as sodium di(2-ethylhexyl) sulfosuccinate; sorbitol esters, such as sorbitol oleate; quaternary amines, such as lauryl trimethylammonium chloride; polyethylene glycol esters of fatty acids, such as polyethylene glycol stearate; block copolymers of ethylene oxide and propylene oxide; and salts of mono and dialkyl phosphate esters. Other adjuvants commonly utilized in agricultural compositions include crystallization inhibitors, viscosity modifiers, suspending agents, spray droplet modifiers, pigments, antioxidants, foaming agents, light-blocking agents, compatibilizing agents, antifoam agents, sequestering agents, neutralizing agents and buffers, corrosion inhibitors, dyes, odorants, spreading agents, penetration aids, micronutrients, emolients, lubricants, sticking agents, dispersing agents, thickening agents, freezing point depressants, antimicrobial agents, and the like. The compositions can also contain other compatible components, for example, other herbicides, herbicide safeners, plant growth regulants, fungicides, insecticides, and the like and can be formulated with liquid fertilizers or solid, particulate fertilizer carriers such as ammonium nitrate, urea and the like. The invention will now be described in more details with reference to the following examples. All the test compounds were evaluated at three rates (x, 2x, 4x) alone and at three rates in mixture (x+y, 2x+2y, 4x+4y). After assessment the observed results were compared with the " Colby " expected results as a test for synergy using the formula: 'Expected' result for (a+b) = a + b - (ab/100), where a and b are the 'observed' results for a and b on there own (ref Colby 1967). A significant improvement in activity is claimed when: either the observed value is 5% or more above the expected value, or if the level of weed control is >90% if the observed value is 2% or more above the expected value. The results are shown in Table 1.