Composiciones activas como herbicidas, que comprenden 3-[5-(difluorometoxi)-1-metil-3-(trifluorometil)pirazol-4-ilmetilsulfonil]-4,5-dihidro-5,5-dimetil-1,2oxazol [nombre comun piroxasulfona] y por lo menos un herbicida B que es seleccionado entre inhibidores de la protoporfirinogeno-IX-oxidasa. Además el uso de una composicion tal como se define en la presente para controlar vegetacion indeseable en los cultivos. Cuando se usan las composiciones para ese fin, el herbicida A y el herbicida B, que es por lo menos uno, pueden ser aplicados simultánea o sucesivamente en cultivos en los cuales pudiese surgir la vegetacion indeseable. Reivindicacion 2: La composicion de acuerdo con la reivindicacion 1, caracterizada porque el herbicida B contiene por lo menos un compuesto de la formula (1) en la que R1 es seleccionado entre el grupo compuesto por los radicales propargiloxi, aliloxi, isopropiloxi, C(=O)NHSO2NR1aR1b, C(=O)N-NR1aR1b, O-CR1aR1c-C(=O)-OR1e, C(=O)OR1aR1c-C(=O)-OR1e, C(=O)O-R1b, C(=O)O-CHR1c, C(=O)NHSO2NR1aR1b, NHSO2NR1aR1b, SO2NHc(=o)NR1aR1b, CH2-CH(Cl)CO2-R1d y el radical de la formula OC(CH3)2-C(=O)-OR1e en el que R1a representa hidrogeno o alquilo C1-4; R1b representa alquilo C1-4; R1c representa hidrogeno o alquilo C1-4; R1d representa hidrogeno o alquilo C1-4 o un cation aceptable en agricultura; y R1e representa alquilo C1-4, propargilo o alilo; R2 representa hidrogeno, fluor o cloro; R3 representa hidrogeno o bien forma, junto con R1 una fraccion -O-C(R3a)=N- o bien -N=C(R3b)-NH- en el que R3a y R3b son seleccionados, independientemente el uno del otro, entre hidrogeno, alquilo C1-6, haloalquilo C1-6, alquenilo C3-6, haloalquenilo C3-6, alquilinilo C3-6, haloalquilinilo C3-6, alcoxi C1-6, cicloalquilo C3-6 y alquilo C1-2 sustituido por un radical cicloalquilo C3-6; R4 representa halogeno o ciano; y Het representa un radical de las formulas (A) o (B), en las que R5 es seleccionado entre hidrogeno, amino, metilo o propargilo; R6 es seleccionado entre difluorometoxi, trifluorometilo y metilsulfonilo; R7 es seleccionado entre halogeno o metilo. Reivindicacion 11: La composicion de acuerdo con la reivindicacion 1 caracterizada porque el herbicida B contiene por lo menos un compuesto de la formula (2) en la que X representa O o NR11; R8 es seleccionado entre el grupo compuesto por propargiloxi, aliloxi, isopropiloxi, el radical de la formula CH2-CH(Cl)CO2-R12 y el radical de la formula NH-SO2-CH3; R9 representa fluor o cloro; R10 representa CH3, ter-butilo; R11 representa CHF2, o junto con R10 puede formar 1,4-butanodiilo; R12 representa hidrogeno, alquilo C1-6 o un cation aceptable en agricultura. Reivindicacion 13: La composicion de acuerdo con reivindicacion 1 caracterizada porque el herbicida B contiene por lo menos un compuesto de la formula (3) en la que R13 representa hidrogeno, fluor o cloro; R14 es seleccionado entre el grupo compuesto por propargiloxi, aliloxi, 1-metil-2-propiniloxi, O-CH2CO2-R16, CH=C(Cl)CO2R16 e isopropiloxi; R15 representa fluor o cloro; o R14 y R15 forman conjuntamente una fraccion O-CH2-C(=O)-NR17, en la que R17 representa un radical propargilo y en la que el átomo de oxígeno está en posicion meta respecto a la posicion de R13; R16 representa hidrogeno alquilo C1-6 o un cation aceptable en agricultura. Reivindicacion 15: La composicion de acuerdo con la reivindicacion 1, caracterizada porque el compuesto herbicida B contiene por lo menos un compuesto de la formula (4) en la que R18 representa cloro o trifluorometilo; R19 es seleccionado entre el grupo compuesto por hidrogeno, alcoxi C1-4, CO2-R21, C(=O)O-CH2CO2-R21, C(=O)O-CH(CH3)CO2-R21, C(=O)NH-SO2-R22; R20 representa hidrogeno, fluor o cloro; R21 representa hidrogeno, alquilo C1-6 o un cation aceptable en agricultura; y R22 representa alquilo C1-4. Reivindicacion 18: La composicion de acuerdo con la reivindicacion 1, caracterizada porque el herbicida B contiene por lo menos un compuesto de la formula (5) en la que R22 representa hidrogeno, fluor o cloro; R24 es seleccionado entre hidrogeno, alquilo C1-6, haloalquilo C1-6, alquenilo C3-6, haloalquenilo C3-6, alquinilo C3-6, haloalquinilo C3-6, alcoxi C1-6, cicloalquilo C3-6 y alquilo C1-2 sustituido por un radical cicloalquilo C3-6; R25 es seleccionado entre hidrogeno. amino, metilo y propargilo; R26 es seleccionado entre hidrogeno y metilo; e Y representa O o S. Reivindicacion 20: La composicion de acuerdo con la reivindicacion 1, caracterizada porque el herbicida B contiene por lo menos un compuesto de la formula (6) en la que m representa 0, 1, 2 o 3; n representa 0, 1, 2, 3 o 4; R27 es seleccionado entre alquilo C1-6, haloalquilo C1-6, alquenilo C3-6, haloalquenilo C3-6, alquilinilo C3-6, haloalquilinilo C3-6, cicloalquilo C3-6, alquilo C1-4 sustituido por un radical alcoxi C1-4, y alquilo C1-2 sustituido por un radical cicloalquilo C3-6; R28 es seleccionado entre halogeno, alquilo C1-3, haloalquilo C1-3, alcoxi C1-3 y alquilo C1-2 sustituido por un radical alcoxi C1-3, siendo posible, cuando m = 2 o 3, que los radicales R28 sean idénticos o diferentes unos de otros; R29 es seleccionado entre halogeno, ciano, nitro, alquilo C1-4, haloalquilo C1-4, alquil C1-4-sulfonilo, alcoxi C1-4, alquilo C1-4 sustituido por un radical alcoxi C1-4, alquilo C1-2 sustituido por un radical cicloalquilo C3-6 y un radical CO2R30, siendo posible, cuando n = 2, 3 o 4, que los radicales R29 sean idénticos o diferentes unos de otros; R30 es seleccionado entre alquilo C1-6, haloalquilo C1-6, alquenilo C3-6, haloalquenilo C3-6, alquinilo C3-6, haloalquinilo C3-6, cicloalquilo C3-6, alquilo C1-4 sustituido por un radical alcoxi C1-4 y alquilo C1-2 sustituido por un radical cicloalquilo C3-6.Active compositions as herbicides, comprising 3- [5- (difluoromethoxy) -1-methyl-3- (trifluoromethyl) pyrazol-4-ylmethylsulfonyl] -4,5-dihydro-5,5-dimethyl-1,2oxazole [common name pyroxasulfone] and at least one herbicide B that is selected from protoporphyrinogen-IX-oxidase inhibitors. In addition the use of a composition as defined herein to control undesirable vegetation in crops. When the compositions are used for that purpose, herbicide A and herbicide B, which is at least one, can be applied simultaneously or successively in crops in which undesirable vegetation may arise. Claim 2: The composition according to claim 1, characterized in that the herbicide B contains at least one compound of the formula (1) in which R1 is selected from the group consisting of the propargiloxy, allyloxy, isopropyloxy radicals, C ( = O) NHSO2NR1aR1b, C (= O) N-NR1aR1b, O-CR1aR1c-C (= O) -OR1e, C (= O) OR1aR1c-C (= O) -OR1e, C (= O) O-R1b, C (= O) O-CHR1c, C (= O) NHSO2NR1aR1b, NHSO2NR1aR1b, SO2NHc (= o) NR1aR1b, CH2-CH (Cl) CO2-R1d and the radical of the formula OC (CH3) 2-C (= O ) -OR1e in which R1a represents hydrogen or C1-4 alkyl; R1b represents C1-4 alkyl; R1c represents hydrogen or C1-4 alkyl; R1d represents hydrogen or C1-4 alkyl or an acceptable cation in agriculture; and R1e represents C1-4 alkyl, propargyl or allyl; R2 represents hydrogen, fluorine or chlorine; R3 represents hydrogen or forms, together with R1 a fraction -OC (R3a) = N- or -N = C (R3b) -NH- in which R3a and R3b are independently selected from each other, from hydrogen, C 1-6 alkyl, C 1-6 haloalkyl, C 3-6 alkenyl, C 3-6 haloalkenyl, C 3-6 alkynyl, C 3-6 haloalkynyl, C 1-6 alkoxy, C 3-6 cycloalkyl and C 1-2 alkyl substituted by a C3 cycloalkyl radical -6; R4 represents halogen or cyano; and Het represents a radical of formulas (A) or (B), in which R5 is selected from hydrogen, amino, methyl or propargyl; R6 is selected from difluoromethoxy, trifluoromethyl and methylsulfonyl; R7 is selected from halogen or methyl. Claim 11: The composition according to claim 1 characterized in that the herbicide B contains at least one compound of the formula (2) in which X represents O or NR11; R8 is selected from the group consisting of propargiloxy, allyloxy, isopropyloxy, the radical of the formula CH2-CH (Cl) CO2-R12 and the radical of the formula NH-SO2-CH3; R9 represents fluorine or chlorine; R10 represents CH3, tert-butyl; R11 represents CHF2, or together with R10 can form 1,4-butanediyl; R12 represents hydrogen, C1-6 alkyl or an acceptable cation in agriculture. Claim 13: The composition according to claim 1 characterized in that the herbicide B contains at least one compound of the formula (3) in which R13 represents hydrogen, fluorine or chlorine; R14 is selected from the group consisting of propargiloxy, allyloxy, 1-methyl-2-propynyloxy, O-CH2CO2-R16, CH = C (Cl) CO2R16 and isopropyloxy; R15 represents fluorine or chlorine; or R14 and R15 together form a fraction O-CH2-C (= O) -NR17, in which R17 represents a propargyl radical and in which the oxygen atom is in a meta position relative to the position of R13; R16 represents C1-6 alkyl hydrogen or an acceptable cation in agriculture. Claim 15: The composition according to claim 1, characterized in that the herbicidal compound B contains at least one compound of the formula (4) in which R18 represents chlorine or trifluoromethyl; R19 is selected from the group consisting of hydrogen, C1-4 alkoxy, CO2-R21, C (= O) O-CH2CO2-R21, C (= O) O-CH (CH3) CO2-R21, C (= O) NH-SO2-R22; R20 represents hydrogen, fluorine or chlorine; R21 represents hydrogen, C1-6 alkyl or an acceptable cation in agriculture; and R22 represents C1-4 alkyl. Claim 18: The composition according to claim 1, characterized in that the herbicide B contains at least one compound of the formula (5) in which R22 represents hydrogen, fluorine or chlorine; R24 is selected from hydrogen, C1-6 alkyl, C1-6 haloalkyl, C3-6 alkenyl, C3-6 haloalkenyl, C3-6 alkynyl, C3-6 haloalkynyl, C1-6 alkoxy, C3-6 cycloalkyl and C1-2 alkyl substituted by a C3-6 cycloalkyl radical; R25 is selected from hydrogen. amino, methyl and propargyl; R26 is selected from hydrogen and methyl; and Y represents O or S. Claim 20: The composition according to claim 1, characterized in that the herbicide B contains at least one compound of the formula (6) in which m represents 0, 1, 2 or 3; n represents 0, 1, 2, 3 or 4; R27 is selected from C1-6 alkyl, C1-6 haloalkyl, C3-6 alkenyl, C3-6 haloalkenyl, C3-6 alkynyl, C3-6 haloalkynyl, C3-6 cycloalkyl, C1-4 alkyl substituted by a C1- alkoxy radical 4, and C1-2 alkyl substituted by a C3-6 cycloalkyl radical; R28 is selected from halogen, C1-3 alkyl, C1-3 haloalkyl, C1-3 alkoxy and C1-2 alkyl substituted by a C1-3 alkoxy radical, being possible, when m = 2 or 3, that the radicals R28 are identical or different from each other; R29 is selected from halogen, cyano, nitro, C1-4 alkyl, C1-4 haloalkyl, C1-4 alkyl sulfonyl, C1-4 alkoxy, C1-4 alkyl substituted by a C1-4 alkoxy radical, substituted C1-2 alkyl by a C3-6 cycloalkyl radical and a CO2R30 radical, being possible, when n = 2, 3 or 4, that the radicals R29 are identical or different from each other; R30 is selected from C1-6 alkyl, C1-6 haloalkyl, C3-6 alkenyl, C3-6 haloalkenyl, C3-6 alkynyl, C3-6 haloalkynyl, C3-6 cycloalkyl, C1-4 alkyl substituted by a C1- alkoxy radical 4 and C1-2 alkyl substituted by a C3-6 cycloalkyl radical.