Use of a compound of Formula or a salt thereof, wherein, X 1 is N or CR 1; R 1 is selected from the group consisting of hydrogen, halogen, cyano, C 1 -C 6 alkyl, C 3 -C 6 cycloalkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 1 -C 6 alkoxy, -C OC 1 -C 6 alkyl, -S p C 1 -C 6 alkyl, NR 6 R 7, C 1 -C 6 haloalkoxy and C 1 -C 6 haloalkyl; R 2 is selected from the group consisting of halogen, cyano, nitro, C 1 -C 6 alkyl, C 1 -C 6 haloalkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 6 cycloalkyl, -C OC 1 -C 6 alkyl, -S p (C 1 -C 6 alkyl), C 1 -C 6 alkoxy, C 1 -C 6 haloalkoxy and phenyl; R 3 is -C X 2 R 12; X 2 is O or NR 10; when X 2 is O, R 12 is selected from the group consisting of C 1 -C 6 alkyl, C r alkoxyC s alkyl, C 1 -C 6 haloalkyl, CralkoxyCshaloalkyl, CralkylthioC s alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, and - (CR a R b)qR 11; when X 2 is NR 10, R 12 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 1 -C 6 alkoxy, C 1 -C 6 haloalkyl, C 1 -C 6 haloalkoxy, CralkylthioC s alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, and -(CR a R b) q R 11; R 10 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 3 -C 6 cycloalkyl; or, R 10 and R 12 together with the nitrogen atom to which they are joined, can form a 5-, 6-, or 7-membered ring, optionally containing 1 to 3 additional heteroatoms each independently selected from O, N or S, wherein when said ring contains a ring sulphur said ring sulphur is in the form S p; R 4 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 1 -C 6 alkoxy, C 1 -C 6 haloalkyl, C 1 -C 6 haloalkoxy, C 3 -C 6 cyloalkyl, C 3 -C 6 alkenyl, C 3 -C 6 alkynyl, -C R 9 and - (CR a R b) q R 5; R a is hydrogen or C 1 -C 2 alkyl; R b is hydrogen or C 1 -C 2 alkyl; R 5 is cyano, -C OC 1 -C 6 alkyl, -C 3 -C 6 cycloalkyl, -aryl or -heteroaryl wherein said aryl and heteroaryl are optionally substituted by 1 to 3 independent R 8; R 6 and R 7 are independently selected from the group consisting of hydrogen and C 1 -C 6 alkyl; each R 8 is independently selected from the group consisting of halogen, C 1 -C 6 alkyl and C 1 -C 6 alkoxy-, C 1 -C 6 haloalkyl, C 1 -C 6 haloalkoxy-, cyano and S p (C 1 -C 6 alkyl); R 9 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 1 -C 6 alkoxy, C 1 -C 6 haloalkyl, C 1 -C 6 haloalkoxy, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, and -(CR a R b) q R 11; or R 4 and R 10 together with the atoms to which they are joined form a 5-7 membered ring system optionally containing from 1 to 3 additional heteroatoms independently selected from S, O and N; or R 4 and R 12 together with the atoms to which they are joined form a 5-7 membered ring system optionally containing from 1 to 3 additional heteroatoms independently selected from S, O and N; R 11 is cyano, -C 3 -C 6 cycloalkyl, or an -aryl, -heteroaryl or -heterocyclyl ring, wherein said ring is optionally substituted by 1 to 3 independent R 8, and wherein when said ring contains a ring sulphur, said ring sulphur is in the form S p; n is 0 or 1; p is 0, 1, or 2; q is 0, 1, 2, 3, 4, 5 or 6; r is 1, 2, 3, 4, or 5, s is 1, 2, 3, 4, or 5, and the sum of r+s is less than or equal to 6, as a herbicide, with the proviso that the compound of Formula is not (i) tert -butyl N-[2-methyl-6-(3-pyridyl)-3-pyridyl]carbamate. (i) tert -butyl N-[2-methyl-6-(3-pyridyl)-3-pyridyl]carbamate.