Use of a compound of formula (I) 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(O)OC 1 -C 6 alkyl, -S(O) p C 1 -C 6 alkyl, NR 6 R 7 , C 1 -C 6 haloalkoxy and C 1 -C 6 haloalkyl; R 2 is methyl or trifluoromethyl; R 3 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 2 -C 6 haloalkenyl, C 1 -C 6 alkoxyC 1 -C 3 alkyl-, C 1 -C 6 haloalkyl- 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 4 is hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 2 -C 6 haloalkenyl, C 1 -C 6 alkoxyC 1 -C 3 alkyl-, C 1 -C 6 haloalkyl- and -(CR a R b ) q R 5 ; R 5 is -C(O)OC 1 -C 6 alkyl, -C 3 -C 10 cycloalkyl, -aryl, or-heteroaryl wherein said aryl and heteroaryl are optionally substituted by 1 to 3 independent R 8 ; or R 3 and R 4 together with the nitrogen to which they are attached, form a saturated or partially unsaturated 4-6 membered ring system optionally containing 1 or 2 further heteroatoms independently selected from S in the form S(O) p , O and N, wherein said ring is optionally substituted by 1 to 3 R 8 ; R 6 and R 7 are independently selected from the group consisting of hydrogen, C 1 -C 6 alkyl and -C(O)OC 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(O) p (C 1 -C 6 alkyl); n is 0 or 1; p is 0, 1, or 2; and q is 0, 1, or 2, and when q is 0, R 5 is not -C(O)OC 1 -C 6 alkyl; as a herbicide. 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(O)OC 1 -C 6 alkyl, -S(O) p C 1 -C 6 alkyl, NR 6 R 7 , C 1 -C 6 haloalkoxy and C 1 -C 6 haloalkyl; R 2 is methyl or trifluoromethyl; R 3 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 2 -C 6 haloalkenyl, C 1 -C 6 alkoxyC 1 -C 3 alkyl-, C 1 -C 6 haloalkyl- 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 4 is hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 2 -C 6 haloalkenyl, C 1 -C 6 alkoxyC 1 -C 3 alkyl-, C 1 -C 6 haloalkyl- and -(CR a R b ) q R 5 ; R 5 is -C(O)OC 1 -C 6 alkyl, -C 3 -C 10 cycloalkyl, -aryl, or-heteroaryl wherein said aryl and heteroaryl are optionally substituted by 1 to 3 independent R 8 ; or R 3 and R 4 together with the nitrogen to which they are attached, form a saturated or partially unsaturated 4-6 membered ring system optionally containing 1 or 2 further heteroatoms independently selected from S in the form S(O) p , O and N, wherein said ring is optionally substituted by 1 to 3 R 8 ; R 6 and R 7 are independently selected from the group consisting of hydrogen, C 1 -C 6 alkyl and -C(O)OC 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(O) p (C 1 -C 6 alkyl); n is 0 or 1; p is 0, 1, or 2; and q is 0, 1, or 2, and when q is 0, R 5 is not -C(O)OC 1 -C 6 alkyl; as a herbicide. <EOS>