The present invention relates to a compound of formula (I): wherein: X is methyl or chlorine; R1 is methyl or chlorine; R2 is hydrogen, methyl, ethyl, n-propyl, cyclopropyl, vinyl, ethynyl, fluorine, chlorine, bromine, methoxy, ethoxy or fluoromethoxy; and G, R3, R4, R5 and R6 are as defined herein; wherein the compound of formula (I) is optionally present as an agrochemically acceptable salt thereof. These compounds are suitable for use as herbicides. The invention therefore also relates to a method of controlling weeds, especially grassy monocotyledonous weeds, in crops of useful plants, comprising applying a compound of formula (I), or a herbicidal composition comprising such a compound, to the plants or to the locus thereof.