The present invention relates to the compounds of formula (I), or their agriculturally acceptable salts or derivatives as herbicides, wherein the variables are defined according to the description, use of compounds of formula (I) as herbicides, compositions comprising them and their use as herbicides, i.e. for controlling harmful plants, and also a method for controlling unwanted vegetation which comprises allowing a herbicidal effective amount of at least one pyrimidine compound of the formula (I) to act on plants, their seed and/or their habitat.