A compound of the formula ##STR1## wherein R.sub.1 is selected from the group consisting of hydrogen, lower alkyl, phenyl and phenyl substituted with halogen or halogenated methyl; R.sub.2 is selected from the group consisting of hydrogen, lower alkyl and lower alkenyloxy; R.sub.3 is selected from the group consisting of lower alkyl and phenyl substituted with halogen; R.sub.4 is selected from the group consisting of lower alkyl, lower alkenyl and benzyl; Or a metal salt of the compound defined herein above; is useful as herbicide.