1. A synergistic herbicidal composition comprising a herbicidally effective amount of (a) a compound of the formula: or a n-butyl or benzyl ester of the compound of formula and (b) a protoporphyrinogen oxidase inhibitor, wherein (b) is at least one compound selected from the group consisting of: carfentrazone-ethyl, flumioxazin, oxadiargyl, oxadiazon, oxyfluorfen, pentoxazone, pyraclonil, pyraflufen-ethyl, and saflufenacil, wherein: the composition comprises the benzyl ester of the compound of formula and carfentrazone-ethyl wherein the ratio between the benzyl ester of the compound of formula and carfentrazone-ethyl is from 2.5:1 to 1:3.2; or the composition comprises the n-butyl ester of the compound of formula and carfentrazone-ethyl wherein the ratio between n-butyl ester of the compound of formula and carfentrazone-ethyl is 1:1.75; or the composition comprises the benzyl ester of the compound of formula and flumioxazin wherein the ratio between the benzyl ester of the compound of formula and flumioxazin is from 1:3 to 1:35; or the composition comprises the compound of formula and oxadiargyl wherein the ratio between the compound of the formula and oxadiargyl is from 1:3 to 1:11; or the composition comprises the benzyl ester of the compound of formula and oxadiargyl wherein the ratio between the benzyl ester of the compound of formula and oxadiargyl is from 1:5 to 1:11; or the composition comprises the benzyl ester of the compound of formula and oxadiazon wherein the ratio between the benzyl ester of the compound of formula and oxadiazon is from 1:1.8 to 1:70; or the composition comprises the n-butyl ester of the compound of formula and oxadiazon wherein the ratio between the n-butyl ester of the compound of formula and oxadiazon is from 1:3.6 to 1:14.3; or the composition comprises the benzyl ester of the compound of formula and oxyfluorfen wherein the ratio between the benzyl ester of the compound of formula and oxyfluorfen is from 1:5 to 1:82; or the composition comprises the benzyl ester of the compound of formula and pentoxazone wherein the ratio between the benzyl ester of the compound of formula and pentoxazone is from 1:6 to 1:26; or the composition comprises the compound of the formula and pyraclonil wherein the ratio between the compound of the formula and pyraclonil is from 1:3 to 1:31; or the composition comprises the benzyl ester of the compound of formula and pyraflufen-ethyl wherein the ratio between the benzyl ester of the compound of formula and pyraflufen-ethyl is from 1:5.5 to 25:1; or the composition comprises the n-butyl ester of the compound of formula and saflufenacil wherein the ratio between the n-butyl ester of the compound of formula and saflufenacil is 1:1.1. the composition comprises the benzyl ester of the compound of formula and carfentrazone-ethyl wherein the ratio between the benzyl ester of the compound of formula and carfentrazone-ethyl is from 2.5:1 to 1:3.2; or the composition comprises the n-butyl ester of the compound of formula and carfentrazone-ethyl wherein the ratio between n-butyl ester of the compound of formula and carfentrazone-ethyl is 1:1.75; or the composition comprises the benzyl ester of the compound of formula and flumioxazin wherein the ratio between the benzyl ester of the compound of formula and flumioxazin is from 1:3 to 1:35; or the composition comprises the compound of formula and oxadiargyl wherein the ratio between the compound of the formula and oxadiargyl is from 1:3 to 1:11; or the composition comprises the benzyl ester of the compound of formula and oxadiargyl wherein the ratio between the benzyl ester of the compound of formula and oxadiargyl is from 1:5 to 1:11; or the composition comprises the benzyl ester of the compound of formula and oxadiazon wherein the ratio between the benzyl ester of the compound of formula and oxadiazon is from 1:1.8 to 1:70; or the composition comprises the n-butyl ester of the compound of formula and oxadiazon wherein the ratio between the n-butyl ester of the compound of formula and oxadiazon is from 1:3.6 to 1:14.3; or the composition comprises the benzyl ester of the compound of formula and oxyfluorfen wherein the ratio between the benzyl ester of the compound of formula and oxyfluorfen is from 1:5 to 1:82; or the composition comprises the benzyl ester of the compound of formula and pentoxazone wherein the ratio between the benzyl ester of the compound of formula and pentoxazone is from 1:6 to 1:26; or the composition comprises the compound of the formula and pyraclonil wherein the ratio between the compound of the formula and pyraclonil is from 1:3 to 1:31; or the composition comprises the benzyl ester of the compound of formula and pyraflufen-ethyl wherein the ratio between the benzyl ester of the compound of formula and pyraflufen-ethyl is from 1:5.5 to 25:1; or the composition comprises the n-butyl ester of the compound of formula and saflufenacil wherein the ratio between the n-butyl ester of the compound of formula and saflufenacil is 1:1.1.