A method for controlling weeds comprising: providing a seed capable of producing a Brassica plant comprising a mutagenized nucleic acid molecule encoding a Brassica napus AHASL III protein comprising an alanine to threonine substitution at the position corresponding to position A122 of SEQ ID NO:23 and a serine to asparagine substitution at the position corresponding to position S653 of SEQ ID NO:23, said AHASL protein being resistant to inhibition by an imidazolinone herbicide, wherein said AHASL protein demonstrates a synergistic level of resistance to inhibition by an imidazolinone herbicide as compared to the additive levels of a Brassica napus AHASL III protein having the single substitution A122T and a Brassica napus AHASL III protein having the single substitution S653N; and contacting the seed with an imidazolinone herbicide before sowing and/or after pregermination; provided that the nucleic acid molecule is not SEQ ID NO:29 of WO2008124495 containing mutation A90T and fused at base 160 of that sequence to the vector tag and translational start site of pTrcHisA; and provided that the nucleic acid molecule is not SEQ ID NO:29 of WO2008124495 containing mutation A90T, comprised in a plant transformation vector; and provided that the nucleic acid molecule is not SEQ ID NO:33 of WO2008124495 containing mutation A122T, comprised in the bacterial expression vector pKK233-2 (Genbank Accession X70478); and provided that the nucleic acid molecule is not SEQ ID NO:34 of WO2008124495 containing mutation A122T, comprised in a plant transformation vector; and provided that the nucleic acid molecule is not pZm UBI + I::c-ZmAHAS L2::t-ZmAHAS L2 of WO2008124495 containing mutations A122T and S653N. providing a seed capable of producing a Brassica plant comprising a mutagenized nucleic acid molecule encoding a Brassica napus AHASL III protein comprising an alanine to threonine substitution at the position corresponding to position A122 of SEQ ID NO:23 and a serine to asparagine substitution at the position corresponding to position S653 of SEQ ID NO:23, said AHASL protein being resistant to inhibition by an imidazolinone herbicide, wherein said AHASL protein demonstrates a synergistic level of resistance to inhibition by an imidazolinone herbicide as compared to the additive levels of a Brassica napus AHASL III protein having the single substitution A122T and a Brassica napus AHASL III protein having the single substitution S653N; and contacting the seed with an imidazolinone herbicide before sowing and/or after pregermination; provided that the nucleic acid molecule is not SEQ ID NO:29 of WO2008124495 containing mutation A90T and fused at base 160 of that sequence to the vector tag and translational start site of pTrcHisA; and provided that the nucleic acid molecule is not SEQ ID NO:29 of WO2008124495 containing mutation A90T, comprised in a plant transformation vector; and provided that the nucleic acid molecule is not SEQ ID NO:33 of WO2008124495 containing mutation A122T, comprised in the bacterial expression vector pKK233-2 (Genbank Accession X70478); and provided that the nucleic acid molecule is not SEQ ID NO:34 of WO2008124495 containing mutation A122T, comprised in a plant transformation vector; and provided that the nucleic acid molecule is not pZm UBI + I::c-ZmAHAS L2::t-ZmAHAS L2 of WO2008124495 containing mutations A122T and S653N.