A method of selecting a polynucleotide which encodes a triketone herbicide resistant HPPD enzyme comprising screening a population of HPPD enzyme encoding sequences and selecting as those which encode a triketone resistant HPPD enzyme those sequences which encode an enzyme which in comparison with a control HPPD enzyme has either at least 2.5 or, preferably, four-fold increased resistance to triketone herbicides selected from herbicides having one of the following formulas and the agronomically acceptable salts thereof, where Ar groups A, B, D and E are independently chosen from optionally substituted phenyl or optionally substituted heteroaryl, wherein optional substituents for the groups A, B, D and E include -C 1 -C 4 alkyl, C 3 -C 6 cycloalkyl, halogen, OH, SH, CN, -NH 2, -NHCOR, -CONHR, -COR, -SR, SOR, - SO 2 R, NHR-SO 2 R, -CO 2 R, -NO 2, CF 3, -SF 5, OR, and CO 2 H, where R=C 1 -C 6 alkyl or aryl optionally substituted with one or more substituents selected from the group consisting of halo or C 1 -C 4 alkoxy, where R 1 or R 2 or both are H and both R 3 and R 4 are H and R 5 or R 6 or both are H, where R 8 and R 9 are both H and R 10 or R 11 or both are H, where R 12 or R 13 or both are H and R 14 or R 15 or both are H, and where R 1 -R 17 are each individually selected from the group consisting of H, -C 1 -C 4 alkyl, C 3 -C 6 cycloalkyl, halogen, OH, SH, CN, -NH 2, -NHCOR, -CONHR, -COR, -SR, SOR, -SO 2 R, NHR-SO 2 R, -CO 2 R, -NO 2, CF 3,-SF 5, OR, and CO 2 H, where R=C 1 -C 6 alkyl or aryl are optionally substituted with one or more substituents selected from the group consisting of halo or C 1 -C 4 alkoxy; as compared to herbicides selected from the group of herbicides having the formula and the agronomically acceptable salts thereof, where R 1 -R 6 are each individually selected from the group consisting of H, -C 1 -C 4 alkyl, C 3 -C 6 cycloalkyl, halogen, OH, SH, CN, -NH 2, -NHCOR, -CONHR, -COR, -SR, SOR, -SO 2 R, NHR-SO 2 R, -CO 2 R, -NO 2, CF 3, -SF 5, OR, and CO 2 H, where R=C 1 -C 6 alkyl or aryl are optionally substituted with one or more substituents selected from the group consisting of halo or C 1 -C 4 alkoxy, or is at least 2.5 or preferably four-fold more resistant to triketone herbicides selected from the group of herbicides having the formula and the agronomically acceptable salts thereof, where group G is chosen from optionally substituted phenyl or optionally substituted heteroaryl, and where R 1 -R 2 are each individually selected from the group consisting of H, -C 1 -C 4 alkyl, C 3 -C 6 cycloalkyl, halogen, OH, SH, CN, -NH 2, -NHCOR, -CONHR, -COR, -SR, SOR, -SO 2 R, NHR-SO 2 R,-CO 2 R, -NO 2, CF 3, -SF 5, OR, and CO 2 H, and where R=C 1 -C 6 alkyl or aryl are optionally substituted with one or more substituents selected from the group consisting of halo or C 1 -C 4 alkoxy, as compared to herbicides having one of the following formulas and the agronomically acceptable salts thereof, where Ar groups K and J are independently chosen from optionally substituted phenyl or optionally substituted heteroaryl, wherein optional substituents for the groups K and J include -C 1 -C 4 alkyl, C 3 -C 6 cycloalkyl, halogen, OH, SH, CN, -NH 2, -NHCOR, -CONHR, -COR, -SR, SOR, -SO 2 R, NHR-SO 2 R, -CO 2 R, -NO 2, CF 3, -SF 5, OR, and CO 2 H, where R=C 1 -C 6 alkyl or aryl optionally substituted with one or more substituents selected from the group consisting of halo or C 1 -C 4 alkoxy, where R 1 -R 4 are each individually selected from the group consisting of -C 1 -C 4 alkyl, C 3 -C 6 cycloalkyl, halogen, OH, SH, CN, -NH 2, -NHCOR, -CONHR, -COR, - SR, SOR, -SO 2 R, NHR-SO 2 R, -CO 2 R, -NO 2, CF 3, -SF 5, OR, and CO 2 H, where R=C 1 -C 6 alkyl or aryl are optionally substituted with one or more substituents selected from the group consisting of halo or C 1 -C 4 alkoxy; wherein said control HPPD enzyme is derived from a dicot, particularly from Arabidopsis or tobacco; wherein the resistance of HPPD enzymes to herbicides is determined by measuring the rate of dissociation of the enzyme/herbicide complex, and; wherein the HPPD enzyme encoded by the selected polynucleotide has a k cat / K m hydroxyphenylpyruvate value in the range from 0.1 to 5 s -1 μM -1 at pH 7.0 and 25°C.