Derivatives of the new class of 3-hydroxy-4-phenyl-5-oxo-pyrazolines were optimized towards both herbicidal activity on key annual grass weed species and selectivity in small grain cereal crops. The generic structure can be separated into three parts for the analysis of the structure-activity relationships, namely the aryl, the dione with its prodrug forms and the hydrazine moiety. Each area appears to play distinct and different roles in overall expression of biological performance which is further beneficially influenced by adjuvant response and safener action. Pinoxaden 6, a novel graminicide for use in wheat and barley incorporating a [1,4,5]oxadiazepane ring, eventually emerged as a development candidate from the discovery and optimization process.