Beyond fixed pre-harvest intervals: a statistical analysis supporting BBCH-based pesticide late application timing in cereals in Europe

Abstract Plant protection product applications in cereals require careful timing to ensure food safety while maintaining efficacy. European regulatory frameworks employ 2 approaches for flowering applications: (a) fixing a growth stage for the last application (BBCH scale) or (b) establishing a fixed pre-harvest interval (PHI), expressed as days before harvest. Understanding the relationship between these approaches is crucial for designing effective supervised residue trials and ensuring reliable residue assessments. This case study uses residue trial data from fungicide applications at growth stages BBCH 61 and 69 in barley and wheat respectively. We evaluated how well-fixed PHI requirements align with natural crop development patterns across European cereal production regions. Results reveal a significant mismatch between fixed PHI of 35 days (window acceptance 26–44 days) and biological crop development, with only 34–50% of trials achieving regulatory compliance depending on crop and region. This biological-temporal disconnect necessitates conducting 15–35 trials to achieve a complete residue package of eight valid trials with reasonable confidence. When crops develop more slowly than prescribed PHIs, stakeholders face the dilemma of collecting immature samples (contradicting the “as low as reasonably achievable” principle) or risking trial invalidation. The consistency of these patterns across Europe indicates the challenge stems from imposing fixed intervals on inherently variable biological processes rather than regional variations. Based on these scientific findings, we propose transitioning to purely BBCH-based label instructions for flowering cereal applications to better align with agronomic reality, improve farmer compliance, and provide more accurate dietary exposure assessments for consumers.