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The relevance of the study is due to the problem of choosing the optimal scale for identifying the phases of plant development in order to determine the best timing of agronomic operations for sorghum plant care. The aim of the experiment was to use a plant growth regulator with higher efficiency during the stages of crop development 21, 31, 37 (BBCH) and III, IV, VI-VII on the Kupperman scale. Field and laboratory research methods were used in the study. The experiment was conducted in the Forest-Steppe zone of Ukraine on two sorghum varieties - Sorghum bicolor and Sorghum saccharatum, which were treated with PGR. Foliar application of PGR (0.5 l/ha) at 21 and 31 stages of BBSH provides 2.8 and 4.9% better results than at III and IV stages according to the Kuperman scale (the latter is based on a comprehensive morphophysiological analysis to identify I-VII CGS). Foliar application of PGR at microstage 21 (BBCH) contributed to faster development and increased grain yield in sorghum varieties (0.19 t/ha in Odesskyi 205 and 0.12 t/ha in Lan 59) compared to application at stage III (according to Cooperman). A similar effect of PGR application at microstage 21 (BBCH) compared to stage III (Kuperman) was recorded for two sugar sorghum hybrids: in Dovista variety, the increase in biomass yield was 1.6 t/ha, dry matter - 0.7 t/ha, and sugar content in stem juice - 0.0%, while in Gulliver variety these indicators were 1.6 t/ha, 0.7 t/ha and 0.2%, respectively. Thus, it is recommended to use the BBCH scale for the application of PPP, and foliar fertilization with PPP should be carried out at 21 and 31 days. The practical value of the work lies in the selection of the scale of growth and development of sorghum crops and the timing of foliar fertilization, which is optimal for the conditions of Ukraine. The study is useful from the practical point of view of foliar nutrition of plants in production and as a theoretical assessment of the scale of plant growth and development for students and scientists.