Post‑flowering dynamics of whole-plant seed weight and nutritional composition for determining reference harvest window in industrial hemp (Cannabis sativa L.)

Industrial hemp ( Cannabis sativa L.) seeds are increasingly valued for their nutritional composition, but asynchronous maturation and seed shattering make harvest timing for maximum seed yield. In this study, four cultivars originating from contrasting latitudes were grown in Yunnan, China, in 2023 and 2024 to investigate post-flowering changes in whole-plant seed weight and nutritional composition, and to estimate a reference harvest window for maximum seed yield. Compared with high-latitude cultivars, low-latitude cultivars had longer vegetative periods, taller plants, thicker stems, and greater stem and leaf biomass. After flowering, actual harvestable seed weight and seed number retained per plant initially increased and then declined in all cultivars, whereas thousand-seed weight increased continuously without a clear plateau. Beta models based on growing degree days (GDDs) and days after flowering (DAF) revealed that fitted maximum harvestable seed weight occurred at 211.60–501.87 GDDs and 35.24–48.17 DAF, with coefficients of variation of 31.26% and 9.51%, respectively. Analysis of the seeds retained on the sampled plants showed that protein content was initially higher than oil content, but rapid oil accumulation later reversed this relationship. Five major fatty acids were identified: palmitic, stearic, oleic, linoleic, and linolenic acids. Across cultivars and post-flowering stages, the proportions of linoleic and stearic acids ranged within 54.11–62.65% and 2.40–3.65%, respectively. The proportions of α-linolenic and oleic acids remained relatively stable over time but differed among cultivars, whereas that of palmitic acid declined from 9.92–17.83% to 6.44–9.13% over time. These results suggest 36–49 DAF as a reference harvest window for maximum harvestable seed weight under the conditions evaluated, although thousand-seed weight and oil content may still increase slightly.