Evaluation of Yield and Morphological Responses of Monoecious Hemp (Cannabis sativa L.) to Topping Across Cultivars and Years: Are There Mechanical Harvest Implications?

The increasing interest in industrial hemp (Cannabis sativa L.) as a sustainable crop has highlighted the need for agronomic strategies to improve seed production and mechanized harvesting efficiency. This study evaluated the effects of apical topping on the growth, morphology, seed yield, and plant traits relevant to the mechanical harvest suitability of three monoecious hemp cultivars (Codimono, Carmaleonte, and Futura 75) under open-field conditions in Northern Italy over three growing seasons (2023–2025). We hypothesized that apical topping modifies plant architectural traits (e.g., plant height) to enhance mechanical harvest suitability without compromising seed yield. A split-plot design (variety as main plots; topping as pub-plot) arranged within randomized blocks was applied. Results indicated that climatic variability and cultivar choice were the primary drivers of morpho-productive variation. Topping mainly affected plant height, reducing it by approximately 14% at harvest, but it did not significantly impact seed yield, impure seed fraction (ISF), or thousand-seed weight (TSW). Climatic variability among years strongly influenced crop performance, with higher yields associated with more favorable water availability during seed ripening. Among cultivars, Carmaleonte exhibited lower yield and higher impurity levels, likely related to its precocity and sensitivity during early growth stages. Correlation analysis suggested that taller and thicker plants tended to produce higher impurity levels, whereas this relationship was weaker in topped plants. Overall, while topping did not affect seed yield under the tested conditions, modifying plant height through topping provides structural adaptations of the plants (specifically height reduction) that would improve suitability for mechanical harvesting.