Growth Dynamics and Nutrient Uptake Modeling in Cut Sunflower for Precision Nutrition

Specific information on the nutritional management of cut sunflower (Helianthus annuus L.) is scarce, particularly considering its biomass dynamics, which differ from grain varieties by prioritizing inflorescence development and stem support. This study aimed to determine dry matter (DM) accumulation and the dynamics of macronutrient and micronutrient uptake in the aboveground biomass of the cultivar ‘Vincent’s Choice’. A field experiment was conducted in a randomized block design, with seven destructive samplings performed between 8 and 49 days after emergence (DAE; R5 stage—commercial harvest). Data were fitted to a three-coefficient Gaussian model to identify the Inflection Point (IP), which indicates the maximum daily uptake rate. The estimated maximum DM accumulation in the whole plant was 53.89 g plant−1, with the stem surpassing the leaves as the main structural sink from 36 DAE onwards. The sequence of total nutrient accumulation at harvest was: K > N > Ca > Mg > S > P > Mn > Zn > B > Cu. Potassium (2.36 g plant−1) and nitrogen (1.10 g plant−1) were the most demanded nutrients, showing synchronized IPs between 32 and 34 DAE. Nutrient remobilization from vegetative tissues to the capitulum was observed during the final stages. In conclusion, the critical window for nutritional demand occurs between 22 and 43 DAE. Modeling the accumulation dynamics of dry matter and of the ten nutrients (N, P, K, Ca, Mg, S, B, Cu, Mn, Zn), together with their inflection points, provides a technical basis for precision fertilization programs, enabling the synchronization of nutrient supply with actual crop requirements.