Ascophyllum nodosum Extract Promotes Vegetative Growth and Sugar Yield in Ratoon Sugarcane

Maintaining the productivity and technological quality of sugarcane during ratoon cycles represents a constant challenge for the sugarcane-energy sector, driving the adoption of biostimulants. This study aimed to evaluate and compare the effects of different biostimulants on the biometric development, yield, and technological quality of second-ratoon sugarcane. The experiment was conducted in a randomized complete block design with four treatments and four replications: (1) Control; (2) Ascophyllum nodosum; (3) A. nodosum + nutrients; and (4) Arbuscular mycorrhizal fungus (Rhizophagus irregularis). Tiller density, stalk height, stalk diameter, stalk weight, productivity, total recoverable sugars (TRS), and sugar yield were evaluated. Data were subjected to analysis of variance (Tukey’s test, p ≤ 0.05) and principal component analysis (PCA). Tiller density was not affected by the treatments; however, biostimulation with A. nodosum alone proved to be the most effective treatment, yielding the highest stalk height (2.35 m), stalk weight (1.48 kg), TRS (146.06 kg t−1), and sugar yield (14.52 t ha−1), representing a 19.5% increase in sugar yield compared to the control. Inoculation with R. irregularis matched A. nodosum in productivity but resulted in lower TRS, indicating a sucrose dilution effect. A. nodosum combined with nutrients did not surpass the control across production variables. PCA confirmed the distinct separation and superiority of the isolated A. nodosum treatment along the growth and productivity axis, further demonstrating that these inputs operate through distinct physiological mechanisms. In conclusion, the A. nodosum-based biostimulant (applied alone) constitutes the most promising strategy to simultaneously enhance yield and technological quality in sugarcane ratoon crops.