Investigation of the last nitrogen application timing and irrigation cut-off on the ripening of sugarcane IRC99-02 variety at the south of Khuzestan

Introduction
Sugarcane is a major crop grown in tropical and subtropical regions and is used for the production of sugar, renewable products, biomaterials, and ethanol. The timing of the final nitrogen application and irrigation cut-off during the ripening stage of sugarcane is critical for improving juice quality in terms of Brix, sucrose, reducing sugars, and purity. Proper water and nitrogen management is essential for achieving optimal outcomes.
 
Materials and methods
To evaluate the effects of final nitrogen application timing and irrigation cut-off on the quantitative and qualitative yield of the sugarcane variety IRC99-02, a field experiment was conducted during the 2023–2024 growing season at the Dabal Khazaei Sugarcane Agro-Industry Company using a split-plot arrangement based on a randomized complete block design with three replications. The main plots comprised four irrigation cut-off timings: September 20, October 1, October 11, and October 21. The subplots consisted of three final nitrogen application timings: June 19, July 5, and July 20. The experiment was conducted on ratoon crops of the sugarcane variety IRC99-02.
 
Results and discussion
The results showed that cane height, number of millable canes, single-cane weight, internode length, cane specific weight, and cane and sugar yield were affected by the experimental treatments. Cane height increased significantly under irrigation cut-off timings of October 1, October 11, and October 21 by 8%, 12%, and 15%, respectively, compared with the September 20 irrigation cut-off. In addition, final nitrogen application on July 5 and July 20 increased cane height by 2% and 8%, respectively, compared with June 19. The greatest cane height (288 cm) was observed under the October 21 irrigation cut‑off combined with the July 20 final nitrogen application. The number of millable canes was 130,250 under the June 19 nitrogen application, while nitrogen application on July 5 and July 20 increased the number of millable canes by 4% and 10%, respectively, compared with the June 19 treatment. The irrigation cut-off treatment on October 21 showed the highest single-cane weight (771 g). Single-cane weight was 691 g under the final nitrogen application on June 19. Nitrogen application on July 20 resulted in the highest average single-cane weight (775 g). Internode length was greatest under the July 20 final nitrogen application and the irrigation cut-off timings of October 1, October 11, and October 21, with values of 17.2, 16.8, and 17.2 cm, respectively. Cane yield increased significantly under irrigation cut-off timings of October 1, October 11, and October 21 by 5%, 8%, and 17%, respectively, compared with the September 20 irrigation cut-off. Significant differences in cane yield were observed among irrigation cut-off timings. The lowest cane yield (89.5 t ha⁻¹) was recorded under the June 19 nitrogen application treatment. Cane yield increased significantly with nitrogen application on July 5 and July 20 by 13% and 20%, respectively, compared with the June 19 application. The lowest sugar yield (9,349 kg ha⁻¹) was recorded under the September 20 irrigation cut-off treatment. In contrast, sugar yield under irrigation cut-off on October 1, October 11, and October 21 was 10,163, 10,736, and 10,755 kg ha⁻¹, respectively. The results indicated that sugarcane juice quality traits changed over time, with a significant increase in sucrose content. The results for quality traits at harvest (February) showed that the October 11 irrigation cut-off treatment had the highest recoverable sugar percentage (10.8%). The highest recoverable sugar percentage (10.75%) was observed under the July 5 nitrogen application treatment. Moreover, the highest recoverable sugar percentage was observed in the combinations of irrigation cut-off on October 1 with nitrogen application on July 5 (11.3%) and irrigation cut-off on October 11 with nitrogen application on July 5 (11.3%).
 
Conclusion
These results indicate that both early (September 20) and late (October 21) irrigation cut-off had negative effects on the recoverable sugar percentage of sugarcane juice. In addition, both early (June 19) and late (July 20) nitrogen application resulted in lower recoverable sugar than the July 5 nitrogen application treatment.
 
Acknowledgements
The authors gratefully acknowledge the financial support provided by Shahid Chamran University of Ahvaz.