Yield response and nitrogen use efficiency of safflower to nitrogen variation under fall and spring planting

Optimizing nitrogen (N) application rates to maximize safflower ( Carthamus tinctorius L.) seed yield and oil productivity remains a critical agronomic challenge, particularly for intermediate cultivars cultivated under variable seasonal conditions. A two-season field experiment was conducted in 2021–2022 to assess the growth, yield, and nitrogen use efficiency of two safflower cultivars (Parnian and Goldasht) in response to different nitrogen rates under spring and fall planting conditions. The experiment followed a randomized complete block design with a factorial arrangement of treatments and three replications. Nitrogen was applied at six rates (0, 45, 90, 135, 180, and 225 kg N ha⁻¹) to both cultivars in both seasons. Analysis of variance revealed that both growing season and N application rate significantly affected all measured traits, while cultivar significantly influenced biomass, seed number per m −2 , seed yield, and agronomic N use efficiency (ANUE). Across treatments, fall planting increased mean seed and oil yields by 14.9 % and 28.3 %, respectively, compared with spring planting. Nitrogen application markedly enhanced seed and oil yields compared to the no-nitrogen treatment, with increases of up to 137.3 % and 142.2 % in fall, and 130.2 % and 115.8 % in spring, respectively. ANUE improved with increasing N rates up to 135 kg ha⁻¹, and the Parnian cultivar consistently showed superior ANUE and yield performance compared to Goldasht. In summary, integrating the high-yielding Parnian cultivar, fall planting, and 180 kg N ha⁻¹ constitutes the optimal strategy to maximize safflower yield and oil content under comparable agro-climatic conditions while ensuring sustainable nitrogen use. • Significant variations exist between safflower cultivars in seed yield and agronomic nitrogen use efficiency (ANUE). • Increasing N application positively influenced seed yield, reaching an optimal at 180 kg N ha −1 . • Nitrogen fertilization enhances safflower seed and oil yields more effectively in the fall season compared with the spring. • ANUE improved with increasing N application rates up to 135 kg ha −1 .