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In order to calibration of nitrogen requirement of dryland wheat in hot regions of the Iran, this study was conducted in four provinces: Golestan (Gonbad), Ardabil (Moghan), Kohgiluyeh and Boyer-Ahmad (Gachsaran), and Lorestan (Kuhdasht and Dora Chegini) under the conditions of farmers' fields in the 2022-2025 crop years.Four experimental treatments were designed for each region, considering the precipitation and crop potential in that area: Experimental treatments include: 1) application of the total recommended urea in fall application (as placement), 2) split application of urea (on average, 70% of the urea from the first treatment at planting time + 30% as topdressing at the first suitable condition), 3) on average, application of 75% of the urea from the first treatment in fall application + foliar application of urea 3% concentration at early stem elongation stage, and 4) regional control. The rotation system, tillage, and the most suitable rainfed cultivar recommended for each region were selected. Wheat was planted in rows at the recommended regional density using a deep tiller has fertilizer placement system. All predicted fertilizers for planting time (especially urea), were applied at planting time in a band and placed. Phosphorus, based on a deficiency below the critical level of 10 mg/kg, was applied at planting from a triple superphosphate. Urea foliar application with of 3% concentration at early tillering was applied at a volume of 500 l/ha. Results showed that applying high amounts of urea, 100 and 150 kg/ha, in fall without placement led to the non-germination of a large number of seeds and a significant reduced grain yield. In areas where high amounts of topdressing (more than 30 kg/ha of urea) were applied without recommended principles (effective and continuous rainfall), thousand-kernel weight (TKW) and grain yield significantly decreased. Two-stage application of urea as topdressing, each at 50 kg/ha (in addition to 50 kg/ha applied at planting), led to severe lodging in the field and a significant reduced in yield. In contrast, no lodging was observed with fall urea applications, even up to 150 kg/ha. Fall urea application increased the number of spikes per m2, while topdressing, especially at high amounts (more than 30 kg/ha), led to a decrease in the number of spikes per m2, contribution of assimilate remobilization in grain by more than 20%, and ultimately, grain yield. An increase of one spike per m2 in fall urea application led to an increase of approximately 5 kg/ha in grain yield. However, topdressing urea application delayed the flowering date by 3 to 8 days, and foliar application of urea delayed the number of days to physiological maturity by 5 to 7 days. Therefore, high rates of topdressing application were not only unhelpful but also became an obstacle to achieving high yields. In total, for Golestan province, applying 100 kg/ha of fall urea as placement + a maximum of 50 kg/ha of urea as topdressing (or 3% urea concentration foliar application at early stem elongation) is recommended. For Lorestan province, applying 100 kg/ha of fall urea as placement + 30 kg/ha of urea as topdressing in spring is recommended. For Moghan, applying 70 kg/ha of fall urea as placement + 30 kg/ha of urea as topdressing (or 3% urea concentration foliar application at the tillering stage) is recommended. Finally, it is inferred from this research that sufficient fall urea application as placement (approximately 100 kg/ha) is essential for dryland wheat production in warm and semi-warm regions. However, in topdressing fertilizer application, the utmost caution should be exercised regarding the amount (maximum 30 kg/ha), timing (early), and climatic conditions (accompanied by effective and consecutive rainfall).
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