Independent effects of sowing date and seeding rate on grain yield, seed quality, and quinolizidine alkaloid content in white lupin ( Lupinus albus L.) under rainfed subhumid conditions

White lupin ( Lupinus albus L.) is a high‐protein and nitrogen‐fixing leguminous plant with potential in rainfed systems, although approaches to maximize its yield, seed quality, and safety of quinolizidine alkaloids (QAs) remain inadequate in subhumid South Asian regions. Three sowing dates (October 1, October 15, and November 1) and four seeding rates (60, 90, 120, and 150 kg ha − 1 ) were evaluated in Lupinus albus ‘Estoril’ using a two‐factor randomized complete block design across two Rabi seasons (2022–2023 and 2023–2024) at Rawalpindi, Pakistan. The main effect was sowing date, which caused 65.9%–92.1% of the phenotypic variance. Delayed sowing reduced grain yield by 32.9% and seed protein by 34.0 g kg − 1 dry matter, while increasing total QA concentration by 47.9%, a pattern consistent with stress‐associated alkaloid accumulation reported in prior lupin studies. Seeding rate also significantly affected grain yield and QA, with no sowing date × seeding rate interaction except for grain yield, where a small, season‐dependent interaction was detected ( p = 0.030). Seed protein and total QA were significantly and negatively correlated ( r = −0.866, p < 0.001), suggesting competitive nitrogen partitioning for biosynthesis. A seeding rate of 120 kg ha −1 and early sowing (October 1) are suggested as independent management strategies to maximize yield and minimize QA accumulation.