EFFECT OF SOWING DATES AND MINERAL FERTILIZER RATES ON THE BIOLOGICAL DRY-MATTER ACCUMULATION OF SUNFLOWER HYBRIDS, WITH A COVARIANCE-BASED ASSESSMENT OF THE STAND-DENSITY EFFECT

Abstract

A three-year field experiment (2024–2026) was conducted on meadow-alluvial soils of the Experimental Farm of Urgench State University (Khorezm region, Uzbekistan) to quantify the effect of sowing date and mineral-fertilizer rate on the biological dry-matter accumulation of three sunflower hybrids — the local ‘KK-52’ F₁ (Uzbekistan) and the introduced ‘Dushko’ F₁ (Serbia) and ‘Svetlana’ F₁ (Russia). Two cropping systems were compared — a spring primary crop (second decade of April) and a summer repeat crop (third decade of June) — each with four fertilizer rates (N₁₂₀P₈₅K₁₂₀ to N₂₁₀P₁₄₅K₂₁₀ kg ha⁻¹) in a three-replicate design. Dry matter per plant was recorded at six phenological phases. Fertilizer rate was the dominant driver of accumulation (r = 0.96), the spring crop out-yielding the summer crop by roughly 15 %, and KK-52 exceeding Dushko and Svetlana. The relationship between stand density and final dry matter was examined by correlation, regression and analysis of covariance (ANCOVA). The simple (marginal) correlation was positive but confounded by fertilizer (spring r = 0.68, p = 0.016; summer r = 0.32, p = 0.31); once hybrid and fertilizer were held constant, the partial effect of density reversed to a small, non-significant negative slope, consistent with mild intraspecific competition. Introducing stand density as a covariate reduced the residual variance and raised the model coefficient of determination to R² ≈ 0.999, markedly increasing the precision and reliability of the treatment comparisons.