Effect of Nitrogen Fertilizer Sources and Redroot Pigweed (Amaranthus retroflexus) Densities on Nitrogen Utilization Efficiency and Quinoa (Chenopodium quinoa Willd) Resistance Index against Weed

The aim of this study was the determine effect of nitrogen fertilizer sourcesand redroot pigweed (Amaranthus retroflexus) densities on nitrogen utilization efficiency and quinoa (Chenopodium quinoa Willd) resistance index. A field study carried out during 2021 growing season in research farm of School of Agriculture, Shiraz University. The experiment was conducted in a split-plot based on a randomized complete block design with 3 replications. Nitrogen fertilizer sources (non‐fertilizer, urea, sulfur coated urea and ammonium nitrate) were assigned to main plots and density of red root pigweed at 6 levels (0, 5, 10, 15, 20 and 25 plants m-2) to the sub-plots. The application of sulfur coated urea in the highest weed density (25 plant m-2) increased dry matter partitioning to grain by 1.2 times, percentage of nitrogen and protein by 1.5 and 1.4 times, photosynthesis rate by 1.2 times and quinoa grain yield by 2.8 times compared to non‐fertilizer. The application of urea, sulfur coated urea and ammonium nitrate in the highest weed density increased nitrogen utilization efficiency by 78.3, 79.8 and 60.4%, respectively compared to non‐fertilizer. In general, the results showed that sulfur coated urea had the maximum grain yield and nitrogen utilization efficiency and was followed by urea and ammonium nitrate, respectively. Therefore, sulfur coated urea is recommended as the most efficient source of nitrogen fertilizer.