Effect of Nitrogen Fertilizer Sources on Growth, Yield and Ability to Withstand Competition of Quinoa (Chenopodium quinoa Willd) in Different Red Root Pigweed (Amaranthus retroflexus) Densities

 Quinoa (Chenopodium quinoa L.) is a highly nutritional seed crop from the Andean region with huge genetic variability, enabling its cultivation across a wide range of environmental conditions. The area and production under quinoa in the world in 2020 was 189000 ha with 175000 tonnes production. There is some evidence for allelopathic activity of quinoa and this potential could be probably used in terms of integrated weed management. Agronomic practices such as nitrogen fertilization influence weed emergence, growth and competition in a crop. Nevertheless, despite the numerous studies on new and promising crops globally, there is a clear lack of information on the combined effect of weed density and nitrogen fertilizer sources on quinoa crop. Therefore, the purpose of this study was to evaluate the effects of nitrogen fertilizer sources and red root pigweed densities on growth, yield and competitive ability of quinoa (Chenopodium quinoa Willd). This information could be helpful for the overall development of crop and weed management strategies in quinoa crop.
A field study was conducted during the 2021 growing season at the research farm of the School of Agriculture, Shiraz University, to assess the impact of nitrogen fertilizer sources on the growth, yield, and competitive ability of quinoa in the presence of red root pigweed at different densities. The experiment was set up in a split-plot design with nitrogen fertilizer sources (control, urea, sulfur-coated urea, and ammonium nitrate) assigned to the main plots, and red root pigweed densities (0, 5, 10, 15, 20, and 25 plants per square meter) assigned to the sub-plots. There were three replications of each treatment. For the quinoa traits and weed traits, a 2-meter square area was harvested from each plot. Quinoa traits included plant height, leaf area index, number of grains per plant, 1000 grain weight, grain yield, biological yield, and harvest index. The quinoa plants were dried in an oven at 75°C for 72 hours to determine seed yield. Weeds were also harvested from a 2 m2 area in each plot to measure plant height, shoot height, panicle length, and leaf area index. The collected data were analyzed using SAS v. 9.1 software (SAS Institute 2003). When significant differences were found among treatments, mean comparisons were performed using Duncan's multiple range tests at a significance level of P < 0.05.
 The results of the experiment indicated that the use of sulfur coated urea had a positive effect on the competitive ability of quinoa. Weed density had a detrimental impact on various growth and yield parameters of quinoa, including plant height, leaf area index, number of grains per plant, 1000 grain weight, grain yield, biological yield, and harvest index. However, the application of sulfur coated urea mitigated the negative effects of weed density. Specifically, when the highest weed density of 25 plants per square meter was present, the application of sulfur coated urea led to a 1.1-fold increase in plant height, a 2.5-fold increase in leaf area index, a 2.5-fold increase in the number of grains per plant, a 1.1-fold increase in 1000 grain weight, a 2.8-fold increase in grain yield, and a 1.8-fold increase in biological yield compared to the control. At different red root pigweed densities (0, 5, 10, 15, 20, and 25 plants per square meter), the application of sulfur coated urea resulted in significant improvements in quinoa performance. It increased the number of grains per plant by 86.5%, 118%, 139.4%, 168.8%, 149.6%, and 153.4% compared to the control at respective weed densities. Additionally, 1000 grain weight increased by 7.9% to 9.9%, and the ability of quinoa to withstand competition increased by 19.6% to 55%. The findings of this study are consistent with previous research that has demonstrated the positive effects of organic nutrients on reducing weed competition in agricultural systems. It has also been observed that weeds tend to produce more biomass in the presence of fertilizer compared to the control. Therefore, it can be concluded that the improved grain yield of quinoa resulting from the application of sulfur coated urea was primarily attributed to its ability to enhance the plant's competitive ability against weeds.
 The application of sulfur coated urea led to a higher quinoa yield compared to using control. However, weed competition was greater with urea fertilization in comparison with sulfur coated urea fertilizer. In addition, most weeds are highly responsive to soil N, so the application of all fertilizer types should be carefully considered to reduce the competitive advantage of weeds over crops.
Acknowledgements
 We would like to thank the School of Agriculture, Shiraz University for their support, cooperation, and assistance throughout this research.