Wheat (Triticum aestivum L.) residues and nitrogen (N) management are the major problems in the southern part of Iran where irrigated wheat-cotton (Gossypium hirsutum L.)-wheat rotation is a common practice. A 2-year (2009-2011) field experiment was conducted as a split plot design with four replications at a cotton field (Darab), Fars Province, Iran, to determine the influence of different rates of wheat residue (0%, 25%, 50%, and 75%) incorporation and N rates (150, 200, 300, and 400 kg ha −1 ) on weed suppression, yield, and yield components of cotton. Results showed that a higher residue incorporation and a lower N rate improved weed suppression in both years. For treatments receiving 150 kg N ha −1 and 75% of wheat residues (2250 kg ha −1 ), weed biomass and density were significantly lower compared to treatments receiving 400 kg N ha −1 . The highest cotton lint yield (about 2400-2700 kg ha −1 ) was obtained by 300 kg N ha −1 in the absence of residue application, in both years. Incorporation of 25% of wheat residue (750 kg ha −1 ) and application of 300 kg N ha −1 are recommended to guarantee an optimum level of cotton lint yield and weed suppression in a wheat-cotton-wheat rotation in this region.

The effects of nitrogen (N) on crop yields have historically been assessed with field trials, but selection and use of the best sources and optimal timing N applications have a significant role in realizing the maximum potential of oilseeds quality and quantity. This study was conducted to determine the combine effects of N sources [ammonium nitrate (AN), ammonium sulphate (AS), sulphur coated urea (SCU), and urea (U)] and split N fertilisation [(1/4,3/4,0), (1/3,1/3,1/3), (1/2,1/2,0), and (1/3,2/3,0)] on safflower (Carthamus tinctorius L.) some growth characters, yield and seed quality, and N use efficiency based on a split plot design with three replications at the experimental research station, Shiraz University in 2015 and 2016. The highest safflower dry matter (5140.93 kg ha–1), seed yield (3303.52 kg ha–1) and protein yield (694.95 kg ha–1) were achieved with the application of AN fertiliser in a split pattern of 1/2,1/2,0 (applying half of the N at sowing time and the rest at stem elongation), while the highest oil yield (753.09 kg ha– 1) was observed by U fertiliser and similar split pattern. Applying AN fertiliser and split patterns of 1/3,2/3,0 (applying one third of the N at sowing and two thirds of the N at stem elongation) and 1/4,3/4,0 (applying one quarter of the N at sowing and three quarters at stem elongation) maximised safflower N uptake efficiency (NUpE) (0.78 kg kg–1). However, the highest N utilisation efficiency (NUtE) (43.70 kg kg–1) was obtained when AN fertiliser in a split pattern of 1/2,1/2,0 was applied. On the contrary, applying AS and SCU fertilisers was less effective on safflower performance by all split patterns. It is concluded that applying AN fertiliser in a split pattern of 1/3,2/3,0 and or U fertiliser in a split pattern of 1/2,1/2,0 not only enhanced safflower growth, yield and seed quality improved, but also increased the N use efficiency of safflower.

Nitrogen (N) effects on crop-weed competition and its information may help to improved integrated weed management systems. A 2-year field experiment (2015 and 2016) was conducted on a silty clay loam soil in semi-arid climatic conditions to determine the combined effects of N sources (ammonium nitrate (AN), ammonium sulfate (AS), sulfur coated urea (SCU), urea (U)) and timing fertilization (sowing, stem elongation, flowering) on weed-safflower (Carthamus tinctorius L.) competition and safflower yield at Shiraz University, Iran. The experimental design was a split split-plot with three replications. Results showed that weeds significantly reduced safflower yield components approximately 39% and caused a decrease in seed yield up to 70%. In weed free, the highest seed yield (3303.52 kg ha ) fertilization at stem elongation and zero N application at flowering (T 1 N 1 3 ⁄ , T 2 N 2 3 ⁄ , and T 3 N 0 ) can be used to advise farmers of the importance of strategic fertilizer management in terms of both weed management and safflower yield.