Response of Sunflower to Nitrogen and Phosphorus in North Dakota

1 Sunflower (Helianthus annuus L.) is native to North America. American natives first cultivated sunflower 5000 yr ago in the present-day southwest United States (Semeslczi-Kovacs, 1975). Sunflower spread to the north and east in North America, where it was domesticated 4000 yr ago (Blackman et al., 2011). Sunflower is a relatively new commercial crop to the United States, with commercial cultivation beginning in the 1970s. Sunflower has since become an important crop in the northern Great Plains regional states of North Dakota, South Dakota, and Minnesota, with additional production in Kansas, Colorado, and Texas. Sunflower production has evolved with innovations in production and genetics that have resulted in improved yield (Fig. 1) (USDA-NASS, 2015; Hulke and Kleingartner, 2014). As of 2015, approximately 85% of US sunflower production is in the northern Great Plains (USDANASS, 2015) and sunflower is considered among the most important crops grown for edible oil in the world (Putt, 1997). Sunflower fertility requirements of N and P in the northern Great Plains were determined through establishment of soil test calibration studies and associated determination critical N and P soil test values, with yield and oil concentration responses when it first became a commercially important crop. Yield components of sunflower include seed number per plant and seed size. Oil concentration is also a consideration for oilseed sunflower (Connor and Hall, 1997). Increasing N supply increases seed number per head, seed mass and overall oil yield (Abbadi et al., 2008; Connor and Sadras, 1992; Hocking and Steer, 1989; Steer and Hocking, 1984). Nitrogen trials were conducted in North Dakota from 1971 to 1983 on dryland sunflower and from 1977 to 1983 for irrigated sunflower (Zubriski and Moraghan, 1983). In these experiments, sunflower yield increased and oil concentration decreased with increasing total known available N. In Minnesota, a seed yield of 2900 kg ha-1 required 135 kg ha-1 of N for oilseed sunflower and 147 kg ha-1 of N for confectionary sunflower (Faulkner, 1977), leading to the recommendation that oilseed and confectionary sunflower required 5 kg N ha-1 to produce each 100 kg ha-1 of seed. In Minnesota, Robinson (1973a) recorded no oil sunflower seed yield increase with N fertilizer; however, subsequent studies found yield increases Response of Sunflower to Nitrogen and Phosphorus in North Dakota