The Effects of Stabilized Urea and Split-Applied Nitrogen on Sunflower Yield and Oil Content

Sunflower is an efficient nitrogen (N) accumulator due to its aggressive taproot and extensive root system. While N rate studies in sunflower have shown a yield response, the response is often highly variable and difficult to predict in many instances. Additionally, since most sunflower production is intended for the oil market, surplus nitrogen tends to decrease oil content. Therefore, it is critical to hone nitrogen rates to maximize both yield and oil production and to incorporate alternative approaches to fertilizer application, which includes timing and method of application. The objective of the present study was to assess the efficacy of a split-application of N at either the V4 or R1 growth stage to increase yield and/or oil content in sunflower. A second objective was to examine whether a urease inhibitor could be used to retain soil N longer and achieve a similar effect as a split-application. Studies were conducted at two locations over two growing seasons in South Dakota, USA. A target rate of 90 kg·ha-1 was applied as urea-ammonium nitrate (UAN) either as an at-planting application or split-applied. Overall, N additions did significantly increase yield over a control. On average, the urease inhibitor tended to increase grain yields over split-applying N at either growth stage, however, there was no statistical effect on either grain yield or oil content. Based on 15N analysis, approximately 27% of the N in the grain was derived from the UAN fertilizer, which indicates a relatively large reliance upon soil N for grain N content. The addition of a urease inhibitor significantly increased average fertilizer uptake by nearly 6% to 32.7%.