Abstract
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%.
Highlights
IntroductionSunflower (Helianthus annuus L.) utilizes N in a fairly
The N treatments were as follows: Control (0N), 90 kg N ha−1 applied at planting (90AP), 90 kg N ha−1 with N-(n-butyl)-thiophosphoric triamide (NBPT) urease inhibitor (90AP + NBPT), 90 kg N ha−1 split-applied with 50% at planting and 50% applied at the V4 growth stage (90 SplitV4) and 90 kg N ha−1 split-applied with 50% at planting and 50% applied at the R1 growth stage (90 SplitR1)
At Bison where rainfall was high early in the growing season, the N derived from fertilizer (Ndff) from the 90AP + NBPT treatment was significantly greater than the 90AP, but not significantly different than the split-applications (Figure 4). These results suggest that there was likely some increased environmental N losses through increased rainfall and that the urease inhibitor likely protected the urea-ammonium nitrate (UAN) against these conditions by reducing the conversion to NO3-N
Summary
Sunflower (Helianthus annuus L.) utilizes N in a fairly. Sunflower is known to have an aggressive taproot, reaching maximum rooting depths of 1 - 3 m by the grain-filling period [2]. Tends to decrease exponentially with depth with up to ten times greater root mass in the 0 - 0.2 m soil layer compared to deeper soil depths [3]. Rooting depth typically correlates with water extraction with maximum depths reached in wetter growing seasons [4]. The majority of N is taken up by the plant through mass flow. N uptake dynamics often mimics water uptake patterns with more N being taken up during wetter years and less during periods of drought
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