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 15 N 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%.
Relative to other plants, sunflower (Helianthus annuus L.) utilizes N in a fairly efficient manner [
Sunflower grain yield has long been known to respond to fertilizer N application, particularly at extractable available soil N levels less than 60 kg∙ha−1 [
Beyond application rate, timing of N application is another important aspect of an effective fertilization program and much less is known in regards to N uptake by sunflower. Previous research indicates that seed weight can be increased by fertilizer N application at various stages of the growing season, but may be most affected when fertilization is timed between floret initiation and anthesis [
Given the nature of sunflower rooting dynamics, the likelihood of excess N post-harvest is high. Indeed, Schatz et al. [
In order to maximize fertilizer N use efficiency (FNUE), it is important to determine when fertilizer N application is most effective and environmentally beneficial. The objectives of this study were to determine FNUE, grain yield, and oil content in sunflower as affected by timing of fertilizer N application (at- planting, V4, R1 growth stages) or through the use of a urease-inhibitor. Both options have been shown to increase N use efficiency, maintain or increase yields and minimize environmental impacts [
This study took place at Bison (45˚30'N, 102˚33'W) and Onida, SD, USA (44˚42'N, 100˚15'W) in 2014 and was reduced to just the Onida site (44˚35'N, 100˚05'W) in 2015. This research was conducted on-farm with the Onida site separated by approximately 24 km between years. Selected soil characteristics by site at the initiation of this research are listed in
Sunflower (Mycogen Seeds MY8H456CL, Size 3, Indianapolis, IN) was planted with a no-till grain drill (Model 750, John Deere Co., Moline, IL) at a population of 4.1 plants m−2 on 9 and 12 June, 2014 for Bison and Onida, respectively. In 2015, the Onida site was planted on 10 June. Thiamethoxam (3-[(2- Chloro-1,3-thiazol-5-yl)methyl]-5-methyl-N-nitro-1,3,5-oxadiazinan-4-imine) (Syngenta, Wilmington, DE) was applied as an insecticide seed treatment at a rate of 0.25 mg a.i. per seed. Each plot consisted of four rows planted at 76.2 cm
Location | Bison | Onida | |
---|---|---|---|
2014 | 2015 | ||
Soil Texture | Sandy Clay Loam | Silty Clay Loam | Silty Clay Loam |
Sand (0 - 15 cm, g∙kg−1) | 550 | 190 | 190 |
Silt (0 - 15 cm, g∙kg−1) | 200 | 420 | 470 |
Clay (0 - 15 cm, g∙kg−1) | 250 | 390 | 340 |
pH (1:1 water) | 5.9 | 6.0 | 6.5 |
Organic Matter (0 - 15 cm, g∙kg−1) | 16 | 31 | 42 |
P (0 - 15 cm, mg∙kg−1) | 21 | 10 | 23 |
N (0 - 15 cm, kg∙ha−1) | 4 | 31 | 10 |
N (15 - 60 cm, kg∙ha−1) | 12 | 65 | 54 |
K (0 - 15 cm, mg∙kg−1) | 464 | 558 | 475 |
Soluble Salts (mmho cm−1) | 0.3 | 0.5 | 0.4 |
between rows and 9.1 m long. For weed control, Sulfentrazone (N-{2,4-Dichlo- ro-5-[4-(difluoromethyl)-3-methyl-5-oxo-4,5-dihydro-1H-1,2,4-triazol-1-yl]phenyl}methanesulfonamide) (FMC, Philadelphia, PA) and glyphosate (N-[phos- phonomethyl] glycine) were applied at planting for each site.
Fertilizer recovery was evaluated using 15N-labeled UAN (1.366‰ 15N atom excess) applied in a micro-plot (3.9 m2) established within the center of each plot. This plot size was assumed to be sufficient to eliminate border effects based on research from other crops [
and
where F is the fraction of total N uptake derived from 15N enriched fertilizer, As is the atom % 15N measured in the harvested plant sample, Af is the atom % 15N in the enriched fertilizer,
Ar is atom % 15N of the reference harvested plant material from non-15N enriched control plots and Ef is the total uptake of 15N enriched fertilizer and Nu is the total N uptake by the plant or plant component [
The study was arranged in a 5 × 2 × 2 split-split-block factorial arrangement with the N treatment randomly applied within each of four replications. Grain yield, oil content, N uptake and residual N were analyzed statistically as a linear mixed-effects ANOVA model with Satterthwaite’s approximation for denominator degrees of freedom using the lme4( ) and lmerTest( ) modules [
Levene’s test was used to check for homogeneity of variance. Residual and Q-Q plots were applied to examine data normality. Further assumptions of the linear package were verified using the gvlma( ) based on Pena and Slate [
In general, the timing of N supply did not have a significant effect on grain yield, N uptake or oil yield. A single application at planting was as effective as a split- application. Hocking and Steer [
Bison (2014) | Bison 30-Yr Average | Onida (2014) | Onida (2015) | Onida 30-Yr Average | ||
---|---|---|---|---|---|---|
Temperature | April | 5.3 | 7.3 | 6.6 | 9.4 | 7.7 |
(C) | May | 12.8 | 13.4 | 13.7 | 12.4 | 13.9 |
June | 15.6 | 18.4 | 17.6 | 19.9 | 19.4 | |
July | 20.4 | 22.6 | 20.8 | 23.4 | 23.3 | |
Aug | 21.1 | 22.1 | 20.8 | 21.7 | 22.2 | |
Sept | 15.5 | 16.1 | 16.8 | 19.1 | 16.7 | |
Oct | 10.5 | 8.7 | 9.8 | 11.2 | 8.3 | |
Average | 14.5 | 15.5 | 15.2 | 16.7 | 16.0 | |
Precipitation | April | 44.7 | 46.7 | 59.4 | 11.7 | 47.5 |
(mm) | May | 38.4 | 78.5 | 63.5 | 138.9 | 77.7 |
June | 205.0 | 73.9 | 136.4 | 84.1 | 84.8 | |
July | 22.1 | 60.2 | 23.6 | 34.3 | 67.3 | |
Aug | 82.8 | 41.1 | 67.1 | 70.9 | 59.9 | |
Sept | 46.2 | 32.3 | 19.1 | 48.8 | 45.7 | |
Oct | 7.6 | 37.3 | 15.0 | 34.3 | 42.4 | |
Total | 446.8 | 370.1 | 384.0 | 422.9 | 425.5 |
Main Effect/Level† | Yield | Percent N in Grain | N Uptake | Yield: N Ratio | ||||
---|---|---|---|---|---|---|---|---|
g plot−1 | % | g plot−1 | ||||||
N Treatment | ||||||||
Control | 84.1 | a | 2.95 | a | 2.08 | a | 35.62 | a |
90 AP | 139.9 | b | 3.23 | a | 4.56 | b | 31.66 | b |
90 AP + NBPT | 147.2 | b | 3.09 | a | 4.42 | b | 32.87 | b |
90 Split V4 | 129.9 | b | 3.04 | a | 3.82 | b | 33.43 | ab |
90 Split R1 | 145.9 | b | 3.23 | a | 4.67 | b | 32.05 | b |
Location | ||||||||
Bison | 66.0 | a | 3.39 | a | 2.35 | a | 29.95 | a |
Onida | 192.9 | b | 2.89 | b | 5.47 | b | 36.31 | b |
ANOVA | ||||||||
N Treatment | * | NS | *** | * | ||||
Location | *** | *** | *** | *** | ||||
N Treatment*Location | NS | * | NS | ** |
*Statistical significance at 0.05; **Statistical significance at 0.01; ***Statistical significance at 0.001; NS, not significant. †Effects are compared within each column and main effect. Mean values followed by the same letter are not significantly different at P ≤ 0.05.
from numerous possible contributing factors including soil type, weather, rotation and application method [
Oil content did not differ by N treatment but did vary significantly by location with Bison averaging 36.3% and Onida averaging 44.6% (
In general, applying fertilizer N increased oil yield. Because oil content was not materially different between treatments, this effect was largely due to the increased grain yield. Contrary to our initial hypothesis, a later N application did not increase either oil content or yield. The addition of a urease inhibitor was as effective as a split-application and in fact produced the highest oil yield on aver-
Main Effect/Level | Oil Content | |
---|---|---|
% | ||
N Treatment | ||
Control | 40.9 | a |
90 AP | 41.0 | a |
90 AP + NBPT | 40.5 | a |
90 Split V4 | 39.9 | a |
90 Split R1 | 39.9 | a |
Location | ||
Bison | 36.3 | |
Onida | 44.6 | |
ANOVA | ||
N Treatment | NS | |
Location | *** | |
N Treatment*Location | NS |
†Effects are compared within each column and main effect. Mean values followed by the same letter are not significantly different at P ≤ 0.05.
age. However, this difference was not statistically different than other N addition methods.
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 final grain N content. The addition of a urease inhibitor significantly increased average fertilizer uptake by nearly 6% to 32.7%. When compared to the standard practice of applying UAN at planting, the urease inhibitor showed a trend toward increased yield as well, which improved the overall efficiency of the fertilizer.
Moreover, the sources for the final N concentration in the grain are a mix of N derived directly from the soil and N mobilized by the photosynthetic apparatus, with little contribution from the stalk [
Excepting grain, location was not a significant treatment effect for Ndff in any of the analyzed plant components (
Main Effect/Level† | Grain Ndff | Leaf Ndff | Stalk Ndff | Head Ndff | ||||
---|---|---|---|---|---|---|---|---|
% | % | % | % | |||||
N Treatment | ||||||||
90 AP | 26.82 | a | 36.46 | a | 29.38 | ab | 30.97 | ab |
90 AP + NBPT | 32.74 | b | 39.57 | a | 32.53 | b | 33.49 | b |
90 Split V4 | 27.15 | a | 31.66 | b | 28.04 | a | 29.59 | a |
90 Split R1 | 26.94 | a | 26.75 | c | 26.38 | a | 27.38 | a |
Average | 28.41 | 33.61 | 29.08 | 30.36 | ||||
Location | ||||||||
Bison | 25.80 | a | 33.33 | a | 29.00 | a | 29.98 | a |
Onida | 30.88 | b | 33.89 | a | 29.16 | a | 30.73 | a |
ANOVA | ||||||||
N Treatment | * | *** | . | * | ||||
Location | * | NS | NS | NS | ||||
N Treatment*Location | NS | NS | NS | NS |
Statistical significance at 0.10; *Statistical significance at 0.05; **Statistical significance at 0.01; ***Statistical significance at 0.001; NS, not significant. †Effects are compared within each column and main effect. Mean values followed by the same letter are not significantly different at P ≤ 0.05.
mental conditions. However, N derived from fertilizer, as a percent of total N in the component part, varied significantly by application timing (
In theory, split-applying N provides a benefit over an at-planting application because the plant can better compete with early season environmental losses (i.e. leaching, denitrification, etc.) [
This is likely due to a substantial reliance upon soil N for the plant’s needs. When averaged across all treatments, fertilizer met only 30% of any sunflower component’s needs. By applying half of the total N fertilizer at planting in the split-application treatments, the plant appears to simply shift its N source to soil reserves. However, this appears to come at the expense of relying more on soil N for its needs. Therefore, the resulting yields are similar between treatments, but they are achieved through different N sources. This dynamic likely played a large role in the location differences. Bison had much lower starting soil N and low organic matter content leaving this site with much fewer N reserves for the plant to draw upon (
However, as shown in
In the drier climate at Onida, the Ndff from the 90AP + NBPT treatment did not differ from the 90AP treatment but was significantly greater than either split-application. Conversely, at Bison where rainfall was high early in the growing season, the Ndff from the 90AP + NBPT treatment was significantly greater than the 90AP, but not significantly different than the split-applications (
throughout the early growing season, which allowed for greater fertilizer N uptake. However, due to a lack of data with respect to split-application and slow- release fertilizers on sunflower, these results should be verified through additional trial replications across a broader environmental gradient.
In general, these data suggest that split-applying N is as effective as an at-plant- ing N application at increasing sunflower grain yield. Based on 15N uptake, lower Ndff in component parts in the split-application treatments suggests that the plant simply shifts its reliance on fertilizer N to soil N depending upon availability.
Meanwhile, the use of a urease inhibitor with UAN does appear to increase fertilizer N uptake. There was a trend towards higher yields with the urease inhibitor; however this was not statistically significant due to high variability. By slowing down the N transformations from urea to
Graham, C.J. and Varco, J.J. (2017) The Effects of Stabilized Urea and Split-Applied Nitrogen on Sunflower Yield and Oil Content. American Journal of Plant Sciences, 8, 1842-1854. https://doi.org/10.4236/ajps.2017.88125