Chilean Journal of Agricultural Research Instituto de Investigaciones Agropecuarias, INIA ISSN: 0718-5820 EISSN: 0718-5839 Vol. 70, Num. 1, 2010, pp. 132-141

Chilean Journal of Agricultural Research, Vol. 70, No. 1, Jan-Mar, 2010, pp. 132-141

Effects of source and rate of nitrogen fertilizer on yield, yield components and quality of winter rapeseed (Brassica napus L. )

Efecto de la fuente y dosis de fertilizantes nitrogenados en el rendimiento, componentes de rendimiento y calidad de semilla de canola (Brassica napus L.) 

Özden Öztürk[1]

[1]University of Selcuk, Faculty of Agriculture, Konya, Turkey. ^*Corresponding author (ozdenoz@selcuk.edu.tr; ozdengulsum@gmail.com).

Received: 04 November 2008.
Accepted: 03 April 2009.

Code Number: cj10014

ABSTRACT

Winter rapeseed (Brassica napus L.) has potential to become an alternate oilseed crop both for edible oil production and energy agriculture (biofuel production) for Turkey. This study was conducted to determine the effect of year, N sources and doses on the yield and quality traits of winter rapeseed in a cereal system in calcareous soils over two seasons, 2000-2001 and 2001-2002, in Central Anatolia. Three N sources, ammonium sulfate, ammonium nitrate and urea, were applied as hand broadcast on the soil surface at five doses (0, 50, 100, 150, and 200 kg N ha^-1). The traits investigated were plant height, number of branches and pods per plant, number of seed per pod, thousand seed weight, seed yield, oil and protein content. There were significantly effects on seed yield, oil and protein content, and other yield components due to N sources and rates. In general, ammonium sulfate and urea gave higher seed yield than ammonium nitrate. Mean values of both seasons indicated that 100 and 150 kg N ha^-1 rate increased significantly yield and quality traits with regard to other N treatments. The present results highlight the practical importance of adequate N fertilization and true N source in seed yield in winter rapeseed and suggest that ammonium sulfate at 150 kg N ha^-1 will be about adequate to meet crop N requirements.

Key words: winter rapeseed, seed yield, oil content, protein content, Brassica napus.

RESUMEN 

El raps (Brassica napus L.) tiene potencial para convertirse en un cultivo oleaginoso alternativo para producción de aceite comestible y agricultura energética (producción de biodiesel) en Turquía. Este estudio fue conducido para determinar el efecto del año, fuente y dosis de N en las características de rendimiento y calidad de raps en un sistema cerealero en suelos calcáreos en dos temporadas, 2000-2001 y 2001-2002, en Anatolia Central. Se aplicaron al voleo tres fuentes de N (sulfato de amonio, nitrato de amonio y urea) en cinco dosis (0, 50, 100, 150 y 200 kg N ha^-1). Las características investigadas fueron altura de planta, número de ramas y vainas por planta, número de semillas por vaina, peso de mil semillas, producción de semilla, y contenido de aceite y proteína. Hubo efectos significativos de fuente y dosis de N en producción de semilla, contenido de aceite y proteína, y otros componentes del rendimiento. En general, sulfato de amonio y urea produjeron mayor producción de semilla que nitrato de amonio. Los valores medios de ambas temporadas indicaron que dosis de 100 y 150 kg N ha^-1 aumentaron significativamente las características de rendimiento y calidad. Estos resultados destacan la importancia práctica del uso de fuente y dosis de N adecuadas en la producción de semilla en raps, y sugieren que el sulfato de amonio a 150 kg N ha^-1 suplirá los requerimientos de N del cultivo. 

Palabras clave: raps, producción de semilla, contenido de aceite, contenido de proteína, Brassica napus.

INTRODUCTION 

Winter rapeseed (Brassica napus L.) is an important agricultural crop, grown commonly for oil or biofuel production. After oil extraction, the high protein seed residue can be used as animal feed. Winter rapeseed is mainly cultivated in Europe, Asia, North America and Australia, but has a limited acreage in Turkey. Presently, over 50% of vegetable oil consumed in Turkey is imported from abroad. Rapeseed production has potential as an alternative income source for the Turkey producer. Although its production is still limited, this crop has large expansion possibilities. It is an alternative principally in areas where wheat (Triticum aestivum L.) is the only winter crop or in marginal areas for this cereal.

In Turkey, 75% of the arable land is devoted to cereals, of which 67% is occupied by wheat. Drought stress is also a serious abiotic stress factor limiting crop production in Turkey, especially in Central Anatolia, which covers nearly 45% (4.5 million ha) of the Turkish wheat-producing area. It is semiarid and the driest region in the country, where the soil is also generally poor in plant-available N concentration. Winter rapeseed is a new and promising oilseed crop for many region of Turkey such as Central Anatolia. In Turkey, winter rapeseed is usually cultivated in a crop rotation including winter wheat and winter barley (Hordeum vulgare L.).

For newly introduced crops, it is necessary to assess the appropriate production technology for different environments. Amongst many others, the nutritional requirements of the crop are considered to be the most important factor. Nitrogen fertilizer plays a vital role in enhancing crop yield (Rathke et al., 2005). Compared to cereals, winter rapeseed requires a higher amount of nutrients, and available N frequently limits seed yield. Hocking et al. (1997) said that rapeseed requires about 25% more N than wheat.

Yield response of rapeseed to increasing N doses varies with different environmental variables, including weather, soil type, residual fertility (especially nitrate), soil water content, and cultivar. Many studies have shown that both growth and yield of rapeseed are enhanced significantly by high doses of applied N (Bilsborrow et al., 1993; Kumar et al., 2001; Cheema et al., 2001). Nitrogen increases yield by influencing a number of growth parameters such as number of branches and pods per plant, seeds per pod and 1000 seed weight by producing more vigorous growth and development (Taylor et al., 1991; Qayyum et al., 1998). Rapeseed needs to accumulate between 200 and 250 kg N ha^-1 to yield 2500 kg seed ha^-1 (Grant and Bailey, 1993). In Argentina it has been reported that seed yield increases with 150 kg N ha^-1 (Sarandon et al., 1996). Sheppards and Bates (1980) noted increased yield with increasing N rates up-to 100 kg ha^-1. Ibrahim et al. (1989) concluded that yield increased with rates of N up-to 213 kg ha^-1. Excess N, however, can reduce seed yield and quality appreciably (Cheema et al., 2001; Laaniste et al., 2004). An excessive N rate or an inadequate moment of N application could increase N content in seeds, decreasing oil content and their commercial value (Chamorro et al., 2002). Besides, excessive application of fertilizers affects negatively the farmer economy and environment.

Choosing the correct dose, source and timing of N fertilizer application is therefore an important aspect of successful rapeseed production. The problem of type of applied fertilizers, rarely taken into consideration by researches and in practice, is even more ambiguous (Wiesler et al., 1999). In spite of the well-recognized effects of the main N fertilizer components, i.e. N sources and/or some other nutrients as a secondary components on soil and plants, the third N factor, i.e. chemical composition of the applied N fertilizers, is seldom treated as an important factor in the rapeseed production system (Wiesler et al., 1999).

Previous study (Öztürk and Akınerdem, 2000) revealed that rapeseed yielded satisfactory and could be grown successfully as winter crop under Central Anatolia conditions. However, there are no published research data on the N rate and sources for winter rapeseed. The objective of this study was evaluate the impact of applying different N rate and sources on rapeseed yield and oil content, results that are critical to promoting winter rapeseed cultivation.

MATERIAL AND METHODS

The research was carried out in the Agricultural Experiment and Research Centre, Faculty of Agriculture, University of Selçuk, Konya, Central Anatolia region (37º35’ N, 32º47’ E, 1013 m.a.s.l.), Turkey, during the growing seasons 2000-2001 (Y1) and 2001-2002 (Y2). Soil samples (0-30 cm) were taken at sowing and analyzed for some parameters. The experimental soil was a clay loam with 0.94% organic matter content and pH of 7.8. Total N content was 0.03%, available P was 3.26 mg kg^-1, available K was 78.0 mg kg^-1, S0₄-N was 3.0 mg kg^-1 and no salinity problems were observed (Table 1). Organic matter was determined by the Modified Walkley-Black procedure; CaCO₃ was determined by Sheibler’s Calcimeter method (Black, 1965); available P was measured by Olsen method according to Black (1965); available K was measured by flame photometry (Knudsen et al., 1982); total N was determined by the Kjeldahl method (Bremner, 1965) and sulfate-S was determined by ICP-AES (Varian Vista Model) following extraction by 0.2 N KH₂PO₄ according to the Soltanpour and Workman (1981).

The average and minimum temperatures, monthly rainfall and relative air humidity data for Y1, Y2 and long term mean (1939-2000) during the rapeseed vegetation period (September-July) are shown in Table 2. In both years, mean temperature was close to the 62 yr average value. There was considerable variability in rainfall amounts and distribution from year to year. The amount of rainfall and average relative air humidity were more suitable for plant growth in Y2 than in Y1. Rainfall received during the vegetation period (September-July) in Y1 was 191.2 mm, which was less than long-term average (321.6 mm) and had an erratic distribution. In early spring (March-April) there was a dry period. In Y2 more rain was received, 375.4 mm in total. The mean Y1 relative air humidity (57.1%) was below to long term mean (60.0%), whereas Y2 relative air humidity (66.0%) was above.

Sowing was done with hand during the 3^rd week of September in both years. The previous crop was winter wheat for each year. The variety used in the experiment was ‘Honk’, a winter type rapeseed variety. Cultivar selection was based on yield performance from earlier trials (Öztürk and Akınerdem, 2000).

The study used a split plot design, with N fertilizer source (ammonium nitrate [AN], ammonium sulfate [AS], and urea) as the main plot and N rate as the subplot. Nitrogen treatments were 0, 50, 100, 150, and 200 kg N ha^-1 (N_0-200). The experiment was replicated three times. Plots were overseeded and subsequently thinned to final plant density of about 50 plants m^-2 at seedling stage. The area of each plot was 9.6 m² consisting of eight rows, 4 m long and 30 cm apart. A 1.0-m alley was left around each plot to avoid plot to plot N contamination. Nitrogen was applied as split in two applications; half was drilled into the soil before sowing and the remaining half was topdressed at the flower-bud-visibility stage (BBCH51; Biologische Bundesanstalt Bundessortenamt and Chemical Industry-BBCH scale; Meier, 2001). All plots received P at 80 kg ha^-1 as triple superphosphate before sowing in both seasons.

Weeds were controlled by hand as needed. Plots were irrigated once at sowing to allow an immediate and homogeneous emergence during both growing seasons. Plots were sprayed with malathion before flowering to protect against a beetle (Omophlus caucasicus) and aphids.

Treatments were hand-harvested at technological maturity stage (BBCH89) at the beginning of July each year. Seed yields were taken at maturity by harvesting the center six rows of each plot for seed yield determination. Subsamples were dried at 105 ºC for moisture determination. Seed yield was adjusted to 9% moisture content (Bilsborrow et al., 1993), and all other measurements were reported on a dry weight basis. Twenty plants were randomly collected from the central six rows and the following growth and yield component variables were recorded for each plot; plant height, primary branches per plant, pod number per plant and 1000 seed weight. The number of seeds per pod was determined on a random subsample of 20 pods. Seed oil content was determined, after drying at 70 ºC for 48 h (Billsborrow et al., 1993), by Soxhlet extraction technique, using diethyl ether, as reported by AOAC methods 920.39 (AOAC, 1980), and seed N concentration by the Kjeldahl procedure, AOAC method 920; N was multiplied by 6.25 to convert to protein content (Rathke et al., 2005). Statistical analysis was conducted using the MSTAT-C statistical package (Crop and Soil Department, Michigan State University, Michigan, USA). Analysis of variance (ANOVA) was performed on experimental data obtained from a randomized complete block in a split plot arrangement. The F test was then applied to examine the statistical significance of differences among treatments. All statistically significant main effects and interactions were considered. Differences among treatments were tested by ANOVA and compared using Least Significant Difference (LSD) test at 0.01 and 0.05 levels of significance.

RESULTS AND DISCUSSION

Year (Y) x N source (F) x N rate interaction was observed for any yield or agronomic traits, except 1000 seed weight. F x N rate interaction was significant for plant height, number of branch number and pods per plant and seed yield (Table 3). Y x F and Y x N rate interactions usually were insignificant for yield and other agronomic traits, except seed protein content.

Yield components

Plant height was affected by Y, F, N rate and F x N rate interaction (Table 3). The highest plant height (126.8 cm) was obtained in Y2 (Table 4). This result could be explained by differences in the weather conditions especially total rainfall amount and distribution between years.

All the N fertilizer treatments significantly increased plant height compared with control. Maximum plant height (131.0 cm) was observed with 150 kg N ha^-1, but thereafter, at the highest N rate (N₂₀₀) it decreased significantly (Table 4). This result agrees with that of Özer (2003).

In this research, AN and urea produced significantly taller plants than AS. Nitrogen sources responded differently to increasing N rates for plant height. This caused significant F x N rate interaction (Table 3). The F x N rate interaction indicated that the maximum plant height (138.5 cm) was observed in plots treated with AN at 150 kg N ha^-1 (Figure 1a).

Year affected branch number significantly (Table 3). Branch number was on average 8.7 in the Y2 and 7.7 in the Y1 (Table 4). This shows that differences in weather conditions over years may affect branch number significantly.

In this research, increasing N rates usually caused increases in branch number of rapeseed plants (Table 4), as has been previously reported (Özer, 2003), but it was not significant. By the other hand, effects of N sources and F x N rate interaction on the branch number per plant were significant (Table 3).

In this research, the highest branch number per plant (8.5) was obtained with AN and the lowest (7.7) with AS (Table 4). The highest branch number (9.1) was obtained with N₂₀₀ application of AN (Figure 1b). The increase in number of branches per plant with increase in N rate may be due to the fact that N promoted vegetative growth and branching on the inflorescence. These results agree with those documented by Uddin et al. (1992), who stated that number of branches per plant significantly increased with N doses from 0 to 150 kg ha^-1.

There was no effect of Y and F on pod number per plant. However, N rate and F x N rate affected the pod number (Table 3). The number of pods per plant increased with increasing N rates up-to N₁₅₀. Thereafter, it declined slightly at the highest fertilizer rate (N₂₀₀). Data showed that maximum pods per plant (283.4) were recorded in those plots which received 150 kg N ha^-1 while the minimum (195.8) was produced by control plots (Table 4). These results agree with findings by Qayyum et al. (1998), Cheema et al. (2001) and Khan et al. (2002). The N rates effects for pod number were inconsistent for N sources, leading to significant F x N rate interaction (Table 3). The highest pod number per plant (372.1) was found with 150 kg N ha^-1 application of AN while the lowest pod number per plant (188.9) was obtained with control application of urea (Figure 1c).

Number of seeds per pod was affected by F (Table 3). Maximum of 28.9 seeds per pod were recorded in AS, however, it did not differ from urea (28.8, Table 4). The N dose was not different (Table 3); however, it increased with increase in N rate from 0 to 50 kg ha^-1, and decreased beyond this level (Table 4). These results are different from Quayyum et al. (1998), who stated that increasing N rate from 0 to 120 kg ha^-1 significantly increased the number of seeds per pod.

Thousand seed weight (TSW) was affected by Y, N rate and Y x F x N rate interaction (Table 3). In Y1, rapeseed had a lower TSW than Y2 (Table 4). This shows that the differences in weather conditions over years may affect TSW significantly. TSW decreases as a result of poor seed filling in years without enough rain and water in the soil. This was the case in the first year of experiment. Similar results have been reported by Özer (2003).

A steady and progressive increase in TSW was observed with each increment in applied N rates up-to 150 kg ha^-1, the best result was at N₁₅₀ with 4.3 g (Table 4). These results are in line with those reported by Uddin et al. (1992), who observed that TSW increased significantly when N was increased from 0-150 kg ha^-1.

When the Y x F x N rate was divided into components: the highest TSW (5.2 g) was obtained with Y2 x AS x N₁₅₀ and Y2 x AN x N₅₀ combinations, and the lowest TSW (3.1 g) was obtained with Y1 x urea x N₀ treatment, whereas Y1 x AS x N₀, Y1 x urea x N₅₀ and Y1 x urea x N₂₀₀ did not differ significantly (Figure 2).

Seed yield

Seed yield was affected by F, N rate and F x N rate interaction (Table 3). Seed yield increased significantly with increase in N rate up-to 150 kg ha^-1. Maximum seed yield (3165 kg ha^-1) was obtained by plots that received 150 kg N ha^-1, while minimum seed yield (2147 kg ha^-1) was obtained by control plots. Seed yield increased significantly by 47.41% with increasing N application from 0 to 150 kg ha^-1 and, thereafter, decreased significantly by 21.82% from 150 to 200 kg ha^-1 (Table 4).

The positive yield response of rapeseed to higher N rates was not surprising. As previously explained, winter rapeseed has a high N requirement (Grant and Bailey, 1993; Rathke et al., 2005). The positive impact of N on the seed yield of winter rapeseed has been reported by Cheema et al. (2001) and Rathke et al. (2005). According to Rathore and Manohar (1989) and Jan et al. (2002), seed yield of winter rapeseed increased significantly when N was increased from 0 to 220 kg N ha^-1 depending on site conditions. Maximum yield at higher N levels than control might be due to the fact that all yield components, i.e., number of branches per plant, number of pods per plant, number of seeds per pod and 1000 seed weight, increased with increase in N. These results agree with those reported by Cheema et al. (2001) and Khan et al. (2002). In this study, the fact that the soil of the experimental field is low in N (0.03%) can be seen as an important factor for the reaction of rapeseed to N fertilizer. When soil N content is inadequate, N addition increases significantly seed yield in rapeseed (Christensen et al., 1985). Nevertheless, some authors noted a stagnation or even reduction in seed yield at high rates of N fertilizer (Hocking et al., 1997; Cheema et al., 2001).

Ammonium sulfate had higher seed yield (2819 kg ha^-1) than other N sources (Table 4). AS fertilizer, which is physiologically acidic, is more efficient than AN -just as in research field soils- especially in alkaline environments where lime content is high. Since AS decreases pH, it leads to the dissolution of many micro elements and thus makes them available for the plant (Kacar and Katkat, 2007). AS contains S different from the other N sources. Sulfur is probably the most important soil fertility factor to consider when growing rapeseed (Franzen, 1997). Sulfur requirements for rapeseed are higher than most crops (Fismes et al., 2000). Rapeseed takes up sulfate-S. The source of S fertilizer may be ammonium sulfate (21-0-0-24S) or another available fertilizer containing sulfate. In this trial, sulfate content of research soils was low (3 mg kg^-1). Based on this result, it was estimated that the positive effect of AS on seed yield of rapeseed comparing the other N sources in this study may be associated with S content in AS. Investigations in the North East of Scotland, where soil S content is low, appropriate S application to rapeseed resulted in quadrupling yield (Walker and Booth, 1992). Similar results were obtained in field experiments of Rathore and Manohar (1989), Fismes et al. (2000) and Khan et al. (2002).

Seed yield increased in response to each increment of added N, but decreased with additional N from 100 to 150 kg ha^-1 in AS while decreased from 150 to 200 kg ha^-1 in other sources (Table 4), which caused significant F x N rate interaction for seed yield (Table 3). The highest seed yield was obtained with AS x N₁₅₀ (3435 kg ha^-1) and the lowest yield with AS x N₀ (1901 kg ha^-1; Figure 1d). Yield increased in a quadratic way for N sources (Figure 3). Regressions between AS, AN, and urea and N application rates were significant (P < 0.01). High R² in N sources (R² = 0.76 for AN, R² = 0.97 for AS, and R² = 0.86 for urea) indicates a close relationship between seed yield and N rates. Jackson (2000) has reported similar results.

Success or failure in the use of N fertilizers depends as much on climate conditions, variety and application method as on the features of the soil. Turkey’s annually average rainfall is 643 mm, but in Konya region is 322 mm. Both annual rainfalls in Konya are lower than Turkey’s average and distribution changes year to year. From total rainfall, 57% correspond to autumn and winter. Winter precipitation is commonly snow and covers whole field and plants. In this research, the amount and distribution of rainfall was more suitable for rapeseed in Y2 than in Y1 (Table 2). Especially, April rainfall, when the second half of N was applied, in Y1 was considerably lower (14.4 mm) than in Y2 (50.4 mm). A disadvantage of spring topdressing N on winter rapeseed is that rainfall is required to move N into the root zone before it is available to the rapeseed plant. If spring season is very dry, N fertilizer on the soil surface may not be available to the rapeseed, and some N may be lost to the air through volatilization. Bayrakli et al. (1995) reported N losses from the soils with lower water content when N is applied in spring. Late N applications in spring or a long drought period after N application may not be useful for the plants, whereas rainfall and increases in temperatures after the N application increase seed yield.

Oil content

Year, F and N rate affected oil content significantly (Table 3). In the first year of the study, the determined oil content was 42.8% and in the second year increased to 44.3% (Table 4). Climatic conditions could be considered as a determining factor for oil production. In Y1, the reduction in oil content was thought to be result of unsuitable weather conditions. It is likely that increased temperature and water stress during seed filling was a major cause of reduced oil concentration. Similar results have been reported by Hocking and Stapper (2001) and Özer (2003).

Effects of N sources on the oil content of rapeseed were significant (Table 3). Oil content obtained from AS fertilization (44.32%) was higher than that of the other N sources and can be attributed to 24% S content, because S plays an important role in the chemical composition of seed and increases the percentage of oil content of seed (Khan et al., 2002). In general, N fertilization without S reduced total oil production due to the decrease in yield (Joshi et al., 1998). Besides, water shortage occurring during flowering or pod-filling stages may favor increased protein content and thereby decreasing oil content (Bouchereau et al., 1996).

Oil content increased in a quadratic way in N sources (Figure 4). High R² in N sources (R² = 0.94 for AN, R² = 0.88 for AS, and R² = 0.96 for urea) indicates a close relationship between oil content and N rates. Different N rates significantly affected percentage seed oil content (Table 4). Seed oil content varied from 39.17% (N₀) to 46.23% (N₁₀₀). The data showed that seed oil content increased 18.02% with increasing N application from 0 to 100 kg ha^-1 and then it decreased significantly. The possible reason for the decrease in oil content with N increase may be due to the fact that N is the major constituent of protein so it might increase the percentage of seed protein, as a result there might be decrease in the percentage of oil content since it has inverse relationship with protein. The results agree with those documented by Jan et al. (2002) and Özer (2003).

Protein content

Seed protein content was affected by Y, F and N rate (Table 3). Mean comparison of the 2 yr data revealed that higher protein contents were recorded in Y2 (Table 4), and can be attributed to plant stress in March-April 2001 when the rainfall and relative humidity were low (6.6 and 14.4 mm, and 53.9 and 53.0%, respectively; Table 1), decreasing seed yield and increasing seed protein content. Fowler et al. (1990) reported a negative correlation between soil water content and seed protein content at all development stages. Besides, water shortage occurring during flowering in Y1 (Table 2) may favor decreased oil content and thereby increasing protein content.

Rapeseed is not only an oilseed crop, but also contains a relatively high seed protein concentration (> 400 g kg^-1 oil-free meal) and its meal is used as a protein supplement for animals and possibly will be for humans in the near future. Because of its high protein content, rapeseed and other Brassica species in general require sufficient N during their growth for protein synthesis (Wang et al., 2008). In this research, the maximum seed protein concentration (23.89%) was achieved at N₁₅₀ rate_, which showed a 17.34% increase over the control (Table 4). Ogunlela et al. (1990) reported that N concentrations in rapeseed seeds increased with increasing N rates. These results are consistent with those reported by Bilsborrow et al. (1993) and Özer (2003).

There was no difference in seed protein content over years and N rates with urea (22.82%) and AN (22.76%) (Table 3 and Table 4). Protein content increased in a quadratic way for N sources (Figure 5). High R² in N sources (R² = 0.91 for AN, R² = 0.86 for AS, and R² = 0.90 for urea) indicates a close relationship between seed yield and N rates. Seed protein content generally increased with increasing N rate, with concomitant decrease in oil content (Asare and Scarisbrick, 1995). Strong negative correlation between oil and protein content in seeds was reported by several authors (Taylor et al., 1991; Cordeiro et al., 1993; Chamorro et al., 2002). Y x F and Y x N rate interactions were significant (Table 3), which means that the effects of fertilizer N sources and doses on the seed protein content differed over years.

CONCLUSIONS

The soils of Central Anatolia are generally alkaline, have low organic matter content and available N, but have high lime content. Because of that, N source is important for rapeseed production. According to our study, yield and quality of winter rapeseed was influenced by N fertilizer sources and doses. As the mean of 2 yr, 150 kg N ha^-1 rate was optimum and ammonium sulfate was suitable N source for winter rapeseed on calcareous soils in Anatolia. Maximum seed yields were obtained with ammonium sulfate at 150 kg N ha^-1, while minimum seed yield was produced by control plots, which clearly suggest the importance of N sources and doses for higher seed production in rapeseed crops. However, significant yield increases were observed only up to 150 kg N ha^-1 in all N sources. The results of the present study may be helpful for the recommendation of optimum N source and rate in winter rapeseed production in similar climatic and soil conditions.

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