ReviewIntegrated nitrogen management strategies to improve seed yield, oil content and nitrogen efficiency of winter oilseed rape (Brassica napus L.): A review
Introduction
Winter oilseed rape (Brassica napus L.) production has undergone a rapid increase in recent decades. Given this development, the question arises whether such an increase might be enhanced by integrated N-management strategies. The economy of winter oilseed rape cultivation is determined primarily by the attainable seed yield and less by the oil content. Processes of yield formation are highly variable and depend on genetic, environmental and agronomic factors as well as interactions between them (Sidlauskas and Bernotas, 2003). The yield potential of a crop is a theoretical assessment of the maximum yield that can be generated when high yielding biological material is grown in an optimum physical–chemical environment. The biological yield of winter oilseed rape is the product of growth rate and duration of vegetative period (Diepenbrock and Grosse, 1995, Diepenbrock, 2000). In addition, the harvest index (HI), the proportion of seed dry matter to aboveground biomass, is a major parameter that limits yield. For winter oilseed rape, it varies between approximately 0.28 and 0.50. Thus, seeds represent 28–50% of total biomass and the remaining aboveground biomass represents 72–50% of total aboveground biomass (Rathke et al., 2005). For a given level of aboveground biomass, a greater HI represents a higher seed yield (Diepenbrock, 2000). In Europe, an economic seed yield of winter oilseed rape between 3 and 4 tons per hectare is normally produced. Under favorable site and production conditions seed yield of 5 t is recorded in central Europe. Based on a simple calculation of the theoretical potential seed yield of winter oilseed rape, Daniels et al. (1986) illustrated a potential for improving yield up to 6.9 Mg ha−1. The yield increase during the last decades is not only due to breeding of high-potential cultivars or sophisticated production engineering but also to higher resource inputs. Here, mineral nitrogen (N) fertilization is a crucial factor (Dreccer et al., 2000, Rathke and Schuster, 2001, Barlóg and Grzebisz, 2004a, Barlóg and Grzebisz, 2004b). Much common and growth stage specific information on N-fertilization of winter oilseed rape is available (e.g. Rodgers et al., 1986, Bilsborrow et al., 1993, Milford et al., 1993, Asare and Scarisbrick, 1995, Andersen et al., 1996, Rathke, 1999, Kappen et al., 2000, Behrens et al., 2001, Behrens, 2002, Rathke et al., 2005, Rathke et al., 2006). However, little is known about N-use efficiency even though winter oilseed rape is a heavy user of N and available N is the most limiting source in many areas of the world. To produce 0.1 t of seeds, the whole crop accumulates approximately 6 kg N. Thus, the N-efficiency of winter oilseed rape, which is defined as produced seed dry weight per unit of accumulated N-fertilizer, is relatively poor (Aufhammer et al., 1994, Schjoerring et al., 1995, Kessel, 2000, Rossato et al., 2001). Moreover, increased N-supply is linked to decreased N-efficiency, mainly due to altered N-uptake during different growth phases. Winter oilseed rape accumulates 25–30% of the total N-uptake (40–80 kg N ha−1) from the soil during autumn (Cramer, 1993). Subsequent N-uptake is high until flowering but low during the reproductive phase associated with an incomplete N-translocation from vegetative organs to seed (Lickfett, 1993, Wiesler et al., 2001a, Wiesler et al., 2001b). Therefore, cropping of winter oilseed rape is characterized by high N-surpluses resulting from N-fertilization exceeding the N-demand of seeds (Lickfett, 1993, Dreccer et al., 2000, Behrens, 2002, Lickfett, 2001). Excessive N-fertilization and other management practices can potentially lead to high nitrate leaching losses (Di and Cameron, 2002). As a result, evidence of increasing nitrate leaching losses from soils under various land-use systems has elevated the interest and need to find better mitigating strategies (Chang and Ents, 1996, Catt et al., 1998, Silva et al., 2005).
Improving N-efficiency of winter oilseed rape will reduce the potential for environmental pollution and improve economic returns. In terms of the environment, the crop should receive optimum doses of N to insure yield development and to avoid subsequent N-leaching from the soil (Aufhammer et al., 1994, Shepherd and Sylvester-Bradley, 1996, Behrens et al., 2001, Barlóg and Grzebisz, 2004b). Proper use of N-sources is also required to optimize the economic return (Grant and Bailey, 1993).
Several integrated N-management strategies have been successfully developed in the past to improve the N-efficiency for numerous agricultural crops (Wiesler, 1998a, Wiesler, 1998b, Horst et al., 2002). Integrated nutrient management strategies that may enhance N-efficiency in plant production are summarized in Table 1. The effects of cultivar and previous crop as well as rate, timing and form of N-application on growth, yield, N-uptake and N-utilization have contributed to a better knowledge of the stage dependent N-demand of several crops (Wiesler et al., 2001a). Based on this information, the present review will highlight the agronomic importance of integrated N-management strategies on seed yield and N-efficiency of winter oilseed rape. Earlier, Grant and Bailey (1993) summarized the most important information on N-fertilizer management in canola production, however much of their summary does not apply to autumn sown winter oilseed rape due to differences in growth and development between the two crops. Walker and Booth (2001) summarized some agricultural aspects of winter oilseed rape, while Diepenbrock (2000) reviewed the most important biological processes that determine the yield of winter oilseed rape.
Section snippets
Definition of N-efficiency
Discussion on N-efficiency is complicated by the fact that there is no sole or generally accepted definition of N-efficiency. Different definitions are used in literature to describe the agronomic and physiological range of N-efficiency referring to external and internal N-status:
N and yield formation
Growth and development of winter oilseed rape is more complicated to describe than that of cereals, for example, because some of the principal stages overlap widely. The life cycle of winter oilseed rape is divided into ten principal stages (Lancashire et al., 1991). The use of numerical keys is useful for timing of management operations (Mendham and Salisbury, 1995). Definitions and codes of stages of development in oilseed rape are contained in Table 2.
For autumn-sown rapeseed the juvenile
Influence of environmental factors on plant growth and N-efficiency
Rapeseed is grown worldwide in different climates. Even extreme conditions for agricultural cropping, e.g. low temperatures or near-drought, are tolerated (Diepenbrock and Grosse, 1995). Independent of the ability to adapt to different environments, it is most important to exploit the full yield potential for a given location. Results of Daniels et al. (1986) indicated a strong relationship between growth and development of winter oilseed rape and environmental factors. Site-specific
Integrated N-management strategies
Fertilizer N-use efficiency in agriculture is generally low (Raun and Johnson, 1999). This might cause severe yield limitations where there is a lack of N-supply. Moreover, this might increase the risk of environmental pollution where high N-fertilizer doses are applied to achieve maximum yields (Horst et al., 2002). In order to improve the use of N-fertilizer and to reduce the environmental pollution in agriculture, a number of integrated N-management strategies have been developed (cf.
Further crop management practices
Apart from environmental factors and integrated N-management strategies as described above, further agricultural management practices are closely interacting with N-formation and N-efficiency of winter oilseed rape. In particular, soil cultivation, seeding, application of plant protection agents and plant growth regulators as well as soil fertilization and harvesting correspond to N-management (cf. Fig. 2).
Conclusions
Further enlargement of the growing area of winter oilseed rape will be obliged to cultivation on marginal lands resulting in reduced N-use efficiency and increased losses of N. More efficient use of N-fertilizer is essential for improving the economic output of the farm and reducing the risk of environmental pollution. Here we have shown that in oilseed rape production using improved N-management strategies will lead to substantial reductions of N-fertilization, representing up to 50% of
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