Cultivated hazelnut is native to the Black Sea coast of northern Europe, and Turkey is one of the few countries in the world with a suitable climate for its production. Hazelnut is grown in both the eastern and western regions of Turkey along the Black Sea, on sharp slopes and in areas where the soil is unsuitable for other crops. The production areas extend up to 30-km inland. Turkey produces over 70% of the world's hazelnut, followed by Italy, Spain, and the United States (Bozoğlu, 2002a), and hazelnut production has shown a general upward trend over the past decade (Bozoğlu, 2002b). The natural form of hazelnut is a multistemmed bush (called an ocak) grown in commercial orchards. In Turkey, hazelnut usually ripens from early to late August depending on the altitude and is harvested mainly by hand. The relatively high rainfall and fertile soils of hazelnut orchards in the Black Sea region also favor establishment of a wide range of annual and perennial weed species.
Weed management is critical in hazelnut production to limit competition, conserve nutrients for the trees, and improve hand-harvesting efficiency. Ocak growing systems allow enough light penetration to the base of rows for extensive weed emergence and growth. Adequate weed control in hazelnut is normally achieved with two glyphosate applications, one in early spring and the other before harvest to facilitate the collection of nuts from the ground. However, alternative weed control strategies are needed for integrated weed management programs, particularly to prevent the development of herbicide resistance among weed populations (Holt et al., 1993).
Cover cropping could be an effective component of integrated weed management in hazelnut. Winter and cool season cover crops are grown for various reasons, including prevention of nitrogen leaching, water runoff, and soil erosion; improvement of soil structure; soil enrichment by nitrogen fixation (for legume species); habitat provision for beneficial insects; microclimate modification; and weed control (Haramoto and Gallandt, 2004; Mennan et al., 2006; Ngouajio and Mennan, 2005; Teasdale, 1996; Yenish et al., 1996).
Many studies have reported positive effects of living cover crops on early season weed suppression and growth either through direct competition (Ngouajio and Mennan, 2005; Yenish et al., 1996) or through allelopathic interactions (Barnes and Putnam, 1986). Furthermore, cover crop residues, whether incorporated or left on the soil surface, can affect weed dynamics by reducing or delaying seed germination, reducing establishment, and suppressing individual plant growth. Hence, cover cropping is recognized as an important tool for weed management. Rye (Secale cereale), ryegrass (Lolium multiflorum), hairy vetch (Vicia villosa), red clover (Trifolium pratense), and annual medics (Medicago sp.) have been used widely as winter cover crops to suppress broadleaf and grass weeds in diverse cropping systems. Recently, brassica cover crops have generated considerable interest because of their potential to inhibit seed germination and seedling growth (Ackroyd and Ngouajio, 2011; Brennan and Smith, 2005; Norsworthy et al., 2011; Petersen et al., 2001). Brassicaceae species contain glucosinolates, which after enzymatic hydrolysis release isothiocyanates–well known allelopathic substances that may suppress weeds.
Weed control without the use of herbicides could be ineffective, expensive, and time-consuming in orchard management and therefore should be investigated before making recommendations to growers. Mulches used to control weeds have been reported to be beneficial for tree growth and yield (Belding et al., 2004; Childers et al., 1995; Stafne et al., 2009). Among these materials, cereal straw is one of the most commonly used products in vegetable production or orchard management. Unfortunately, little is known about hazelnut husk residues used as mulch and their allelopathic properties, which could contribute to weed suppression. Each year, ≈400,000–500,000 Mg of hazelnut husk residue is burned or left in the field after harvest (Çöpür et al., 2007). This important organic waste could be incorporated into weed management programs in hazelnut orchards if proven effective.
Hazelnut growers need alternative or supplemental weed management strategies that can provide adequate weed control while maintaining optimum yield. Therefore, the goal of this study was to improve weed management programs in hazelnut production. Specific objectives were to determine the impact of brassica cover crops on weed control and to assess possible contributions of hazelnut husk as organic mulch for weed control.
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