Leek is a major vegetable crop worldwide. Most of its production and consumption is in western Europe, with France and Belgium being the world's greatest producers (De Clercq et al., 1999). In Turkey, ≈290,000 tons of leek was produced in 2002 from an estimated 12,083 ha. Dry leek is the most consumed vegetable in Turkey after potato (Ermenli and Yoldaş, 1999).
Like many other vegetables, leek is a weak competitor against weeds, thus requiring high costs for weed management (Baumann et al., 2000). Growth and yield of leek are substantially reduced by weed competition for nutrients, water, and light. Yield reduction depends on multiple factors, including weed species, weed density, time of weed emergence relative to crop emergence, weed distribution, soil type, soil moisture, pH, and soil fertility level.
In Turkey, there are currently no herbicides registered for leek. Cultivation and hand hoeing are the common practices used. These techniques are time-consuming and require considerable labor; therefore, they are not usually cost-effective and appropriate (Ngouajio et al., 1997).
Environmental and health concerns and growing markets for high-value and ecologically produced vegetables have encouraged farmers to search for production techniques that could help reduce or eliminate synthetic herbicide use (Baumann et al., 2000). Integrated weed management (IWM) combines preventive and curative weed control methods, based on ecological principles, to address environmental and economic concerns. Under IWM, herbicides may be applied as corrective measures if other weed management tactics fail to protect a crop adequately. In organic farming, nonchemical methods are generally preferred and synthetic chemicals are not allowed. In organically grown crops with weak competitive ability, like onion, leek, and carrot, more than 400 h·ha−1 of hand weeding may be necessary to reach a level of weed control comparable with that obtained with herbicides (Vereijken and Kropff, 1996). Because labor-intensive methods of weed control are not sustainable, preventive strategies, decision-making tools, and improved control technologies have been suggested (Kropff et al., 1996).
With respect to improved decision making, application of the critical period threshold model (Nieto et al., 1968) has been suggested for reducing losses resulting from weeds in vegetable production systems (Roberts, 1976; Zimdahl, 1988, 1993). By controlling weeds during the critical period, reductions in the yield and quality can be minimized (Müller–Schärer and Baumann, 1993).
The critical period for weed control is important for the development of alternative weed management strategies (Swanton and Weise, 1991). It allows identification of appropriate timing for weed management and aids in understanding the affect of weed populations on crop yield. Research on the critical period for weed control is usually performed by measuring the affect of early-season weed competition and late-season weed competition (Nieto et al., 1968). Early-season weed competition is achieved by allowing weeds to emerge and grow with the crop for certain predetermined times, after which all weeds are removed in a timely manner until harvest. For late-season weed competition, the crop is kept free from weeds until certain predetermined times, after which weeds are allowed to emerge and compete with the crop for the remainder of the growing season.
Many studies have been conducted to determine the critical period for weed control in various crops under various environmental conditions (Bryson, 1990; Buchanan et al., 1980; Dawson, 1970; Evans et al., 2003; Knezevic et al., 2003; Rogers and Buchanan, 1986; Van Acker et al., 1993). However, there are limited published studies on the critical period for weed control in vegetables in general and leek in particular (Amador–Ramirez, 2002; Baumann et al., 2000; Buckelew et al., 2006; Tursun et al., 2004; Weaver, 1984). Because of the differences in climatic conditions and weed populations, result of studies conducted in different environments or on different crops may not apply to other systems (Bukun, 2004; Evans et al., 2003; Knezevic et al., 2003; Van Acker et al., 1993). The critical period for weed control in leek has not been determined in many regions, including Turkey. This information could help leek producers improve the efficacy of their current weed management systems and reduce yield loss resulting from weed competition. The objectives of this study were, therefore, to determine the critical period for weed control in leek under conditions found in Turkey and to investigate the effect of weed interference on leek yield.
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