Weed Biology and Weed Management in Organic Farming

Abstract

Weed biology, including the ecology, physiology and population dynamics of weed species, does not differ from plant biology apart from the notion that the plants under investigation are considered to be “unwanted”. Weeds are unwanted and undesirable plants which interfere with the utilization of land and water resources and thus adversely affect human welfare (Rao, 1999). Weed biology research consequently aims to generate knowledge that is expected to be applied in the practical control of weeds, and should include integrated research, from basic to applied, with all elements contributing to real improvements in weed management (Moss, 2008). Management of weeds is performed for the benefit of different interests, ranging from clean and non slippery pavements, to minimizing yield losses in agriculture. The occurrence of weeds in agricultural crops leads to substantial yield reductions causing economic losses all over the world. Crop damage from weeds generally is larger than from other pests (Oerke, 2006). According to FAO (the Food and Agriculture Organization of the United Nations) and the environmental research organization, Land Care of New Zealand, weeds caused yield losses corresponding to $95 billion in 2009. This may be compared with yield losses caused by pathogens ($85 billion), and insects ($46 billion). The economic losses may even be larger if the costs for weed control measures are included (FAO, 2011). The main reason for controlling weed abundance in agricultural crops is the risk for qualitative and quantitative reductions in crop yields. Black Nightshade (Solanum nigrum L.) is a problematic weed in crops such as peas (Pisum sativum L.), beans (Vicia faba L.) and soybean (Glycine max (L.) Merr.), where it not only causes a yield reduction in the crop, but also reduces crop quality by means of contamination with its poisonous seeds (Defelice, 2003). Common ragwort (Senecio jacobaea L.) is another poisonous species which does occur in temperate grasslands and pastures where it may lead to death of cattle and other livestock (Suter et al., 2007). Not only the fresh herbage is poisonous, but also its hay and silage remains toxic (Luthy et al., 1981; Candrian et al., 1984). A quantitative reduction in crop yield due to weeds foremost is ascribed to the ability of weeds to compete for resources such as light, water and nutrients, at the expense of the crop. The relative competitive ability of weed species is determined by two groups of interacting factors. The first one consists of species characteristics, such as propagation and dispersal features and other life cycle characteristics, and potential growth rate. The second