Matrices unitarias Matrices ortogonales

The increase of the world population will demand a higher food production, which can be achieved by increasing crop yields and applying a sustainable approach through a responsible use of land and water and enhanced food diversity. Relying solely on chemical weed management can be unsustainable both to the environmental impact of herbicides and their residues and when weed populations develop resistance to herbicides (Mace et al., 2007). No single management technique is perfect for all weed control situations, a multiple management actions is preferable for an effective control. One alternative is to apply current knowledge of agricultural practices on weed populations in order to design new cropping systems that would require small quantities of herbicides to manage weeds. Since the mid-1960s, integrated pest management has been promoted by the FAO worldwide as preferred strategy for pest control (Singh et al., 2006). Integrated pest management is “a decision support system for the selection and use of pest control tactics singly or harmoniously coordinated into a management strategy, based on cost-benefit analyses that take

into account the interests of and impacts on producers, society, and the environment” (Norris et al.,

2003). According to this definition it is necessary to evaluate all the available pest control techniques and the integration of appropriate measures that discourage the development of pest populations and keep pesticides and other interventions to levels that are economically justified and reduce or minimize risks to human health and the environment. Consequently an integrated pest management aids the growth of a healthy crop with the least possible disruption to agro-ecosystems and encourages natural pest control mechanisms (FAO, 2003). The strategy of using an integrated selection of management techniques has been developed for use in a variety of pest control situations, including plant pests, or weeds. Integrated weed management is what is known as integrated pest management with the focus on weeds. Integrated weed management relies on weed management principles that have proved to be suitable for long-term weed containment (Barberi,

34

IWM

2002), by combining the use of cultural, mechanical, thermal, and chemical means based on ecological approach (Figure 3).

According to an integrated weed management, a farmer needs to predict the likely outcome of different strategies, so that rational decisions can be made. Critically, weed management advice needs to be tailored to the individual field conditions, because weed infestations vary substantially, both within and between fields (Moss, 2008). From the standpoint of crop protection, integrated weed management has three principal objectives (Liebman et al., 2001):

a. Weed density should be reduced to tolerable levels. Several experimentations have demonstrated that the relationship between crop yield loss due to weed competition and weed plant density could be described by a rectangual hyperbola (Cousens, 1985, Moffitt and Bhowmik, 2006). However, this relationship is strongly affected by different factors

35

such as weather and soil conditions (Lindquist et al., 1996; ). Despite this relationship, the total eradication of weeds may be excessively expensive depriving farmers of the ecological services that certain weeds provide.

b. The amount of damage that a given density of weeds inflicts on an associated crop should be reduced. The crop yield damage caused by weeds could be reduced, not only by reducing weed density but also by minimizing the resource consumption, growth, and competitive ability of each surviving weed. These objectives can be reached by delaying weed emergence compared to crop emergence and increasing the proportion of available resources captured by crops.

c. The composition of weed communities should be shifted toward less aggressive easier- to-manage species. Weeds act differently in their relationship with the crops. As a consequence, it is desirable to balance the weed community composition within a agro- ecosystem toward a preponderance of species that crops and farmers can tolerate. This can be achieved by selectively and directly suppressing undesirable weed species, while manipulating environmental conditions to prevent their re-establishment.

An integrated weed management strategy minimizes the affect of weeds but certainly does not eliminate them all (Liebman et al., 2001). Weeds will be accepted as a normal and manageable part of the agricultural community. One goal of integrated weed management is to maintain weed populations below an economic threshold level by reducing emphasis on strategies of eradication and promoting a strategy of containment for potential increase in weed diversity (Cousen, 1987). The level of effectiveness depends on the balance between the characteristics of the weeds present and the management tools available to growers. The balance between weed communities and management tools could deteriorate if attention is not given to management of weeds at landscape level, and to preservation of herbicides and ecological control tactics in the face of evolutionary response of weeds. Management of weeds over large areas and long periods of time requires an expanded perspective on weed community dynamics and weed evolution (Cardina et al., 1999). Integrated weed management attempts to address the underlying causes of a weed infestation, rather than just focusing on controlling visible weeds. This is done by targeting the different stages of the weed’s lifecycle and undertaking measures that will prevent weed reproduction, reduce weed emergence, promote seed bank depletion and minimize weed competition with the desired crop (Streibig, 1979; Ferron and Deguine, 2005). This point of view provides an opportunity for producers to develop weed management systems that integrate prevention with control tactics

36

(Figure 4). According to this approach, an integrated weed management may provide a more sustainable techniques for crop production by reducing the reliance on external inputs that characterizes conventional agriculture.

In order to reach the desired level of weed suppression it is necessary to use specific weed management practices. An integrated weed management plan must be individual, practical, economically sound and flexible. It is important to note that the most effective and economical weed control plan always requires several types of approach. In an ideal integrated weed management strategy it is essential to consider the cultural, mechanical, biological and chemical methods contained in the weed management toolbox and each component contributes to the overall level of weed control like several “little hammers” (Liebman and Gallandt, 1997). All successful weed control programs analyze the various tactics available to control the existing weed problem (Figure 5) and this is followed by making a plan to integrate the various tools into an effective weed control system according to the crop, the environment, and the objectives of the farm. Without this knowledge it is impossible to evaluate the impact of weed control tactics on a given weed population. When the strategy is well defined, growers must consider “what can be done” (scientific question) and “what should be done” (moral question). Indeed, some methods that could be scientifically possible may not be socially, culturally, politically, or environmentally desirable (Zimdhal, 2007). A final important step is to evaluate the yearly success of a weed management program to verify and identify tactics to consider in the future. As expected the long term objective

Figure 4. A conceptual framework for developing a broader perspective with weed management that

37

Cultural factors and pre-drilling treatments - Cultivations

- Non selective herbicides - Current crop and variety - Future crops

Field history - Previous crops - Previous cultivations - Previous herbicides use - Soil type

-Weed resistance status

Identifies risks and sets defaults for control strategy

Decision