This volume is a supplement to the previous publication Weed Management for Developing Countries (1994, FAO) hence the title Addendum, and its aim is to provide up-to-date information on new developments and progress made in weed management.
Many people involved in agriculture are of the opinion that weed management consists solely in the use of herbicides. This is not true, because in reality there are a number of weed problems that need to be controlled by the use of various strategies. Chemical control may indeed play a role, but certainly other methods exist. In the case of aquatic and parasitic weeds, for example, it can be seen that chemicals may help, but they are not the only solution to the problem. Again, with regard to the present problem of weedy/red rice in direct-seeded rice it is clear that herbicides form only a part of the solution in a raft of measures to be implemented.
This material once again emphasizes the importance of weed ecology in understanding weed problems, the need to evaluate seed banks in the soil, the importance of evaluating weed competition and the ability of crops to compete with weeds.
Some troublesome weeds and the problems they cause are also described. Invasive plants are posing serious limitations to agriculture in several countries around the world. One of these is Spathodea campanulata (African tulip tree), which is affecting several crop and forest areas in the Pacific islands. As mentioned previously, parasitic weeds continue to be a serious constraint to food legume production in the Near East and North Africa as well as to cereal production in Africa south of the Sahara. Large fallow areas are also being invaded by Imperata cylindrica, again in Africa south of the Sahara, while weedy rice is becoming a major problem in direct-seeded rice areas. All these issues are discussed here and some solutions are proposed, based on what has been achieved up to now.
Particular attention is given to water hyacinth, which, in the opinion of this author, is the most prolific weed in tropical and sub-tropical water bodies. The weed is present in nearly all water bodies in Africa south of the Sahara, in Egypt, in South and Southeast Asia, and in North America and the Caribbean. Water hyacinth is another example of the need to implement integrated weed management, where various control methods should be alternated rationally in order to reduce its stand. Although biological control seems to be the most effective control method, it alone cannot reduce the infestation of the weed to the level required.
In the chapter on management options, there are some very interesting contributions on the use of various preventive and cultural methods for weed control, with particular emphasis on cover crops. Tillage systems have gained enormous importance, particularly zero and minimum tillage, which protect the soil from erosion and preserve available moisture, but some weed species find a very favourable habitat for their growth when these operations are practised. Other solutions need to be applied where these tillage systems continue to be used.
Other control strategies are also discussed, some of which are not practised frequently, as is the case for allelopathic cultivars, which can successfully depress the growth of a number of common weeds, as well as ways to develop such cultivars. Another strategy, widely extended in hot, dry, arid and semi-arid zones, is the use of soil solarization. This is a good example of using solar radiation for agricultural processes. In several countries this method has effectively replaced the use of methyl bromide as a soil fumigant.
Visiting the web site International Survey of Herbicide-Resistant Crops (http://www.weedscience.org/in.asp), it can be observed that day by day we have more problems of herbicide-resistant weed species not only in developed nations where herbicides are currently used, but also in various crops in several developing countries where the usual applications of herbicides are requested. This site reveals that there are 276 resistant biotypes, 166 species, (99 dicots and 67 monocots) and over 270 000 fields affected by these resistant weeds. At this stage there is no other option than to adopt specific methods, firstly to predict and avoid the evolution of resistance, and secondly to provide effective solutions to farmers once the resistance has evolved. The contributions in this volume provide this advice, which obviously will have to be validated locally.
Another important paper relates to the benefits and risks in the use of herbicide-resistant crops (HRCs). Instead of establishing a series of polemics bereft of any scientific basis, the authors of this section have carried out an in-depth technical discussion. Core issues to be addressed when assessing risks from HRCs are explained in detail, and the adverse effects on weed management and the environment by the transfer of genes from one population to another are well illustrated. Gene flow may enable the resistance genes to move between HR and non-HR cultivars and thus pollute a crop which is not genetically modified.
There are a great number of cases in the world of exotic, invasive weed species, introduced either accidentally or purposely, and this phenomenon may increase along with intensive trade. Actions taken to exclude a plant species, including potentially invasive ones arriving from any country, must be consistent with the international standards regulating the movement of trade goods. The present volume introduces the topic of weed-risk assessment and provides guidelines for countries wishing to strengthen their own quarantine protocols.
It will be noted that no information is provided here on new herbicide molecules, and this is because the main developments during the last eight years have taken place through new technologies and approaches. We hope that this material will be another important tool for weed researchers and agricultural extension workers in their efforts to develop new and alternative methods of weed control, and to help farmers in the developing world to reduce unnecessary drudgery and at the same time to increase their productivity.