There has been and is an intimate, bidirectional relationship between molecular biology—a most basic biological science—and weed control, the most base as well as basic agricultural practice. Weed control, as practiced for millennia, is base because it has employed the majority of mankind on this planet since crops were domesticated yet has been the lowest paying mass employment known. Weed control can be one of the most physically taxing tasks, yet typically more women are employed in weeding than men. Without weed control there is virtually no crop to harvest. The advent of mechanization (tractor plowing and cultivating) replaced much of the hand labor in the developed world as well as the developing parts of the third world. Mechanical weed control is fraught with high energy costs, facilitates

International Standards Organization (ISO) accepted common names of herbicides are used throughout. The chemical names can be found in Tomlin (1994). Occasionally, molecular biologists have used nonaccepted common names for herbicides. Their synonyms will be given in parentheses at first use in this chapter.

soil erosion and compaction, and has been largely replaced by chemical weed control using herbicides that can selectively eradicate weeds from crops. As countries industrialize and develop economically, cheap farm labor becomes unavailable, increasing the necessity for cost-effective chemical weed control. Selectivity between a crop and its associated weeds is no mundane trait to find in a chemical; it is based on the immense biodiversity of metabolic pathways abounding in the plant kingdom. Too often there is no selective chemical that can control a particular weed in a particular crop, and the weed infestations understandably worsen based on simple ecological, genetic, and biochemical principles.

No single agricultural practice used extensively in the past to control weeds has remained as useful as initially presumed. After one weed problem is solved, there are other weed species that will fill the ecological vacuum left behind. Thus, either weeds have evolved resistance to most commonly used herbicides or weed species that had never been controlled by the particular herbicide replace the weed species controlled. This is most apparent in monoculture, where weeds closely related to crops are becoming the greatest problems: grass weeds in grain crops, legumes in soybeans, bras-sicas in oilseed rape, etc. As most selectivities between crop and weed are due to catabolic degradation of the herbicide by the crop, closely related weeds are to be expected to have catabolic pathways similar to those of the crop. This is one major reason that transgenic herbicide-resistant crops (T-HRCs) have become so useful and that biotechnology has been utilized to produce such crops as well as to find new herbicide targets (see the excellent review by Cole and Rodgers 2000). Selectivity is enhanced by inserting exogenous resistance genes into the crops or by selecting natural mutations. This also leads to one major concern about T-HRCs, that the transgene will genetically introgress into related weeds.

These and other issues are reviewed in this chapter. A call for new concepts of using molecular biology for weed control that do not necessarily utilize HRC, with some possible examples for consideration, is made in the article from which this chapter is derived (Gressel, 2000). Indeed, it is highly unfortunate that a World Bank report (Kendall et al., 1997) dealing with how molecular biology can alleviate constraints to world food production has delineated only insects, pathogens, and lack of water as the major constraints where molecular biology can assist. That report ignored weeds, even though the greatest variable input into agriculture is the cost of weed control, whether (fe)manual, mechanical, or chemical, with or without transgenics. And there are many unsolved weed problems lowering yields over vast areas of the world. If the weeds could be controlled, crop yields would increase without additional inputs.

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