1
Weeds: History, Definition, Classification, Harmful and Beneficial Effects

1.1 Role of Weeds in Human's Activities

Weeds have played an important role in almost everyone's lives and activities, from Biblical times to the present. In the Bible, Jesus spoke of the parable of a man who "sowed good seed but his enemy sowed tares among the wheat" (Matthew 13:24-30, Holy Bible, King James version). The tare referred to the weed "darnel" (Lolium temulentum), or "zizania" (botanical term for wild rice), or ryegrass, a weed in wheat (Zimdahl 2013). Weeds play a most important role to workers in agriculture, particularly in farmers' lives. Jethro Tull, a British farmer, called weeds "hurtful and uninvited guests that devour crops," and his inventions, the grain drill and horse-drawn hoe, revolutionized agriculture and enabled removal of weeds between rows, a highly significant contribution to weed control (Tull 1731). Rice farmers in the Philippines observed that a few days after sowing rice, the rice seedlings that emerge are "lost in the profusion of the green cover of grass weeds which dominate the field after the first rains in May" (Velasco et al. 1961). Rice farmers in Japan observed that "after transplanting, the farmer's duty is weeding" (Miyazaki 1697, cited by Matsunaka 1983). Corn, soybean, and cotton farmers in southern and midwestern United States complain of the formerly harmless Amaranthus spp., Amaranthus palmeri, and Amaranthus tuberculatus that turned into "monster weeds" by thoroughly covering their fields during late August, with some fields completely lost (Bensch et al. 2003; Hampton 2009; Davis et al. 2015; Sfiligoj 2021). Poets, however, regard weeds in a more pleasant and positive manner. James Russell Lowell (1819-1891) refers to a weed as "no more than a flower in disguise," while Ralph Waldo Emerson (1878) looks at weeds as "plants whose virtues have not yet been discovered." No matter what weeds mean to different people, weeds have one thing in common. Weeds are pests, like insect pests, plant pathogens, nematodes, mites, rodents, and vertebrate pests. When growing with crops, weeds compete for water, nutrients, sunlight, and other growth resources. If left uncontrolled, weeds will deprive the farmer of its ultimate objective, which is to get high yields from his crops.

Weed science is the scientific discipline dealing with the study of weeds and their relationship with, and effects on, crops. It determines practices and strategies on how to control and/or manage weeds to minimize the adverse effects that result from crop-weed relationships. In order to control weeds effectively, one has to know how the weed grows, its morphology, and physiology, how it reproduces, what are its strengths and weaknesses, when is it most vulnerable to control, how do chemical or nonchemical control methods affect its morphological and physiological properties, and why and how does it adapt to the various methods of controlling them? These studies are an integration of the various disciplines and research areas involving plant or weed biology, chemistry, ecology, genetics, physiology, evolutionary biology and ecology, and molecular biology and genomics, as well as economics and social sciences. Weed management deals with various methods to manage or control weeds, ranging from cultural, mechanical, chemical, and biological practices. The best strategy is to combine any of these methods in a diversity of approaches and not rely on just one control method.

This chapter introduces weeds, weed science, and weed management and presents some historical perspectives of weed science in the world and in the Asian tropics. It also discusses the various definitions of weeds from agriculturists' and ecologists' viewpoints, and some thoughts about the origin and evolution of weeds, what are their harmful and beneficial effects, and how they are classified. Examples of some of the common and most troublesome weeds in rice, wheat, corn, soybean, and various cropping systems are also presented.

1.1.1 Brief Historical Perspective: History of Weed Science in the World

Although the origin of weeds began with the start of agriculture some 10,000 years ago, the science of weeds and weed management began much later, only about 1000-2000 years ago. During ancient times, farmers were aware of weeds that compete with crops for food and growth resources. But because their effects were not as visible and damaging as those caused by insects or plant pathogens, weeds were regarded as part of agriculture which farmers had to live with and which had to be removed when they get in the way of growing crops (Timmons 1970). Farmers during early times removed weeds growing with their crops with handweeding or with simple hand-held implements to help facilitate removal of weeds such as sticks, hoes, bolos, scythes, sickles, and machetes. As agriculture progressed, labor-saving devices were invented beginning in the 18th century. In 1701, Jethro Tull, a British farmer, invented the seed drill (or grain drill), which revolutionized agriculture. It also marked a significant event in weed control, because the grain drill enabled seeds, formerly broadcast, to be planted in rows, which made easier recognition of weeds and enabling faster handweeding (Timmons 1970; Sayre 2010). Jethro Tull also invented the horse-drawn hoe or cultivator in 1722 to enable cultivation between the crop rows and remove weeds between the rows. In 1731, Jethro Tull published the book, Horse-hoeing Husbandry, in which he called weeds as "inutiles herbae or herbae noxiae," and described them as "plants that come up in any land of a different kind from the sown or planted crop and are unprofitable, hurtful, uninvited guests that hurt and spoil the crop by devouring what the farmer has provided for its (crops) sustenance; the best way to destroy them is to pull them up, roots and all, out of the rows, while they are young." In inventing the hoe, Jethro Tull wrote in his book, "the hoeing husbandry will probably make an utter riddance of all sorts of weeds" (Tull 1731, original from University of California, digitized September 2007).

Transition from horsepower to tractor power occurred in the nineteenth century with the invention of the tractor-mounted cultivators and mechanical weed control (Timmons 1970). From 1920s to 1990s, more diverse and modern forms of mechanical tractor-powered machines were invented for mechanical weed control such as the flame cultivator, electric cultivator, heat or steam cultivator, and devices that kill weeds with steam or electricity. These were used starting in the 1940s for selective weeding in cotton and other row crops. Other nonchemical methods such as crop rotation, time and method of planting, and use of competitive cultivars were also practiced. However, intensification of agricultural production leading to monoculture and growing of a single crop in vast areas of lands consequently resulted in increasing weed populations infesting crops each season.

But the most significant event that advanced the discipline of weed science was the discovery of herbicides for weed control. Use of chemicals started in the 1890s and early 1900s, initially with inorganic chemicals such as sulfuric acid, arsenicals, sodium chlorate, sodium borate, ammonium sulfamate, and dinitrophenols (Timmons 1970). In the 1940s, the organic herbicides, the phenoxys, were discovered starting with 2,4-D, signaling the start of selective chemical control. The first phenoxy herbicides, 2,4-D, 2,4,5-T, and MCPA, were discovered simultaneously and independently by four groups of workers in the United Kingdom and the United States in the 1940s (Troyer 2001). Because of wartime and commercial secrecy during World War II, publications pertaining to the discovery were kept secret causing some confusion on who are the original discoverers. According to Troyer (2001), all four groups of workers deserve credit for this significant discovery that revolutionized weed science and clarified the chronology of the events of discovery of the hormone herbicides.

The discovery and development of the different chemical groups, starting with phenoxys in the 1940s, to the most recently developed herbicide groups such as pyridinecarboxamides, benzoylpyrazoles, and carbamoyltriazolinones in the 2010s up to 2020s, are discussed in Chapters 6 and 7. The first herbicides developed from 1950 to 1970 were listed by Timmons (1970), and herbicides developed from 1971 to 2005 were listed by Appleby (2005). Herbicides developed after 2005 up to the present are also discussed in Chapters 6 and 7, together with their modes of action, uses, and application rates and times. From 15 herbicides in the 1940s, the number of herbicides developed and used increased to 25 herbicides in the 1950s to 281 herbicides in the 2020s, covering 68 chemical families and 31 target sites (HRAC 2020; WSSA 2021; Heap 2022).

Several developments in agricultural research made a significant impact and contributed to the critical role of weed science in crop production. In the 1960s, the Green Revolution and discovery of dwarfing genes for wheat and rice gave rise to nitrogen-responsive high-yielding rice and wheat cultivars which more than doubled...