What is weed – Definitions of Weed, Nature and Characteristics

What is weed – Definitions of Weed, Nature and Characteristics
Nweke Jerry Anayo
Department of Crop Production and Landscape Management
Ebonyi State University Abakaliki
For PROJECT Materials, Term Paper and Assignment call 08037454204

INTRODUCTION

Because of the importance of weed in agriculture, plant weeds have been the subject of much research, largely directed towards the discovery of methods for their elimination. Nevertheless, weeds are excellent subject for the study of adaptation and microevolution, for they are abundantly available, usually grow fast, and reproduce quickly and easily. Two overall strategies for management of changes in weed communities are apparent. One is to continue the present essentially responsive approach in which shifts in weed composition and development of herbicide resistance are attacked with newly developed herbicides and complex mixtures of existing materials. This approach guarantees a continuing market for new chemical technologies, but leaves the grower with a generally increasing bill for weed control.

The alternative is to take a more methodical approach in which principles are elucidated that predict the response of weeds to control measures and strategies are developed to intercept problems before they become severe. The growing interest among weed scientists in modeling the dynamics and genetic composition of weed populations is a first step in implementing this alternative approach to the management of incipient weed problems.

weedHowever, new categories of higher-level models are needed to understand and predict phenomena like species shifts, the spread of weeds within and between regions, and the evolution of herbicide resistance in taxa that are currently susceptible. Such phenomena occur at spatial and temporal scales that exceed the boundaries of farms and the attention span of individual growers. Consequently, the extension of human understanding of weeds into larger scales will make management decisions at the community, regional, and national levels both practical and desirable. Developing higher-level theory of weeds probably represents the greatest challenge for weed science in the coming century. Implementation of that understanding by farmers, communities, and government agencies may prove equally challenging. C.L. Mohler, 2001.

THE NATURE OF WEEDS

Biblical Reference

To Adam he said, Because you listened to your wife and ate from the tree about which I commanded you, You must not eat it.‘

Cursed is the ground because of you; through painful toil you will eat of it all the days of your life. It will produce thorns and thistles for you and you will eat the plants of the field. By the sweat of your brow you will eat your food until you return to the ground, since from it you are taken; for dust you are and to dust you will return.

Genesis 3:17-19, Anonymous, 1984.

The Parable of the Weeds

Jesus told them another parable:

The kingdom of heaven is like a man who sowed good seed in his field. But while everyone was sleeping, his enemy came and sowed weeds [tares, darnel, Lolium temulentum] among the wheat [Triticum sp.], and went away. When the wheat sprouted and formed heads, then the weeds also appeared.

The owner‘s servants came to him and said, Sir, didn‘t you sow good seed in your field? Where then did the weeds come from?‖

An enemy did this, he replied.

The servants asked him, do you want us to go and pull them up?

No, he answered, because while you are pulling the weeds, you may root up the wheat with them. Let both grow together until harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn.

Matthew 24-30, Anonymous, 1984.

The Parable of the Weeds Explained

Then he left the crowd and went into the house. His disciples came to him and said; explain to us the parable of the weeds in the field.

He answered, the field is the world, and the good seed stands for the sons of the kingdom. The weeds are the sons of the evil one, and the enemy who sows them is the devil. The harvest is the end of the age, and the harvesters are angels.

As the weeds are pulled up and burned in the fire, so it will be at the end of the age. The Son of Man will send out his angels, and they will weed out of his kingdom everything that causes sin and all who do evil. They will throw them into the fiery furnace, where there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. He who has ears, let him hear.

Matthew 13:36-43, Anonymous, 1984.

The concept of a weed plant is inherently human in two different ways. In an immediate sense, they are the plants we define as weedy. In a historical sense, they are plants that arose as a consequence of agricultural crop domestication, an inevitable result of the singular human act of planting a seed (or propagule). Understanding the intimate relationship of humans and plants in both of these ways can provide a more comprehensive understanding of the nature of these plant species.

Definitions of Weed

Weeds are defined as a plant out of place, thriving in habitats disturbed by humans, possessing competitive behavior, and capable of mass movement from one area to another. Human values related to disturbed and agricultural habitats, appearance, utility and biological traits dominate how we define a plant as weedy.

What is the relationship between human nature and the nature of weeds? The nature of weeds is an inevitable evolutionary consequence of agricultural natural selection under the influence of human nature. Human nature includes inherent intuitions about the natural world: taxonomy and the classification of plants we observe, eat and utilize. Human cognition is finely tuned to discern both good and bad qualities about the specific plants with which we interact. We humans are very sensitive to plant behaviors whose form and powers‘ we do not appreciate, especially in our managed landscapes. The evolutionary consequences of these human traits are the major crop-weed groups of contemporary world agriculture. Unfortunately for us, separating weedy species from desirable species is often genetically impossible.

The nature of weeds can be understood at a deeper level than definitions and human nature by observing the biological and adaptative characteristics that lead to their success. Defining what a weed is, and appreciating the traits they possess, is a good start to understanding the nature of these plants. Evolution acts on individuals in a population, so understanding the nature of the weeds we have today requires an understanding of where and when particular plants species became weeds.

Most of the common and widespread weed species we now have came as a consequence of crop domestication, planting and cultivation. These agricultural processes began about 12,000 years ago. They occurred on different continents and involved different native species available for selection as crops. Since those early origins both crops and their weeds have spread throughout the world. These crop-weed groups are the most successful invasive species in human history. The processes of plant domestication, planting and cultivation created new plant communities featuring the crop genotypes they desired. These domesticated species typically came from preexisting wild relatives selected for their crop qualities. The wild relatives interbred with their new crop derivatives and new variants joined these heterogeneous communities, forming metapopulations extending across the landscape. Some of these new weeds were in turn again domesticated, others not. Over 12 millennium this promiscuous inter-fertility and gene flow led to the world crop-weed groups we now have. The most common pattern for the origins of agricultural plants is the inter-fertile wild-crop-weed (w-c-w) plant complex in which both crop and weed were derived from the same wild progenitor species.

What plant species evolved under such close human scrutiny and management? The nature of weeds is ultimately revealed in the particular weed species that have survived to plague human-managed ecosystems to the present day. It is the properties and stability of these successful weeds that define the nature of weeds most precisely.

What exactly are these crop-weed groups? A comparison of the origins of specific crop species with the current weed flora infesting contemporary agriculture reveals the close genetic relationship of most of our major weed species and crops: crop-weed groups. It is to these crop-weed groups that we shall look throughout this book to understand the nature of weediness. The major crop-weed species groups are described here: local native cultivated and/or domesticated plant species reported in seminal publications of the original hearths of agriculture are compared to contemporary world weed species of the same genus. The crops and their centers of origin, diversity, cultivation and domestication as reported in older classical weed flora taxonomy and identification sources.

1.1 What is a weed?

The concept of a weed plant is inherently human in two different ways. In an immediate sense, they are the plants we define as weedy. In a historical sense, they are plants that arose as a consequence of agricultural crop domestication, an inevitable result of the singular human act of planting a seed (or propagule). Predictable, systematic disturbances in agricultural and other human-managed habitats create opportunity for colonizing plant species over vast areas of the earth. Understanding the intimate relationship of humans and plants in both of these ways can provide a more comprehensive understanding of the nature of these plant species.

What is the relationship between human nature and the nature of weeds? What plant species evolved under such close human scrutiny and management? To begin to answer these questions we need to look closely at the most successful invasive plant species of the last 12,000 years: crops, and their genetic fellow-travelers weeds. The roots of the nature of weeds can be found in human sociobiology, plant biogeography, archaeology, anthropology and ethnography. These scientific disciplines can provide clues to the impact of human nature on plant communities. A comparison of the origins of specific crop species with the current weed flora infesting contemporary agriculture reveals the close genetic relationship of most of our major weed species and crops.

The definitions of a weed by different Authors 

Human desires, values, and most importantly economic needs are what drive a plant being defined as a weed. The qualities by which humans define plants as weeds include disturbance, aesthetics, utility or biological characteristics. All of these definitions are the consequence of interactions with humans. Many of these definitions are anthropomorphic, plant qualities as perceived by humans. As such they reveal the plants’ relationship to us, and tell us much of how we view the nature of weeds. Below are15 definitions of weeds by different author based on their value to human, disturbances and plant behavior.

Based on human values:

Weed:

1: any plant that is objectionable or interferes with the activities or welfare of man (Anonymous, 1994)

2: a plant out of place, or growing where it is not wanted (Blatchley, 1912)

3: a plant growing where it is not desired (Buchholtz, 1967)

4: a very unsightly plant of wild growth, often found in land that has been cultivated (Thomas, 1956)

5: useless, unwanted, undesirable (Bailey and Bailey, 1941)

6: a herbaceous plant, not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering growth of superior vegetation (Murray et al., 1961)

7: a plant whose virtues have not yet been discovered (Emerson, 1878)

Based on human disturbance:

Weed:

8: a generally unwanted organism that thrives in habitats disturbed by man (Harlan and deWet, 1965)

9: opportunistic species that follow human disturbance of the habitat (Pritchard, 1960)

10: a plant that grows spontaneously in a habitat greatly modified by human action (Harper, 1944)

11: a plant is a weed if, in any specified geographical area, its populations grow entirely or predominantly in situations markedly disturbed my man (without, of course, being deliberately cultivated plants) (Baker, 1965, p. 147).

Based on plant behavior:

Weed:

12: pioneers of secondary succession of which the weedy arable field is a special case (Bunting, 1960)

13: competitive and aggressive behavior (Brenchley, 1920)

14: appearing without being sown or cultivated (Brenchley, 1920)

15: persistence and resistance to control (Gray, 1879)

Weeds and human nature

Weeds have been constant and intimate companions of man throughout his history and could tell us a lot more about man, where he has been and what he has done, if only we knew more about them.‖ J. Harlan, 1992.

The dominant theme that emerges from these definitions of weediness is human: plant behavior as a consequence of human values and behavior. The sociobiology of human nature can provide insight into the nature of weeds. Brown (1991; in Wilson and Keil, 1999) has compiled a list of human universals‖, universal behaviors exhibited by all humans and human cultures noted in ethnographic studies. Two may be relevant to the nature of weeds: classification of flora‘ and taxonomy‘. A universal human trait includes behaviors to classify and organize the plants with which we observe, eat, utilize and interact. These traits arise from the way the human brain conceptualizes the world.

Saying a plant is out of place is an essential cognitive function, or faculty, of the brain. Cognitive functions of the human brain are our core intuitions, our reasoning faculties, the way we conceptualize our world. The brain consists of multiple reasoning faculties, each based on a core intuition that is suitable for analyzing the world in which we evolved. Examples include a spatial sense, a number sense, language and a sense of probability (Pinker 1994, 1999, 2002). Among these core intuitions we are born with is:

An intuitive version of biology or natural history, which we use to understand the living world. Its core intuition is that living things house a hidden essence that gives them their form and powers and drives their growth and bodily functions. S. Pinker, 2002.

We humans are very sensitive to plant behaviors whose form and powers‘ we do not appreciate, especially in our managed landscapes. Unfortunately for us, separating weedy species from desirable species is often genetically impossible. After defining particular plant species as weeds, the next step in defining the nature of weeds is discerning their biological and adaptative characteristics. What functional traits they possess makes them so successful?

CHARACTERISTICS OF WEED

  1. Strong competitive ability

Weeds compete successfully with crops by being more aggressive in their growth habit. (Reproductive potential and vegetative spread etc.) They compete better in obtaining and utilizing the essentials of growth and development at the expense of the crons some weeds also exhibit alleopathy e.g  Euphorhia sp.

  1. Adaptation for seed dispersal

(Adaptation for both long and short distance dispersal ).

It is important that weed that produce seed have to dispersal them for better establishment and growth. As weeds are associated with human activity, thus their dispersal is still associated with human ability. (This mean that man is the better agent of weed dispersal) for example Boerhavia coccinea weeds have fruits that are very sticky and when the fruits stick on the body of animals there are dispersed. Some weeds when taking by animal remain undigested in the alimentary canal of the animals. These weed seeds are t us dispersed in the field when they pass dung e.g Fumaria officinals. Some weeds have seeds that are dispersal by water especially by Cyperus rotundus are carried to long distance places.

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