Weeds and weed control
Paraquat is used to control a huge range of weeds worldwide, but to control weeds effectively and sustainably it is important to understand them. Why does a plant become a weed? How can different types of weeds be described? What are the features of weeds and the way they grow which can be targeted by herbicides for successful control? Why is paraquat such a useful tool for farmers?
What are weeds?Weeds are usually described as unwanted plants. Weeds grow on arable land which is waiting to be planted and then a new flush of weed seedlings emerge with the crop. In perennial crops like fruit, vines, rubber and oil palm, weeds grow continuously with new growth prompted by the weather and changing seasons. Weeds are unwanted for many reasons:
- They compete with crop plants for sunlight, water and soil nutrients, reducing yields and quality.
- They may provide a habitat for pests and diseases from which these can attack the crop.
- Large, climbing or spiny weeds can make it difficult to get into the crop for pest and disease control, fertilizer application, harvesting and other operations.
DescriptionDisplacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement.
Authoritative On-line References and Resourceshttp://soilerosion.net/ This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation. and provide habitats for beneficial insects and wildlife, increasing biodiversity
DescriptionThe variety of life in all its forms, levels and combinations. Includes ecosystem diversity, species diversity, and genetic diversity (IUCN, UNEP and WWF, 1991).
Authoritative On-line References and Resourceshttp://earthtrends.wri.org/ EarthTrends is a comprehensive online database, maintained by the World Resources Institute, that focuses on environmental, social, and economic trends. Statistics on biodiversity indicators are available.. However, it is not only the effects on the current crop that count and weeds must be managed. “One year’s seeding means seven years’ weeding” the saying goes. Weeds become a problem when they reach a critical size or number, and these will depend how aggressive a particular species is. Weed management is part of any farmer’s job and paraquat is a very economic, environmentally sound and flexible tool.
Weed typesWeeds are classified on the basis of leaf shape, on their life-cycle, and on their climate or seasonal preference.
- Broadleaves or grasses
DescriptionThe leaves are "narrow" as opposed to the "broad" leaves of broadleaved weeds. Also called 'monocots' having one seed leaf opposed to 'dicots' having two seed leaves.
Authoritative On-line References and ResourcesThe International Weed Science Society represents individual associations around the world. ? Weeds have leaves in a huge variety of shapes but the grasses with long narrow leaves readily stand-apart and almost all the rest are grouped as broad leaves. Broadleaved weeds
DescriptionThe leaves are "broad" as opposed to the "narrow" leaves of grasses. Also called 'dicots' having two seed leaves, while grasses are 'monocots' having one seed leaf.
Authoritative On-line References and Resourceshttp://www.iwss.info The International Weed Science Society. have seeds with a pair of storage organs which after germination become the first ‘leaves’, actually the cotyledons – hence the other name often used: dicotyledons, or dicots.
- Grasses are monocotyledonous (monocots). There are a few exceptions in that the odd monocot can have broad leaves, like the important tropical weeds in the Commelina genus. Another grass-like class with relatively few members is the sedges. These are important because they are difficult to control. In fact, purple nutsedge (Cyperus rotundus) has been called the “world’s worst weed”.
DefinitionWeeds that complete their life cycle within one growing season, or year. From seed to flowering to seed before the year ends.
Authoritative On-line References and ResourcesThe International Weed Science Society represents individual associations around the world. or perennials? Annuals germinate, flower and set seed in a single season. Perennials have underground storage organs, often rhizomes, which enable them to grow for many years. They can reproduce both from seed and by extending their rhizomes from which daughter plants grow. A third type germinates in one season and flowers in another. These are biennials Passing through winter prompts them to ‘bolt’ by elongating a tall flowering stem.
- Cool season or warm season, etc.? Weeds have evolved to grow best in particular temperature and daylengths. These tend to define the crops they are found in and the time at which they germinate, eg winter annuals or summer annuals. Also, in tropical climates with dry seasons and rainy seasons, some species tend to be more prevalent in one season than the other.
Features of weedsWeeds are vulnerable to herbicides if their internal biochemical processes can be accessed. Once inside a plant cell an effective herbicide will disrupt normal functioning leading to death. However, to kill the plant itself, all the various growing points – at shoot and root tips, and the buds on stems and rhizomes - must be killed too. Herbicides enter plants by two main routes:
- Directly into shoots
- Through the soil into seeds, roots or rhizomes