Classification of weeds
1. Annuals – complete their life cycle in less than one year
2. Perennials- plants that may live more than two years
1. Aquatic – plants that emerge and grow in very wet or flooded soil
2. Semi-aquatic- dry land plants that has some tolerance to sub mergence
3. Terrestrial – dryland plants
4. Parasitic – depend on other plants to take nutrient
1. Grasses – round hollow stems with solids nodes
2. Sedges – with solid, triangular or round stem, usually no nodes
3. Broadleaved weeds –expanded leaf blade and have various shape
Distinguish Characteristics of Weeds
- Rapid vegetative growth and seed production
- Propagation by plant parts: Bulbs, Rizome, Stolon, Tuber in perennial weed
- Abundant Seed production and almost viable
- Plastic (ability to germinate and grow in a wide range of environmental conditions)
- Effective means of seed dispersal
- Dormancy((impermeable seed coat, unfavorable condition, environmental factor)
Crop and Weed Competition
Component of competition and weed control
- Light- – One plant shade another, Limited available radiation
- Water – Competition for water is greater where moisture is limiting
- Nutrients – Weeds have a much requirement for nutrient. They may absorb as much as or more than the crop plant.
Crop and weed growing independently of each other or the supply of growth factors is excess of the needs of both. Crop and weed are growing in close proximity to one another and the supply of growth factors fall below the demand of both. Weed competition is greatest in the first 30-40 days after seeding or transplanting. During this period weeds should be removed to prevent yield losses due to weed competition.
Thus the first 30% of the life cycle of the crop is the best time to control weeds to prevent yield reduction due to competition.
- preplant foliar applied
The herbicides is sprayed on the existing vegetation to kill weeds before planting .eg. Glyphosate,Paraquat
- preplant soil incorporated
The herbicide is incorporate into the soil.
2. Pre emergence
The herbicide is applied to the soil surface before emergence of the crop or weeds. eg.Pretilachlor, Butachlor.
Herbicide application after emergence of particular crops or weeds. eg.2,4-D
- Foliar applied herbicides
These are sprayed directly on the foliage.
- Soil applied herbicides
Herbicide is applied germinated weed seeds on the soil surface.
- According to mode of action
a. contact herbicides
These herbicides are applied to the foliage and kill plant tissue at or very close to the site of application. Eg. (Paraquat)
b. translocated (systemic) herbicides
Herbicides are capable of movement within the plant .eg. (bispyribac sodium)
- According to selectivity
a. Selective herbicides
Herbicides that kills or stunts some plants with a little or no injury to others. eg. (Propanil, Cyhalopfop butyl)
b. Non – selective herbicides
They are toxic to all plants. eg. (Glyphosate, Paraquat)