(a) Weed is any plant that grows in a place where it is not expected to grow, or a plant which grows out of place. Weed can also be define as a plant that grows where it is not desired in such a way that it constitutes nuisance either to man, livestock or crops.
(b) Methods of weed control are: (i) Mechanical/Physical: (a) involves the use of tools, implements and machines (b) could be hoeing, ploughing, harrowing, roughing (hand picking), etc.
(ii) Chemical: (a) involves the use of herbicides (b) herbicides could be selective and non-selective depending on their mode of action. (c) herbicides can be pre-ernergence or post-emergence (d) it can be systematic in nature.
(iii) Biological: (a) involves the use of parasites and predators to control weeds (b) involves killing weed with their natural enemies e.g. cactoblactis cactorum (c) suppression of weeds with creeping or sprawling crops such as melon, potatoes, catapogonium or cover crop etc.
(iv) Soil sterilization: (a) heating or steaming of earth (b) limited to small areas such as in bush burning (v) Cultural: (a) routine farm practices not involving tools/equipment e.g. cover cropping, crop rotation etc.
(c) Reasons for controlling weeds: (i) weeds cause loses in crops yield
(ii) weeds lead to increased cost of farm operations, thereby leading to inefficiency of farm operations
(iii) increase cost of harvesting
(iv) reduce the quality of crop products
(v) reduce the quality of animal products, such as milk by imparting bad odour and taste to it
(v) certain weeds are poisonous and hence kill animals that feed on them
(vi) form alternative hosts for pests
(vii) form alternative hosts for pathogenic organism
(viii) competes with crop plants for nutrients, space and light
(ix) reduce land value
(x) poisonous weeds are dangerous to man.