Weeds: Characterstics and their Effects on Crop Production

Bathua (Chenopodium album) is a popular plant weed that is also part of diet in several South Asian Countries

Weeds are “out of place” plants in cultivated areas, kitchen gardens, lawns and other areas of domesticated plant growth. One can simply say that  Weeds are unwanted and undesirable plant that interfere with utilization of land and water resources and thus adversely affect crop production.

Origin of Weeds
Weeds are no strangers to mankind. They have been there ever since man first started to cultivate crops for his own needs around 10000 BCE and recognized as a problem from the beginning. Weeds have similar wild origin as of the cultivated plants. For example, modern day wheat plant is a derivative of wild grass. Man has improved crop plants by concious or unconcious selection according to his own needs. But in this process of crop development, weed plant species grew consiously or unconciously over the period of time. Hence we divide the weeds in two group. These are

1. By Man's Effort
2. By Invasion.

Around the globe, there are more than 30,000 weeds species listed. Out of which 18,000 cause serious damage to agricultural production. Eighteen are considered to be the most serious invasive weeds in the world and about 26 species have been listed as principal weeds in crop fields of India. Some of the most common weed plants found in the crop fields of India are

Weed(Scientific Name)
Barnyard grass(Echinochloa crusgalli)
Goat weed(Ageratum conyzoids)
Crabgrass(Digitaria sp.)
Pig weed(Amaranthus sp.)
Foxtail(Setaria sp.)
Black jack(Bidens pilosa)
Sandbur(Cenchrus sp.)
Cox comb(Celosia argentia)
Wild oat(Avena fatua)
Lambsquarters(Chenopodium album)
Goose grass(Eleusine indica)
Wild carrot weed(Parthenium hysterophorus)
Torpedo grass(Panicum repens)
Common purslane(Portulaca oleracea)
Canary grass(Philaris minor)
Horse purslane(Trianthema portulacastrum)
Crowfoot grass(Dactyloctenium aegyptium)
Bermuda grass(Cynodon dactylon)
Canada thistle(Circium arvense)
Thatch grass(Imperata cylindrical)
Day flower(Commelina benghalensis)
Johnson grass(Sorghum halepense)
Field bind weed(Convolvulus arvensis)
Quack grass(Agropyron repens)
White horse nettle(Solanum elaeagnifolium)
Nut grass(Cyperus rotundus)

Charactersitics of Weeds
Weeds are highly competitive and are highly adaptable under varied adverse situations. Their reproductive mechanism is far superior to crop plants particularly under unfavourable side. They produces larger number of seeds compared to crops. Most of the weed seeds are small in size and contribute enormously to the seed reserves. Weed seeds germinate earlier and their seedlings grow faster. They flower earlier and mature ahead of the crop they infest. They possess the phenomenon of dormancy, which is an intrinsic physiological power of the seed to resist germination till favourable conditions. They do not lose their viability for years even under adverse conditions. Most of the weeds possess C4 type of photosynthesis, which is an added advantage during moisture stress. They possess extensive root system, which go deeper as well as of creeping type.

Harmful Effects of Weeds

1. Compete with crop plant for space, light, moisture and soil nutrients thus reducing crop yield.
2. Affect quality of farm produce, livestock products such as milk, meat and skin as some weeds are poisonous to livestocks eg. Lochnera pusilla and Abrus precatorius.
3. Act as a host for many pest and diseases.
4. Can cause serious health problems to human beings. e.g., Parthenium hysterophorus (congress weed).
5. Increase the cost of cultivation due to weeding operation.
6. Aquatic weeds transpire large quantity of water, obstruct flow of water; thus affecting fishing, swimming and recreation.
7. Thorny weeds can reduce movement of farm animals and workers.

Beneficial Effects of Weeds
Though weeds are harmful, but some of the weeds also have beneficial properties and an agriculturist can gain in terms of both productivity and capital. Beneficial effects of weeds are:
1. Weeds as fodder: Weeds like Rynchosia aurea, R. capitata and Clitoria terneata are very good fodder legumes and preferred by cattle and livestock.
2. Weeds as vegetables: Weeds like Amaranthus viridis (chaulai) and Chenopodium album (bathua). are directly taken by humans as part of the diet.
3. Weeds as soil binders: Panicum repense is an excellent soil binder; keeps bunds in position and prevents soil erosion in high rainfall regions and hilly slopes. Hariyali, kikuyu grass, kollukattai grass (Cenchrus sp.) etc., can be used as soil binders.
4. Weeds as manure - When weeds are ploughed in, they add to the soil plenty of humus.  In wetlands, weeds are said to form a sort of rotation with paddy and are valuable in preventing loss of nitrates.
5. Weed as fuel - Prosopis juliflora very invasive in nature and notorious tree weed commonly used as fire wood.

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