In this article we will discuss about the harmful effects of weeds. Learn about the harmful effects of weeds on: 1. Agriculture 2. Animal Husbandry 3. Human Health 4. Aquatic Ecosystems 5. Industry and Public Utilities 6. Woodlands and Forests.
1. Weed Menace in Agriculture:
i. Weeds Reduce Crop Yields and Production Efficiency:
Growing with the crop plants, weeds cause tremendous reductions in crop yields and elevate their production costs in varied ways. Several scientists have estimated such losses in crop yields in different parts of India.
A very broad-based average of these estimates shows that weeds reduced productivity of wheat by 15- 30%, of rice by 30-35%, and of maize, sorghum, pulses, and oilseeds by 18-85% each. There were also frequent cases of complete crop failures due to weeds, particularly in the upland rice and vegetable crops.
Besides the direct reductions in crop yields inflicted due to the presence of weeds, there are many indirect ways by which the weeds may be troublesome in agriculture. For example- in weedy fields farm operations like application of fertilizers, pesticides, and irrigation become cumbersome. Even when a crop is made despite the presence of weeds, it may be difficult to harvest it, particularly when prickly weeds, like wild safflower (Carthamus oxyacantha), Canada thistle (Cirsium arvense), and cocklebur (Xanthium strumarium) invade the fields.
Cowage (Mucuna pruriens) annoy the harvest labour by causing itching. Bindweed (Convolvulus arvensis) and morningglories (Ipomoea spp.) bind the crop plants together so well that their harvesting becomes troublesome. Weedy crops harbour insect pests and disease organisms in larger intensity than a weed free crop.
This they may do by either providing shelter or acting as alternate hosts to the pest and disease organisms. To cite a few examples, Cenchrus ciliaris is a confirmed alternate host of ergot disease of pearlmillet. Commelina benghalensis in banana plantations inhabits nematodes. Agropyron repens is an alternate host of black rust. Euphorbia hirta, Brassica kaber, Cleome viscosa and Carthamus oxyacantha form alternate hosts of rootknot nematodes which infest several vegetable crops.
The leaf caterpillars of lucerne use Achyranthus aspera as its alternate host. Rice stem borer breeds on Echinochloa spp. There are several other examples of weeds acting as alternate hosts of crop pests and diseases available in literature. The weeds at harvest time also bring about excessive wear and tear of farm machines Add to this the cost of separating weed seeds and fruits from the grain and other farm produce.
There is yet another way the weeds limit our annual agricultural production. This is by permanently occupying thousands of hectares of otherwise productive land and taking these out of cultivation. Recent estimates show that weeds cause an annual loss of almost Rs. 2000 crores to Indian Agriculture, which is more than the combined losses caused by the insect pests and plant pathogens.
ii. Weeds Reduce Crop Quality:
Weeds mar the quality of farm produce in many ways. Contamination of food grains with weed seeds, particularly of poisonous nature, fetches low price. The weedy grains produce flour with bad odour. In warehouses the weed seeds and weed fragments continue respiration and thus cause the grain to heat and rot. In tea, the presence of loranthus (Dendrophthoe falcata) leaves impair its quality.
In cotton, the dry weed fragments adhere to its lint and hinder the spinning process. In dryland agriculture weeds cause severe moisture stress and force the food grains to shrivel. The vegetables and fruits are discoloured and deshaped in the presence of weeds. The foliage of Cirsium arvense, a common weed of mint plantations, often gets crushed with the mint leaves which lowers its oil quality.
2. Weed Menace in Animal Husbandry:
Growing with the forage crops, weeds often help to fill the carts and silos but it is in the milk and meat returns that their damage becomes apparent. The chemical analyses of the weedy and weedfree forages may sometimes not show differing nutritive values, particularly when there are leguminous weeds in the mixture, but in the feeding tests the animals exhibit clear preference for the weedfree forages.
When some hungry animals are forced to feed upon the weedy forage, their meat and milk are odd-falvoured and tainted. Weeds like Cichorium intybus, Allium vineale, Argemone mexicana, Brassica kaber, Anthimis cotula, Thalapsi sp. Ambrosia sp., Helemium sp., and Oxalis acetosella are particularly notorious in this respect.
Certain weeds cause sickness in animals while others may prove fatal due to high levels of specific alkaloids, tannins, oxalates, glucosides, or nitrates. Halogeton (H. glomeratus, a weed of arid and semiarid regions, has brought death to herds of sheep with its high oxalate content. Johnsongrass (Sorghum halepense) at its tillering stage and Xanthim pun gens at its cotyledon stage are poisonous to animals due to their high prussic acid contents.
Corncocle (Agrostemma githago) seeds have been found poisonous to horses, cattle, and poultry. Their toxic principle is githagin (or agrostemine). Puncturevine (Tribullus terrestris), a weed of drylands, induces in sheep extra-sensitivity to light. Also, its thorny fruits cause sores in the hooves of animals. The spiny fruits of Xanthium strumarium, Achyranthus aspera, Cenchrus settigerous, Cirsium arvense, and Cenchrus incertus stick on to the mouth, tail, and body of animals, which annoys them badly.
The leaves of lantana (Lantana camara) induce acute photosensitivity and jaundice in animals due to their toxic principle ‘Lantradene- A’. Carrotgrass (Parthenium hysterophorus) causes contact dermatitis in livestock and it is reported to be poisonous to sheep. Ageratum spp. growing in grazing grounds in abundance in Himalayas and Nilgiris, are often hazardous to livestock.
In Rajasthan there is growing concern about rapid spread of the weed golden crownbeard (Verbesina enceliodes) since it is poisonous to sheep and goat. Locoweeds (Astragalus and Oxytropis spp.) inflict abortive and teratogenesis effects on sheep and cattle. Such weeds possess the toxic alkaloid swainsonine. Lupinus sericeus induces crooked calf disease in cattle.
Rhododendron sp. has been found responsible for causing diarrhoea in milch animals in Kashmir. It also stains the milk of the affected animals with blood spots. Leafy spurge (Euphorbia esula) causes scours and weakness in cattle and it is fatal to sheep. Crotolaria sp. is fatal to chick. Sweetclover (Melitotus Alba) contains a dicumarin which acts as an anti-blood coagulant. Heliotropium amplexicaule, Helenium spp., and Datura stramonium are other sickening weeds for animals.
Under drought conditions weeds like Chenopodium, Amaranthus, Cirsium, and Polygonum spp. develop nitrate levels as high as 1000 ppm or more, which causes asphyxia in animals. Spines of Tribullus terrestris can puncture animal skin. When ingested, it can injure the animal stomach. The hair of neetle (Urtica urens) cause severe itching and inflammation in animals.
3. Weed Menace to Human Health:
Health, comfort, and work efficiency of men are adversely affected by weeds. Numerous people are plagued year after year with hay fever and asthma aggravated by pollens of Ambrosia artemisiifolia and Franseria sp. The airborne pollens and other biotic particles of several other weeds belonging to diverse families are known to cause rhinitis and conjuctivitis. Carrotgrass (Parthenium hysterophorus), poison ivy (Rhus spp.), common regweed (Ambrosia artemisiifolia), and sneezeweed (Helenium spp.) are held responsible for different kinds of dermal allergies in humans.
Weeds provide food, protection, and habitat for the reproduction of vectors of fatal, human diseases. Aquatic weeds like waterlettuce (Pistia lanceolata), salvinia (Salvinia auriculata) and alligatorweed (Altemanthera sp.) shelter alternate hosts and vectors of malaria, yellow fever, encephalitis, dengue fever, and filariasis. Tsetse fly, which causes African sleeping sickness (trypanosomiasis) flourishes under the cover of brush weeds. Eyegnats, serving as vectors of mastitis and tropical ulcer also breed in certain weeds.
Then there are weeds which cause direct food poisoning. Wheat flour contaminated with seeds of corn cockle (Agrostemma githago) gives bread a bitter taste and irritates the gastro-tract of its consumers. Mexican poppy (Agremone mexicana) seed crushed with mustard seeds have brought death and blindness to thousands of people in India. Milk from animals feeding upon this weed can cause glaucoma in humans.
The Argemone toxicity is due to alkaloids called sanguinarine and 11-Oxotriacontanoic acid. Some other poisonous weed seeds are from darnel (Lolium temulentum), wild mustard (Brassic kaber), saponaria (Saponaria vaccaria), ragwort (Senecio vulgaris), and wild garlic (Allium vineale). Their seeds contaminate the food grains.
4. Weed Menace to Aquatic Ecosystems:
Not only on land, weeds are nuisance in and around waterbodies too. Aquatic weeds make the appearance of waterbodies repulsive and decline their recreational values. In other waterbodies they hinder navigation and fishing. Waterflow in irrigation canals and channels is slowed. The potable and drinking waters are fouled by the presence of decomposing aquatic weeds.
5. Weed Menace to Industry and Public Utilities:
Weeds growing on industrial sites and air fields form potential source of fire hazards, besides being unseemly. They hide industrial pipelines and valves and contribute to deterioration of stores, equipment, and material. Weeds force the fence lines to corrode and obstruct road signs and curves on highways.
The working road widths are reduced by weeds. Some weeds penetrate through even asphaltic surfaces, which get weakened. Weeds growing around electric poles prevent their ready access to the work crews. Weeds also weaken the rail tracts and air strips.
6. Weed Menace to Woodlands and Forests:
In forests dry weeds offer potential source of fire hazards. A weed like lantana (Lantana camara) can catch fire even when green. Also, the presence of unwanted brush weeds reduce tree growth and prevent recreational and other uses of woodlands. In extreme cases the brush weeds take over land from the young forest trees.