Beware of invasive weed

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Discovered in Kenya in the 70s, the Parthenium hyteroporus weed can prevent the germination and growth of other crops. It suppresses livestock pastures, sustains malariacausing mosquitoes and is harmful to humans and the environment.

Agriculture is not only the country’s main employer, but also the economic pillar. It employs over 70 percent of Kenya’s working population. Farmers, mainly aim at maximizing farm yield. Some of the major methods practiced by farmers to increase farm yield include: irrigation, improving soil fertility, reducing crop pests and weeding. There are several methods used to control weeds in the organic farm. They include mulching and uprooting. However, some weeds may be problematic to manage and can be very harmful not only to crops, but also to human beings and the environment at large. Such weeds include the parthenium weed - one of the world’s top ten noxious (poisonous) weeds.

Parthenium hysterophorus

Commonly known as Santa Maria, white top, famine weed or feverfew, Parthenium hysterophorus was identified in Kenya around the 1970s. It was gazetted as a harmful weed after spreading to other parts of the country. It has not been possible to contain it. It is spreading at a faster rate than the water hyacinth and much way faster than Prosopis juliflora (mathenge weed). While it’s not clear how the harmful weed got into Kenya, it has serious negative effect to crops, animals, human and the environment.

It suppresses pasture and spoils milk taste

There is great concern about the weed contaminating the food and fodder crops. The weed is foul-tasting to livestock, so its invasion results to fodder shortages. When mixed with fodder, it spoils meat and milk. Moreover, it produces certain substances that prevent other plants from germinating and growing near it. This inhibits the germination and later growth of a wide variety of crops including pasture grasses, cereals, vegetables, other weeds and tree species growing near the weed.

Prevents seed germination

As a result, the weed may reduce crop yields besides displacing edible species in natural and improved pasture- which would be used as livestock feed. Furthermore, the pasture carrying capacity can be reduced by 90%. The weed’s pollen, too, are allelopathic (they produce chemicals that kill other crops). Thus, heavy deposits on nearby crops may result in failure to set seed. Also, Parthenium hysterophorus also acts as an alternate host for several crop pests and diseases such as the tomato leaf curl virus. This results in reduced crop yield in tomatoes.

Avoid handling weed with bare hands

The negative effect of the weed is not limited to field crops and livestock; human beings are also at great health risk when exposed to the weed. Parthenium hysterophorus has been reported to cause skin rashes (dermatitis), on those parts of the body that come in contact with the weed on a regular basis, watery eyes, swelling and itching of the membranes of the mouth and nose, constant coughing especially at night, continually running nose and sneezing, itching of the roof of the mouth and fatigue. Allergy-prone people are particularly susceptible to both the dermatitis and respiratory problems. Recent research by ICIPE has also shown that the weed has the ability to sustain the malaria-transmitting mosquito - Anopheles gambiae - by extending its life even in the absence of a blood meal. “Our results show that when female Anopheles mosquitoes feed on Parthenium, they survive much longer, and they also accumulate substantial energy reserves.” Says Prof. Baldwyn Torto, an ICIPE scientist. As a result, the spread of the Parthenium hysterophorus weed may lead to higher disease transmission, such as malaria.

Management of Parthenium hysterophorus weed

The precise management measure of any invasive weed is prevention. However, if invasion has already occurred it may be necessary to treat the infestation rapidly when it is just starting- to prevent them from establishing. Early detection and management is necessary. Controlling the weed before it produces seeds may reduce problems in the future. To control the weed, uproot the plant before it flowers. This should be done carefully to ensure no regrowth occurs. Direct skin contact with the weed may result to skin diseases. Therefore, lightweight, long sleeved garments and cotton gloves should be worn to prevent skin contact with the weed. The weed can also be controlled by digging it out, before it seeds. However, this must be followed up by sowing a crop or direct seeding of perennial pasture (eg pasture grasses).

Stop livestock from spreading weed

To stop spreading the weed through animal droppings, always confine livestock in the same area to contain weeds carried in contaminated fodder. Assign livestock into small paddocks until seed has dropped from their coats and tails. Establishing several paddocks complete with their watering points, then practicing rotational grazing between the paddocks can prevent them from spreading the weed seeds.

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