Weed Management in Horticultural Crops

COMMON WEED DISPERSAL AGENTS

(a) Wind

Many seeds are well adapted to wind travel. Cottony coverings and parachute-like structures allow seeds to float with the wind. Examples of wind-dispersed seeds include common milkweed (Asclepias syriaca), common dandelion, Canada thistle, and perennial sowthistle (Sonchus arvensis). Weed seeds and fruits that disseminate through wind possess special organs to keep them afloat. Such organs are:

• Pappus – It is a parachute like modification of persistent calyx into hairs e.g. Asteraceae family weeds - Tridax procumbens
• Comose - Some weed seeds are covered with hairs, partially or fully e.g. Calotropis sp.
• Feathery, persistent styles - Styles are persistent and feathery e.g. Anemone sp.
• Baloon - Modified papery calyx that encloses the fruits loosely along with entrapped air e.g. Physalis minima
• Wings - One or more appendages that act as wings e.g. Acer macrophyllum

Factors that influence wind dispersal:

• seed weight
• seed shape
• structures (wings or pappus)
• height of release
• wind speed and turbulence

(b) Water

Aquatic weeds disperse largely through water. They may drift either as whole plants, plant fragments or as seeds with the water currents. Terrestrial weed seeds also disperse through irrigation and drainage water. Weed seed often moves with surface water runoff into irrigation water and ponds, where it is carried to other fields. Weeds growing in ditch banks along irrigation canals and ponds are the major source of weed seed contamination of irrigation water.

Weed seed often remains viable in water for several years, creating a "floating seed bank" and allowing weeds to disperse over large areas in moving water. Field bindweed seed, for example, remains over 50 per cent viable after being submerged in water for more than 4 years. Some seeds have special adaptations that aid in water travel. The seed pod of curly dock, for example, is equipped with pontoons that carry the floating seed.

(c) Animals

Several weed species produce seeds with barbs, hooks, spines, and rasps that cling to the fur of animals or to clothing and then can be dispersed to long distances. Farm animals carry weed seeds and fruits on their skin, hair and hooves. This is aided by special appendages such as Hooks (Xanthium strumarium), Stiff hairs (Cenchrus spp), Sharp spines (Tribulus terrestris) and Scarious bracts (Achyranthus aspera). Even ants carry a huge number of weed seeds. Donkeys eat Prosophis julifera pods.

Weed seeds are often ingested and passed through the digestive tracts of animals. Animal droppings provide an ideal nutrient and moisture environment for weed germination. Only a small percentage of the seed remains viable after exposure to an animal's digestive enzymes. The ingested weed seeds are passed in viable form with animal excreta (0.2% in chicks, 9.6% in calves, 8.7% in horses and 6.4% in sheep), which is dropped wherever the animal moves. This mechanism of weed dispersal is called endozoochory e.g. Lantana seeds by birds, Loranthus seeds stick on beaks of birds. Viable weed seeds are present in the dung of farm animals, which form part of the FYM. Besides, addition of mature weeds to compost pit as farm waste also act as source.

(d) Dispersal by Man

Man disperses numerous weed seeds and fruits with raw agricultural produce. Weeds mature at the same time and height along with crop. Due to their similar size and shape as that of crop seed man unknowingly harvest the weeds also, and aids in dispersal of weed seeds. Such weeds are called “Satellite weeds” e.g. Avena fatua, Phalaris minor.

(e) Dispersal by machinery

Weed seeds often are dispersed by tillage and harvesting equipments. Seeds move from field to field on the soil that sticks to tractor tires, and vegetative structures often travel on tillage and cultivation equipment and latter dropping them in other fields to start new infestation. Disc-type cultivation equipment is less likely to drag vegetative plant parts than are shovels or sweeps.

(f) Intercontinental movement of weeds

Introduction of weeds from one continent to another is through crop seed, feed stock, packing material and nursery stock e.g. Parthenium hysterophorus

(g) Crop mimicry dispersal

Weed seed adaptations to look like crop seed: plant body or seed same size, shape, and morphology as crop e.g. barnyard grass bio-type looking like rice escapes hand weeding and is dispersed with rice. Nightshade fruit (berries) are same size, shape as dry beans, harvested and dispersed with beans.

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( h) As admixtures with crop seed, animal feed, hay, and straw

Weeds probably are spread more commonly during the seeding of a new crop or in animal feed and bedding than by any other method. Seed labels often indicate a tiny percentage of weed seed, but consider this example. If a legume seed contains 0.001 percent dodder (a parasitic annual; Cuscuta campestris) seed by weight, there will be eight dodder seeds per 2 kg of legume seed. If the legume seed is sown in a field despite an extremely low dodder seed percentage by weight, the small size of the seed, combined with rapid early-season growth, could result in an infested legume field within a single season.