The economic importance of weeds

There are over 250, 000 plant species in the world. About 30, 000 of them occur as weeds in various agro-ecosystems and about 18, 000 of these cause serious damage to crops throughout the world.

Weeds today are considered to be the most important pest group which interferes with utilization of land and water resources. The losses caused by them to horticulture are greater than any other pest category.

Definition

Some of the several definitions given to the term “weed” are listed below.

A weed is a plant:

1.    Growing where it is not desired;

2.    Out of place;

3.    Growing where something else should grow or nothing should grow;

4.    Whose potential for harm is greater than for good;

5.    Extremely noxious, useless or poisonous;

6.    Whose economic value has not been discovered.

Careful examination and analysis of these definitions indicate that the term “weed” is very much relative to a situation, time, and to an individual concerned.

In general, due to the following reasons, weeds are undesirable plants at any place and time and have to be managed:

1. Weeds cause yield reduction of crops through competition and allopathic effect

a.   Competition for nutrients - Analysis of mineral composition of common weeds and crop plants indicates that the former accumulate high concentration of plant nutrients in their tissues and in many cases, they are more than those found in common crop plants.

b.   Competition for solar energy - About 99% of the dry matter of plants are composed of organic matter which depend upon solar energy for production. Production potentials of crop plants shaded with weeds are greatly reduced and they may even appear chlorotic and weak.

c.   Competition for water - For producing equal amount of dry matter, weeds in general transpire more water than most crop plants. This may be of importance in dry land agriculture where moisture is very often a limiting factor.

d.   Competition for space - Competition for space may be both in the atmosphere as well as in the rhizosphere preventing the crop plants respectively from photosynthesis and moisture and nutrient uptake.

e.   Allopathic effects - Allelopathy involves any direct or indirect harmful effect on another plant through the release of chemical compounds on the other. These chemical compounds may inhibit seed germination or reduce growth of other plants.

2. Weeds harbour other pests

Weeds growing in the crop fields, harbour insects disease organisms and other pests. Outside the fields, weeds often act as alternate or parallel hosts to crop pests which migrate to the main fields. Control of these pests should also be undertaken, which enhances the cost of plant protection.

3. Weeds reduce the quality of farm produce

Weeds reduce the quality through admixture of weed seeds which produce odd odours, increase moisture contents and cause the produce in warehouses to heat and rot. Fruits and vegetables are discoloured and malformed under the impact of weeds.

4. Weeds increase the cost of labour and equipment

Because of weeds, more time and labour are spent on land preparation and cultivation, cleaning, irrigation and drainage channels and harvesting and cleaning produce. Likewise, maintenance and repair costs tools and equipment also would increase.

5. Weeds contaminate water bodies and increase water management problems

Weeds impede navigation, reduce flow rate of irrigation water by 50% or more, and clog irrigation and drainage channels, increase evapo-transpiration by 130 – 250%.

Therefore, for the formulation of a sound weed management programme, thorough knowledge of the weed problem in an area or a farm, their prevalence, density, frequency, association with crops and their changing patterns under field conditions is required. Hence, a floristic survey of the region or one’s own farm should be able to help identify the weeds.