Plants and Weeds as Herbicides

Weeds are basically unwanted and undesirable plants that interfere with utilization of land and water resources and thus, adversely affect human welfare. In croplands, weeds compete with natural resources such as light, water, moisture and nutrient which directly results in decreased yield as well as quality of produce. 

An eco friendly and environmentally safe method of weed management is the use of botanicals (from plants, weeds and trees) to control weeds. Here, the leachates, oils and alleochemicals from plants, weeds and trees are isolated and applied as herbicide as result of which plants are killed due to its allelopathic effect. 

How the plants and weeds are used as herbicides?

1. Allelopathy b. Crop residue c. Catch crop d. Trap crop

2. Intercropping f. Crop rotation g. Botanicals (Trees and other plants)

1. Allelopathy

It refers to any direct or indirect, harmful or beneficial effect by one plant on another through production of chemical compounds that escapes into the environment.

Allelopathy is divided into two types:

• True allelopathy

• Functional allelopathy

The active substance may be released in a form from same plant organ (True allelopathy) or be released which is transformed into an active substance by more or less closely associated microorganisms (Functional allelopathy).

Source and routes of liberation of allelochemicals:

The allelochemicals involved in allelopathy occur in roots, rhizomes, stem, leaves, flowers, fruits and seeds. Leaves seem to be most consistent source of allelochemicals. 

The allelochemicals are commonly liberated from donor plant by four general routes

1.Volatalization 2. Leaching 3. Exudation 4. Decomposition

Mechanisms of action of allelochemicals 

Allelochemicals play a major part in the basic metabolisam of plants. They affect numerous physiological and biochemical process in plants such as 

1. Inhibition of cell division and cell elongation

2. Inhibition of gibberellins or IAA/growth hormones

3. Mineral uptake

4. Retardation of photosynthesis

5. Inhibition of respiration

6. Inhibition of opening of stomata

7. Inhibition of protein synthesis and organic acid metabolism

8. Inhibition of activity of specific enzymes

Allelopathic plants as natural herbicide sources 

Synthetic herbicides based on natural plant compounds are called as natural herbicides 

Advantages:  

• New target sites

• Water soluble

• Perceived as more environmentally friendly

Disadvantages: 

• Chemically complex

• Difficult to isolate and produce

• Not stable

• Expensive to synthesize 

• Potential mammalian toxicity, carcinogenic, allergenic

Examples-  

Sorgoleone from sorghum, inhibits photosynthesis better than atrazine, but is short-lived and not stable 

Callisto is a plant based herbicide the active ingredient of which is mesotrione. It is derived from a compound (leptospermone) isolated from the callistemon (bottle brush) plant. It is commonly used to control broad leaf weeds in corn and fodder maize. It generally controls common rage weeds, Chenopodium album and velvet leaf. 

Rape-turnip: Isolated compounds is allelopathic to several weeds like spiny sow thistle, may weed, smooth pigweed, and barnyard grass germination. At higher concentrations, germination was completely eliminated. 

Allelopathic effect of weeds on crops (Examples) 

1. Seed exudates of Avena fatuaaffect the germination and early growth of wheat. 

2. Leaf and inflorescence ofAmaranthus spinosa affect the vegetative growth of finger millet and maize. 

3. Leaves and inflorescence extract of Parthenium hysterophorousaffects the germination and seedling growth of sorghum 

Allelopathic effect of crops on weed (Examples) 

1. Root extraction of maize inhibit the growth of Chenopodium album and Amaranthus retroflexes.

2. Cold water extracts of wheat straw reduces the germination and growth of Ipomea spp. and Abutilon indicum.

B. Crop residue 

Crop residues are the plant parts remaining after the harvest of economic yield which for economic or social reasons are not fully utilized within the confines of production units and allowed to burden the natural environment. Some of the studies have shown that incorporation of crop residues in to the soil affected the germination of weeds seeds.  

For example, incorporation of sunflower stalk in to the soil has affected weed seed germination to an extent of 50% as compared to without sunflower stalk incorporation. Similarly, incorporation of alfalfa reduces the germination of many weeds due to presence of allelochemicals called saponin, caffeine, chlorogenic acid, isochlorogenic acid, coumaric acid and ferulic acid in root exudates and residues. Therefore, we can use alfalfa as an natural herbicide. 

C. Catch crop

Here, the crop is raised which favours the germination of parasitic weed seeds. Once the parasitic weeds germinate and start emerging, the crop is incorporated along with the weeds in to the soil there by reducing weed seed bank in the soil of main crop. 

Eg: Growing of sorghum encourages the germination of striga. After few days of its emergence, incorporation of biomass in to the soil reduces the striga weed seeds in soil. 

D. Trap crop

It is also similar to catch crop, where in a suitable crop is raised in order to stimulate germination of parasitic weed seeds. But due to non availability of host plants these weed seedlings die eventually.  

Eg: Growing of cotton, cowpea, bengal gram etc., stimulates the germination of parasitic Striga seeds. However, the seedlings do not sustain in the field due to the non availability of host plants. 

E. Intercropping 

Intercropping also helps in the control of many weeds in cropping system as well as parasitic weeds especially Cuscuta in case of lucerne crop. Here, by growing two to three rows of cereal crops like maize, sorghum and bajra we can reduce its problem by restricting its movement from one plant to another. 

Eg: Growing of green gram or white clover in sweet corn or tomato as an intercrop has shown a reduction in weed population up to 90% as compared to control. 

F. Crop rotation 

Crop rotation indirectly helps in the control of weeds. Here, the roots of previous crop exudates allelochemicals which affects the germination of weed seeds. 

For eg, when sorghum is rotated with alfalfa, it reduces the weeds population up to 30 percent. Similarly, wheat rotated with legume crop reduced the germination of Amaranthus spp., Portulaca oleracea and Chenopodium album etc.  

G. Botanicals 

Extraction of leachates, oils and allelochemicals from trees and other plants can be used as herbicides for controlling weeds. 

Eg: An alkaloid called 1, 3, 7 trimethylxanthise isolated from coffee seeds at various concentration affected the germination of many weeds like Amaranthus spinosa, Avena fatua, Eichnachlolva colonum, Eichanachlova crusgalli, Vicia sativa etc.  

Author details

Dr. Sunil Kumar K^# and Dr.Madhurima Vinod*  

^#Research Associate (Agronomy), *Research Associate (Agril. Entomology), ZBNF Project, ZAHRS Brahmavar, Udupi Karnataka - 576213