Weed Management in Horticultural Crops

ANTIDOTES AND ADJUVANTS

ANTIDOTES

Chemicals which are used to inactivate the applied herbicides are called as antidotes e.g. Paraquat spray can be inactivated by spraying 1% ferric chloride

SAFENERS / PROTECTANTS

Substances used for protecting crop plants, which are otherwise susceptible or less tolerant to some herbicides at doses required for good weed control.
e.g., Naphthalic anhydride (NA) – 0.5g / kg of seed for rice to protect against molinate and alachlor

Mode of Action: Safeners enter the target plants and compete there with herbicide molecules for a binding site on some native enzyme.

ADJUVANTS
Adjuvants are chemicals employed to improve the herbicidal effects, sometimes making a difference between satisfactory and unsatisfactory weed control.

Mode of Action: Adjuvants aid the herbicide availability at the action site in plants. Some important kinds of adjuvants are:
1. Surfactant ( Surface active agents) are:

i. Wetting agents- Aid in wetting the waxy leaf surface with aqueous herbicide sprays
ii. Spreaders - spreading the hydrophilic herbicides uniformly over the foliage
iii. Penetrates - the penetration of herbicide into the target leaves and stems

A water drop is held as a ball on a waxy leaf surface. (Take water in a beaker, if you dip a leaf of Cynodon dactylon and pull it back, you can see the leaf without wetting. But if you add a drop of surfactant you can readily wet the foliage.). With the addition of surfactant, the water drop flattens down to wet the leaf surface and let the herbicide act properly.

2. Stabilizing agents:
These include:

1) Emulsifiers:

A substance which stabilizes (reduces the tendency to separate) a suspension of droplets of one liquid which otherwise would not mix with the first one. It substitutes for constant agitation of spray liquids during field operation e.g. ABS, Solvaid, 15-5-3, 15-5-9.

2) Dispersing agents:

They stabilize suspensions. They keep fine parricides of wettable powder in suspension in water even after initial vigorous agitation has been withdrawn. They act by increasing the hydration of fine particles of WP laden with the herbicides.

3) Coupling agents (Solvents and co-solvents):

Chemical that is used to solubalize a herbicide in a concentrated form; the resulting solution is soluble with water in all proportions e.g., 2,4-D is insoluble in water, but it can be dissolved in polyethylene glycol to make it water soluble.
Common solvents: Benzene, acetone, petroleum ether, carbon tetrachloride

4) Humicants (Hygroscopic agents):

Humicants prevent rapid drying of herbicide sprays on the foliage, thus providing an extended opportunity of herbicide absorption e.g. glycerol.

5) Deposit builders (Stickers or filming agents)

Chemicals added to herbicide concentrates to hold the toxicant in intimate contact with the plant surface. They also reduce washing off of the toxicant from the treated foliage by rain e.g. several petroleum oils, Du Pont spreader sticker, Citowett.

6) Compatibility agents

Used to intimately mix fertilizers and pesticides in spray liquids e.g. Compex

7) Activators (Synergists)

These are the chemicals having cooperative action with herbicides. The resultant phytotoxicity is more than the effect of the two working independently.

e.g., Paraffinic oils, Ammonium thiocyanate, Urea and Ammonium chloride to enhance 2,4 –D phytotoxicity

8. Drift control agents

Herbicide spray drifts may pose serious hazards to non-target plants e.g., 2,4-D on cotton. Solution is to spray herbicide liquids in large droplets. Drift control agents are:

i. Thickening agents:
It is a large molecules organic compound which in aqueous systems behaves like a gel making the spray liquid viscous. The thickened sprays produce large sized drops that are less susceptible to spray drifts than the usual aqueous sprays e.g. Sodium alginate, hydroxyethyl cellulose and dacagin.
ii. Particulating agent:
Comprise particles of a lightly cross linked swell-able polymer. It imbibes liquids both water and oil to form a particulate aqueous and particulate oils e.g. Norbak.
iii. Foams:
Foams are liquid inter phase surrounding small packets of gases usually air in herbicide spray. Low expansion foams are under trial as drift control agents.