sprayed in combination with other soil-surface (residual) herbicides; these kill off the
weed seedlings that germinate some days after application of the herbicides. This dual
effect can eradicate weeds for long periods.
damaged. This check in the growth rate of the weeds allows the crop plants to become
dominant and thus overcome the weeds.
Some Commonly Used Herbicides
MCPA and 2,4-D—very selective, used against broadleaf weeds in cereals, grass and
sugarcane. The application rate is about 1 litre/hectare of the commercial solution. Can
be mixed with contact herbicides such as sodium chlorate for circle weeding or on
weeds between rows of tree crops. Some crops such as the palms and coconuts are very
sensitive to them.
2,4,5-T—for woody plants; it kills most trees. Principal constituent of the defoliant
Agent Orange. Banned in most forward thinking countries due to its persistence and
toxicity.
Amiben (Chloramben)—used in legume and vegetable crops, and also in paddy rice
just after transplanting the seedlings.
Dalapon—kills grasses by direct contact, at 4–12 kg/ha. It should not be sprayed
directly onto crops. Similar to TCA, both of which persist for several months.
Simazine, Atrazine and Ametryne—selective, used in maize, pineapples and in circle
weeding as pre-emergence sprays at 1– 4 kg/ha. Persist for 6 months or longer.
Monuron (CMU) and Diuron (DMU)—mainly used for young weed seedlings and
germinating seeds. Used either after cultivation or with contact herbicides. Persist for up
to 6 months.
Paraquat (Gramoxone)—contact, pre-emergence. Becomes inactivated when in
contact with clay particles in the soil. Used widely in minimal (zero) cultivation
systems, when the land is not ploughed before planting. Toxic to animals and kills all
plant tissues, especially grasses. Rate of application is about 0.25–2 litres/ha of the
commercial solution.
Diquat (Reglone)—similar to Paraquat but more effective against broadleaved weeds.
Used to remove potato haulm. Both Diquat and Paraquat persist (ie remain active) for
only a few days.
Glyphosate (Roundup)—slow-acting, translocated. Used for clearing grass and
broadleaved weeds before planting, and for some tree crops. Becomes rapidly
inactivated in the soil.
TOK (CMU)—Selective. Used in rice in a granular form (7% a.i.—active ingredient).
Applied after transplanting, but before weeds have germinated, at about 20 kg/ha. It is
claimed that TOK is relatively harmless to fish.
into the plant tissues, and move about within the plant. They are most effective on
weeds that are growing rapidly. Normal plant growth is upset, and the plants become
twisted or distorted but not always killed. The underground plant parts are also often
91
3.Hormone—a form of translocated or growth regulator herbicide. They are absorbed