Environmental Engineering Reference
In-Depth Information
Post-emergence herbicides can be either
contact
or
systemic
. Contact
herbicides affect only those parts of the plant that they touch and are not
translocated through plant's vascular system to other portions of the
plant. In contrast, systemic herbicides are taken up by roots or foliage
and translocated throughout the plant, so they can affect parts of the
plant that were not directly exposed to the herbicide (Fig. 11.7). Contact
herbicides kill plants quickly, often within a few hours of application,
whereas systemic herbicides kill slowly and usually require several days
or sometimes a few weeks to be fully translocated and effective. Contact
herbicides require adequate spray volumes and complete coverage of the
weed foliage for effective control, and repeat applications are usually
needed to kill regrowth from underground parts of the plant. In contrast,
systemic herbicides do not require complete coverage of weed foliage; if
any portion of the weed receives the spray, herbicide molecules are
translocated throughout the plant.
Chemical control of broad-leaf weeds
The selective post-emergence herbicides most commonly used to control
broad-leaf weeds in the tropics include: 2,4D, 2,4DP (dichlorprop), MCPP
(mecoprop), dicamba and triclopyr. Many different commercial formula-
Nozzle
Sprayed with contact pesticides
Sprayed with systemic pesticides
Unaffected part of plant
The entire plant is affected
Affected (sprayed) part of plant
Fig. 11.7. Contact pesticides (herbicides, fungicides, insecticides) affect only those
parts of the plant that they actually touch. Systemic pesticides can affect parts of the
plant that were not directly exposed.
