Section 10
Selectivity of Herbicides
&
Use of Selectivity in Ecosystem Management
Selectivity of Herbicides
Selectivity of herbicides may be a result of any one of the several factors in the
following outline:
l. Components of herbicide action leading to selectivity
A. Biochemistry of action
Processes specific to plants (photosynthesis, cell wall formulation,
etc.)
Rate of degradation by the plant.
Threshold of effect lower in plants when chemical is general
toxicant.
B. Physical processes affecting herbicide action.
Soil active herbicides (atrizene, etc.) move in soil only to limited
extent; root distribution effects sensitivity. Foliage uptake
influenced by wetability of surface, "wetness" of spray mixture.
Translocation affected by weather, physical injury (e.g., glyphosate
has little effect on injured plants.)
Droplet size, volume applied, influence coverage, absorption.
C. Seasonal differences in phenology of crop and weed species vs.
sensitivity.
2. Components of ecosystem structure leading to selectivity.
A. Canopy density
B. Layered communities
C. Ecosystem diversity
Use of Selectivity in Ecosystem Management
1. A large number of silvicultural problems, including insects, diseases, and
weed plants are influenced in a useful way by managing vegetation. Many
pests can be managed simply by host control (e.g., bark beetle control by
thinning with MSMA. Rots can be stopped by killing host). Crop trees are
more pest resistant, in general, when in good vigor; vigor is maintained by
weeding and thinning. Trauma to ecosystem is least when chemical practice
is selected for maximum specificity.
2. Biological activity of herbicides tends to run in groups of species. Pests
tend to run in groups. Some herbicides are specific to few groups, or even
a single species (asulam on bracken fern), or broadly effective on many
groups (glyphosate, picloram). Pin down the weed problem as specifically
as possible, and choose the chemical to control target plants. Specificity is
improved by precise timing of application and dosage calibration.
3. Animal populations are not directly affected by herbicides. Habitat is.
Understanding animal habitat requirements and herbicide specificity will
permit management of animals in ways not possible with direct control
measures.