ENVIRONMENTAL SAFETY OF FORESTRY HERBICIDES
ANR-846, Reprinted Mar 1997. Ken McNabb, Extension Forester,
Associate Professor, Forestry, Auburn
Forestry herbicides are a cost-effective tool for the control
of undesirable vegetation in forest stands. They can be
used by landowners in several ways to increase forest productivity.
Forestry herbicides help prepare sites for tree planting
by reducing unwanted vegetation and providing conditions
for prescribed fire. They also reduce competition from herbaceous
weeds so that newly planted trees are given a boost in survival
or early growth. They may also be applied to improve the
growth rates in established stands by selective removal
of non-crop trees. The vast majority of herbicides are used
for growing pines, although some are applied for herbaceous
weed control and timber stand improvement in hardwood forests.
The practice of chemical vegetation control in forestry has
progressed significantly in the last decade. Research and development
efforts have produced new compounds that are more effective and
more environmentally sensitive. Better application techniques
have increased the efficiency of chemical weed control by forest
managers. And, recent emphasis on applicator training by state
regulators and professional organizations has helped to ensure
that these chemicals are safely and effectively applied.
Even though herbicides may be effective and safely applied,
some people have concerns as to the long-term environmental effect
of using these chemicals in forest management. Unfortunately,
much of the information the public sees regarding silvicultural
herbicides is misleading and inflammatory. Some view all herbicides
as indestructible toxic compounds that are applied at high rates
over vast acreages, inevitably finding their way into the food
chain and water supplies to become a threat to the general public.
Such extreme views should not be simply dismissed. Everyone, particularly
those who are most directly dependent upon the health and productivity
of our forests, must be sure that our management techniques are
environmentally sound. In this publication let us examine the
case for using herbicides in forestry.
Herbicide Use in Forestry
First of all, what quantities of herbicides are used in forest
management and at what intensity? The USDA Forest Service has
completed a number of Environmental Impact Statements (EIS) that
are excellent references in regard to the environmental effect
of using silvicultural herbicides. In their EIS for the southeastern
Coastal Plain and Piedmont, the Forest Service stated that approximately
0.5 percent of its total land base in the Southeast is treated
with herbicides annually. At this rate, it would take 200 years
for the Forest Service to spray all of the national forests only
once. The forest industry, on the other hand, usually manages
their forests more intensely than the Forest Service and may use
herbicides more frequently. Even so, an intensively managed plantation
will receive a maximum of three herbicide applications over a
rotation, which may be anywhere from 20 to 60 years in the Southeast.
Regardless of the management philosophy, herbicide use in forestry
does not approximate the intensity of chemical use in agriculture.
Not only are forestry herbicides used infrequently, but they
are generally applied in very small amounts. Specific application
rates will vary with herbicide, vegetation, and soil type but
range from 24 ounces to a maximum of 3 gallons of product per
acre for chemical site preparation. Even lower rates of chemicals
are used for herbaceous weed control in young plantations, ranging
from as little as 1 ounce of product per acre to a maximum of
1 gallon per acre. These low application rates indicate the efficiency
of existing vegetation control products under forestry field conditions
where plant biomass and diversity are considerable.
Many people have the misconception that all compounds whose
name end in "cide," such as insecticide, rodenticide,
or fungicide, can be lumped together as dangerous, highly toxic
chemicals, and unsafe at any application level. This is simply
not the case for the vast majority of agricultural pesticides
and is certainly not true of forestry herbicides. Table 1 provides
the acute toxicity of the active ingredient in several forestry
herbicides for comparison to some other common chemicals. The
table lists the LD50, which is a rating system for chemical toxicity.
A low LD50 indicates that a small amount of chemical is toxic
and is a more dangerous substance. Likewise, the larger the LD50
the less toxic the chemical. All of the forestry herbicides have
active ingredients that are less toxic than caffeine. And, the
active ingredient is diluted to make the herbicide product sold
on the market. All over-the-counter formulations of the products
listed in Table 1 have LD50s above 1,700 mg/kg (milligrams of
chemical per kilogram of body weight) and so are therefore less
toxic than aspirin!
Table 1. The Relative Toxicity of Commonly
Used Silvicultural Herbicides
||LD50* of the Active Ingredient mg/kg
|| sulfometuron methyl
| For Comparison:
|| Table Salt
|| Malathion (insecticide)
|*LD50 is the dose that is lethal
to 50 percent of a test animal population, expressed as milligrams
(mg) of chemical per kilogram (kg) of body weight.
How can this be so? How can a chemical with such low toxicity
be so effective at killing plants? Imazapyr, for example, has
an LD50 above 5,000 mg/kg, making it practically non-toxic. Yet
this compound is a very effective herbicide and can control many
of the largest trees. The secret to understanding this apparent
contradiction comes from realizing that herbicides work on biochemical
pathways that are specific to plants. For example, only plants
photosynthesize (produce food from carbon dioxide and water),
so, if a compound inhibits one or several of the steps in the
long biochemical pathway that is photosynthesis, that compound
is then toxic to plants. At the same time, this compound may have
no effect on animal systems because the biochemical pathway for
photosynthesis does not exist in animals. As another example,
some herbicides work on amino acid pathways that are specific
to plants and not found in animals. All of these types of compounds
can be very effective herbicides yet are safe for animals because
the biochemical basis for toxicity does not exist.
Effect on Wildlife
Given the low toxicity and application rates of forestry herbicides,
game or non-game animals would have to consume a great deal of
treated biomass for a toxic effect. In an area sprayed with hexazinone,
for example, a deer weighing 150 pounds would have to ingest all
the chemical applied to an area 54 feet by 54 feet to consume
enough herbicide to reach the LD50 level (application rate of
2 gallons of product per acre). This consumption would have to
occur within a few hours and before natural elements begin to
breakdown the herbicide. This is assuming, of course, that the
deer would consider herbicide-treated foliage to be palatable.
Not only are silvicultural herbicides very non-toxic to wildlife,
they also do not bioaccumulate (accumulate in the food chain).
These chemicals pass very quickly through the body when ingested
and are eliminated through urine and feces. Laboratory studies
have shown that 95 percent of ingested glyphosate is eliminated
within 5 days, 93 percent of hexazinone is eliminated in 24 hours,
and 93 percent of 2,4-D is eliminated within 2 hours. In this
respect, forestry herbicides are substantially different from
some of the older pesticides, such as the insecticide DDT, which
would accumulate in fatty body tissue. Silvicultural herbicides
belong to a class of compounds that do not remain in the body
and are eliminated within a short period of time. So, herbicides
show no tendency to accumulate in the food chain.
Although the danger to wildlife from toxic herbicide effects
are virtually non-existent, there is a real--although indirect--effect
on wildlife through habitat modifiation. A large diversity of
plant and animal species quickly move in to occupy the site after
a forest tract is harvested. Herbicides are used to delay plant
succession so crop trees can get a good start and effectively
compete with the many other plant species present. Chemical site
preparation normally increases the amount and diversity of herbaceous
plants (forage) like grasses and forbs, because residual pine
and hardwood sprouts are reduced. And, when larger hardwoods are
killed and left in place they may improve habitat for bird species
that nest and feed in dead standing trees. While herbaceous weed
control results in a significant reduction in wildlife forage
and cover species during the first growing season after application,
research has shown that this effect is temporary, and many species
begin to reappear in the first year. By the end of the second
growing season, the diversity and quantity of herbaceous plants
are comparable to untreated areas.
Environmental Fate and Water Quality
What happens to silvicultural herbicides when they are released
into the environment? Do they perpetuate and remain in the ecosystem,
slowly accumulating over time until reaching hazardous levels?
Forestry herbicides neither move very far nor do they survive
very long. The Forest Service, for example, in its "Federal
Environmental Impact Statement for Vegetation Management in the
Coastal Plain/Piedmont," gave the half-life of picloram as
63 days, of 2,4-D as 28 days, and of triclopyr as 46 days. This
means that for picloram, one-half of the applied amount decomposes
during the first 63 days after application, followed by one-half
again in the following 63 days. One year after application, less
than 2 percent of the original picloram applied, less than 0.01
percent of the 2,4-D, and 0.4 percent of the triclopyr will remain
in the soil. Although the actual environmental persistence of
a chemical depends upon the application rate, application method,
soil type, weather, and characteristics of the chemical, all these
herbicides are subject to the relentless and effective process
of biological decomposition.
In addition, silvicultural herbicides do not move very far
from where they are placed. The same EIS calculated leaching fractions
for several forestry herbicides when applied to a sandy loam soil.
For nine of the most commonly used chemicals, five had "non-significant"
leaching fractions. As for the remaining four chemicals on the
list, the highest leaching potential would still be less than
12 percent of the total amount applied 90 percent of the time.
Although it is very unlikely that properly applied forestry
herbicides move through the soil and into ground and surface water,
a possibility exists for their movement on top of the ground during
heavy storms that move soil and debris into streams. This could
occur if a heavy rain came immediately after application, something
an effective and conscientious applicator might prevent by monitoring
weather forecasts. In fact, when comparing the use of chemicals
to the use of large machines for site preparation, herbicides
positively affect water quality by reducing sedimentation rates.
Chemical site preparation normally results in less runoff, since
there are more roots, stems, and leaves left on the site to slow
water flow and physically hold the soil in place, particularly
if the site is not burned prior to planting.
The Issue of Risk
One of the most discussed aspects of forestry herbicides is
whether or not they pose a long-term health risk to the public.
Some feel that exposures to even infinitesimal amounts of these
chemicals will eventually result in adverse health effects, particularly
cancer. This is a complicated and often emotional issue. Even
though we are living longer and healthier lives than at any period
in our country's history, much of the public has come to believe
that the use of agricultural pesticides has introduced hazardous
chemicals into the environment at unacceptable levels. Forestry
herbicides have been caught up in this debate and are viewed by
some segments of the public as posing a hazard. But, there are
several things we should keep in mind when reviewing the potential
health hazard of herbicides.
First, there is nothing we do that is totally risk free. We
could, if desired, calculate the risk of the building falling
in on us as we read this publication. While the possibility of
such an occurrence is extremely small, the risk is not zero, as
some buildings do occasionally fall on their occupants. Common
activities like driving a car, climbing a ladder, or getting an
X-ray all have associated risks. An X-ray, for example, carries
a 7 in 1 million chance of causing a cancer. Those who would expect
zero risk for any human activity are not living in the real world.
Second, calculations of cancer risk to the public have shown
forestry herbicides to be an extremely low risk. The Forest Service
calculated cancer risk to the general public from herbicide use
on Forest Service lands in the Southeast to be 1 in 10 million.
These estimates are based on an extremely conservative approach,
which assumed that the herbicides were carcinogenic (cancer causing)
and exposure levels were high over long periods of time--70 years.
The fundamental assumption of carcinogenicity is subject to much
debate and to date no forestry herbicide has been conclusively
shown to be carcinogenic.
Finally, when evaluating a perceived risk we cannot assume
that its elimination will result in a higher margin of public
safety. The cure can often be worse than the disease. We use fewer
manual methods of vegetation control because we have herbicides
that are much safer for workers than long hours of swinging brush
axes or machetes through uneven terrain and thick vegetation.
The use of chain saws and bulldozers for vegetation control would
likely increase the consumption of hydrocarbon fuels, whose effect
on the environment are well known and documented. In addition,
machinery requires considerable capital investment, which could
increase the cost of forest regeneration and therefore decrease
its implementation. The issue of risk evaluation is complex and
should be based on a review of the health risks of the activity
in question and compared with an accurate evaluation of the costs
and risks of the alternatives.
All of us should be aware and concerned about the long-term
environmental wisdom of our forestry management practices, including
the use of forestry herbicides. But, after reviewing the use pattern,
chemical properties, and safety associated with these chemicals,
we must conclude that their continued use in forest management
not only improves forest productivity but does so in an environmentally
sound manner. The following five statements summarize the environmental
safety of silvicultural herbicides.
- Small amounts of forestry herbicides are used on a very small
percentage of forest land, a maximum of two or three applications
over a 20- to 30-year period.
- Forestry herbicides are very low in animal toxicity, and
they are significantly less toxic than most insecticides and
other chemicals commonly found in the home and environment.
- Forestry herbicides do not bioaccumulate and are quickly
eliminated from animal tissue.
- Forestry herbicides biodegrade relatively fast after field
- Potential public health risks from using forestry herbicides
are negligible and are most certainly less risky than their alternatives.
Although these statements make a strong case for the use of
silvicultural herbicides, this logic can be entirely undone if
these chemicals are used in an irresponsible or unlawful manner.
Forest managers and landowners have an obligation to use this
important tool properly to ensure its continued availability.
It is wise to remember, "if you abuse it, you lose it."
The single most important thing to remember about the use of forestry
herbicides is to always read and follow the label instructions.
The label is a legal document and to disregard it may result in
penalties under the law. Disregarding label recommendations could
also reduce application effectiveness. Chemical companies have
invested considerable time and effort into developing label recommendations
that maximize the effectiveness of their product.
Another management technique important to the proper use of
silvicultural herbicides is to leave streamside management zones
(SMZs) along permanent streams. These are buffer strips that are
neither harvested nor sprayed. The utility of these zones for
protecting water quality is well documented and all forest managers
should employ them around permanent bodies of water. Most states
have written Best Management Practices (BMPs) for silvicultural
operations that include SMZs. By following BMPs, the label, and
a conscientious approach to forest management, silvicultural herbicides
will continue to be an effective and environmentally sound forestry
Use pesticides only according to the directions on the label.
Follow all directions, precautions, and restrictions that are
listed. Do not use pesticides on plants that are not listed on
the label. The pesticide rates in this publication are recommended
only if they are registered with the Environmental Protection
Agency and the Alabama Department of Agriculture and Industries.
If a registration is changed or cancelled, the rate listed here
is no longer recommended. Before you apply any pesticide, check
with your county Extension agent for the latest information.
Trade names are used only to give specific information. The
Alabama Cooperative Extension System does not endorse or guarantee
any product and does not recommend one product instead of another
that might be similar.
For more information, contact your county Extension
office. Look in your telephone directory under your county's name
to find the number.