HERBICIDE TOLERANCE IN PLANTS
Homer M. LeBaron
President, Weed Science Society of America
Senior Research Fellow, New Technology and
Basic Research, CIBA-GEIGY Corporation
Herbicide Resistance in Plants1
The first discovery of a triazine-resistant weed (common ground-
sel) was in western Washington in the late 1960s.I hasten to add that there will be
very useful tools and technology developed to help us do better in se-
lecting the more acceptable herbicides, using lower rates, reduce the
leaching and environmental impact of those used, getting more of
them to their sites of action, improve the integration of other control
methods for best management practices, use of biologicals for control
of major or nixous weeds which cannot be adequately controlled with
herbicides, and other improvements for the protection of both crops
and the environment.The Environmental Protection
Biotechnology and Sustainable Agriculture: Policy Alternatives
Agency (EPA) and the public are increasingly concerned about pesti-
cides in groundwater since analytical advances have allowed us to mea-
sure very low and often meaningless levels of synthetic chemicals in
water, mostly traceable to point-source contamination.In the U S. the total
area of land or crops infested with triazine-resistant weeds is still rela-
tively limited (estimated to be about 3,000,000 acres) and does not
seem to be expanding rapidly, except in a few states where continuous
corn or no-tillage farming is being practiced or good alternative herbi-
cides are not used.Some scientists need to study the
effects of herbicides versus tillage practices on the movement of natu-
ral toxins (e.g., organics, inorganics, and microorganisms) into our
water and air, including their potential mutagenic or health effects,
identification, characterization, and quantification.Conservation tillage not only protects 50 to
90 percent of essential topsoil that would otherwise be permanently
lost by water and wind erosion, but it prevents much more than just
inert soil moving into streams, rivers, lakes and air.Although weeds have taken longer to evolve herbicide-resistance
compared to insect pests and pathogens, biotypes of 40 broadleaves
and 15 grass weed species are known to have developed resistance to
triazine herbicides somewhere in the world.There are many ways that
herbicides can and are being used to protect and enhance the environ-
ment for use by humans, birds and animals, and in most cases, they
will be safer and have less environmental impact than other weed con-
trol tools such as mechanical tillage, biological (live organism) con-
trols, etc.In summary, nothing will come out of biotechnology, biocontrol
organisms, or other presently perceived and much talked about techno-
logy that will substantially replace chemicals for weed control in the
foreseeable future (20 to 40 years).Much more soil will be preserved if the pro-
jected 40 million acres (11 percent of total cropland) is set aside over
the next two years and better herbicide programs could be developed
to make farmers more confident that the weeds can be controlled with-
out tillage.A total of 45 weed bio-
types (29 broadleaves and 16 grasses) have evolved resistance to 14
other types of classes of herbicides, making a grand total of 100
herbicide-resistant weed biotypes to date.In most areas of the U.S. where triazine-resitant
weeds have evolved it has not even been necessary or desirable to cease
using the triazine herbicide of choice, due to the many susceptible
weeds that are still usually prevalent.The subsequent
widespread and frequent occurrence of other triazine-resistant weeds
over the past 20 years have made triazine herbicide-resistance the best
known and most studied case of herbicide-resistance.The Low Input Sustainable Agriculture (LISA)
philosophy promotes conservation tillage, as do all weed scientists.Soil conservation programs, agricultural sustain-
ability and production efficiency are, and will continue to be, abso-
lutely dependent upon herbicides.The switch from the moldboard plow and cultivator to conserva-
tion tillage systems makes us more dependent on herbicides, but the
benefits more than compensate for the risks.This reduced ero-
sion, combined with the erodable cropland that has been planted to
grasslands or woodlands have already saved more than half a billion
tons per year of top soil.We will depend more and more on imported
foods, our surpluses will disappear, we will have less control on the
quality of our food, and the greatest agricultural technology in the
world that is responsible for providing by far the highest quality and
variety of food at the lowest prices that this country or any other has
ever known, will be in jeopardy.If other single target site residu-
al herbicides (e.g., diuron) were used as extensively and continuously
as the triazines, they would have almost certainly led to resistant bio-
types.Only 21 of the triazine-re-
sistant biotypes and 16 biotypes resistant to other herbicides have
been found in the U.S., but one or more of these resistant biotypes
have invaded 39 states, six provinces of Canada and 27 other countries.'LeBaron noted that this paper was represented as the President of the
Weed Science Society of American and as a weed scientist, rather than
a representative of CIBA-GEIGY Corporation.It is only because of our surplus
and efficient production without farmers always being the economic
benefactors that we have such vocal opponents to herbicides.Where herbicides have been extensively used, some
species have almost disappeared and the weed seed density in the soil is
often much reduced.Herbicide Resistance in Plants
PUBLIC AND POLITICAL PERCEPTIONS
The main problem in agriculture today is not the technology, but pub-
lic perceptions.There is the option of dropping herbicide use and purchas-
ing only food produced without them, but we will not remain competi-
tive in world agriculture.Triazine-resis-
tance has also been of great interest because of the importance and ex-
tensive use of this group of herbicides.It is very important for nonbiologists to understand that an essen-
tial requirement of herbicides is that they control all weeds through-
out the season.Furthermore, it must not be assumed that LISA or alternate farming
methods have no environmental impacts.They may, in fact, cause
exposure to more toxic or objectional contaminants than do herbi-
cides.We do
not know everything that manure contains, and it may be that very
little of it reaches groundwater, but there could be contaminants in
runoff water.With
only two percent of our population on the farm, our graneries and
supermarkets full, and people who do not have an appreciation for
how sensitive the balance is between feast and famine, some difficult
choices lie ahead.But just do not try to do it without chemical
herbicides or agriculture will fail to be competitive, profitable, sustain-
able, or environmentally sound.HERBICIDES AND LISA
Herbicides have already made great contributions to low input and
sustainable agriculture.This does not mean that mechanical
(e.g., tillage, moving), biological (e.g., mycoherbicides, allelopathy,
cover crops), and other tools are of no value.This trend should be con-
tinued and increased where it can be advantageous to agriculture, as
well as the environment.Some watershed studies in recent years have shown a rever-
sal from major losses to net gain in soil.This is not obvious or easy to measure, especially
to nonbiologists, because there are still many weed seedlings that ger-
minate each spring.Nature will adapt and take advantage of any nitch available, and
the weed infestations and species will likely get worse with time.There is no significant exposure or risk to human
health or the environment from herbicides in food or groundwater; we
are at serious risk of solving the wrong problem.Past experience has shown that weeds resistant to triazines can be
managed or restrained within a reasonable limit.This may be from 5 to 25 species, not just the one or
two pests that insecticides and fungicides usually try to control.However, without herbicides, there would be little or no conservation
tillage in most crops.However, these methods
will continue over the next 20 to 30 years, at least, to be very limited
in application, even though their development and use needs encour-
agement wherever they fit the problem.Herbicides have
had beneficial effects on water quality through conservation tillage;
the whole picture needs to be seen.For example, I would prefer to drink water coming off of or from
under a field treated with herbicides and commercial fertilizers rather
than a field treated with 10 to 20 tons per acre of cow manure.We need to know what the effects and comparative risks
are, and not assume that LISA is a safer way to farm than using syn-
thetic chemicals.Another concern about LISA, or any arbitrary reduction in the use
of herbicides, is the phenomenon of biological changes with time.This phenomenon is even contributing to some of
the short-term successes from cutting back on herbicides.With considerable misgivings, I am prompted to say that what we
most likely need in this country and some others in the developed
world is to experience a little famine.Biotechnology and Sustainable Agriculture: Policy AlternativesVirtually no-
thing is known about natural pollutants or mutagens that have been
and may still be in drinking water.But, the full
effect of reducing or eliminating herbicides will not be seen the first
year.Ignorance, fear and
emotions must be replaced by education, reason and rational thought
and action.In a few cases, the resistant bio-
types have even disappeared.If all
but one species is controlled, little has been accomplished because that
species will take over.In
many situations, the weed populations and pressure are not the same
as 30 years ago.There is not, never has been, and never will be zero-
risk agriculture or life.We need to learn to live with herbicides and solve the
right problems.