Transgenic Crops and Issues in Weed Management
Alex R. Martin, Professor
Department of Agronomy
University of Nebraska
Lincoln, Nebraska
U.S.A.
No-tillage crop production leaves crop residue on the soil surface. The residue
protects the soil from raindrops and wind which detach soil particles, a first step in soil
erosion. Residue reduces soil surface crusting thereby increasing moisture infiltration.
Surface residue slows the velocity of runoff water, increasing the time available for infiltration
and reducing the amount of soil carried in runoff water.
Crop residue remaining on the soil surface has a positive effect on surface water
quality. Because water runoff is reduced in quantity and velocity, off site movement of
sediment, pesticide and fertilizer is reduced. This results in less deposition in surface water
and therefore benefits surface water quality.
Surface residue protects the soil from sun and wind reducing evaporative water loss.
Standing residue traps snow in the winter increasing soil moisture. The resulting increased
soil moisture is beneficial to crop yield in limited moisture environments.
Weed control is a challenge to successful no-till crop production. Weeds established at
planting time as well as those that develop later must be controlled. Reliance is placed
completely on herbicides to control weeds in no-till crop production. Effective economical
weed control has been a continuing challenge in no-till crop production. Crop producers have
systematically progressed through a series of weed management strategies in an effort to
combat constantly evolving weed populations. Herbicide resistant crops (HRCs) used with
the appropriate herbicide(s) are effective tools in a weed management program.
Herbicide resistant crops (HRC) allow the “in crop” use of herbicides for selective weed
control that would otherwise seriously injure or kill the crop. There are several HRCs
available or under development (Table 1). The Roundup and Liberty HRCs were developed
utilizing genetic engineering techniques while the bromoxynil, IMI, Poast and STS crop
varieties were developed utilizing only conventional plant breeding techniques. Crops
developed through genetic engineering may be subject to distribution and marketing
limitations in some countries.
Table 1. Herbicide resistant crops (HRC) and their date of registration.
Crop
Alfalfa
Canola
Corn
Cotton
Flax
Potato
Rice
Soybean
Sunflower
Sugar beet
Wheat
Bromoxynil
IMI group
Glufosinate
1999*
1992
1999*
1997
1998
Glyphosate
2003
1999*
1998
1997
Sethoxydim
SU group
Canada
1995/96
Canada
2001
2002
2001
1998#
2002
2003
1996
2000**
2005
2000**
2003
1993
*Registration in certain states and Canada.
**Registered, but will not be planted commercially.
#Registered, but not sold.
Herbicide resistant crops exhibit excellent tolerance to the herbicide involved
essentially eliminating crop injury. Thus, application rate, timing, and environmental
conditions are not concerns with respect to crop injury although these factors are important
with respect to weed control. These characteristics of HRCs are great advantages to
producers and resulted in the rapid acceptance of this technology. Over 50% of the soybeans
in the United States in 1999 were Roundup Ready. Volunteer herbicide resistant crops are
also resistant to the herbicide involved. Specific plans must be developed for controlling
volunteer herbicide resistant crop plants. As with any practice, use of HRCs and the
corresponding herbicide should be part of an integrated weed management program.
Currently there is a major controversy surrounding the use of genetically modified
organisms (GMOs) in agriculture. The term GMO although widely accepted is probably not
the most descriptive from a scientific perspective. The term GMO is applied to organisms
(including crop plants) that have had a gene or genes from some other organism, the gene
could come from any living organism, inserted into the crop’s chromosomes through genetic
engineering techniques. These “new” genes are called transgenes. “Transgenic” is a more
appropriate term for plants that contain transgenes. However, the term GMO has become
widely used in referring to plants containing transgenes.
The GMO controversy impacts weed management in that some but not all of the HRCs
that have become widely used are GMOs. The non-GMO HRCs are not involved in this
controversy. From a market perspective it is important to understand which HRCs are
classified GMO and which are non-GMO (Table 2).
Table 2. HRC Classification.
Trait
IMI-Clearfield
Roundup Ready
SR-Poast Protected
Liberty Link
STS
Crop
Several
Several
Corn
Several
Soybean
Description
Resistance to Imidazolinone Herbicides
Resistance to Roundup (glyphosate)
Resistance to Poast (sethoxydim)
Resistance to Liberty (glufosinate)
Resistance to Sulfonylurea Herbicides
GMO
No
Yes
No
Yes
No
There is a great deal of confusion on the GMO issue in the public arena fueled in-part
by a lack of understanding. Often positions taken are founded on misunderstanding and
confusion of the issues involved. In some cases the issue of market consolidation and
integration of crop production and marketing are commingled with biological issues when the
GMO issue is examined. The GMO issue has been portrayed as contributing to an
undesirable integration in agriculture. It is important to systematically examine the key issues
in order to evaluate the merits of various positions.
Market limitations of transgenic crops is impacting the adoption and economics of their
use. Market place resistance to GMOs is perhaps greatest in Europe and relatively low in
several countries including the U.S. and Canada although this could change quickly. The
market acceptance issue is composed of two dimensions, registration or approval of the
GMO, and consumer acceptance. GMOs are approved on a case by case basis. It is
common for one GMO to be approved and another not approved within the same country.
Equal in importance to regulatory approval is consumer acceptance regardless of approval
status. In fact consumer resistance is becoming a more restrictive force in the market place
than registration. Recently in the United States, Gerber Foods, a manufacturer of foods for
infants and small children, decided not to use any GMOs in their products. This position was
driven by consumer acceptance.
Most current GMOs contain “input” traits rather than “output” traits. Input traits are
useful to the producer in the production of a crop. Examples of input traits are herbicide
resistance and insect resistance. Output trains influence the end use value of a crop.
Examples of output traits include high oleic acid corn and low saturate soybean. Output traits
may bring nutritional or quality attributes to the consumer of the crop.
Impart the market resistance to GMOs is due to the fact that most current GMOs
contain input traits rather than output traits. The benefits associated with input traits are
realized by the producer. The consumer does not realize a benefit from an input trait GMO to
offset any perceived risk associated with its use.
Market resistance to GMOs has its basis in three broad areas, food safety concerns,
ecological concerns, and moral/philosophical concerns.
Food safety concerns associated with GMOs include potential allergenic response;
antibiotic resistance, nutritional concerns, and concerns regarding unknown effects. These
concerns have their foundation in the belief that the current regulatory/registration process for
GMOs is inadequate to protect the public. GMOs are subject to carefully testing before they
are approved. However, a segment of the public believes the testing and examination of
GMOs must be more rigorous. To date there have been no confirmed occurrences of
adverse impact of GMOs on consumers.
Ecological concerns regarding the use of GMO crops include gene escape to wild
relatives, pest resistance resulting from widespread use, impact on non-target species and
gene trespass. It is important to note that any ecological risks associated with transgene
escape are related to the traits they impart and not to the transformation techniques. Gene
escape to wild relatives could occur if the GMO crop had a sexually compatible weedy relative
with which it could readily hybridize (Table 3). The likelihood of hybridization is influenced by
several factors including the degree of cross-pollination, pollen mobility and pollen viability of
the species involved. In the case of an escaped transgene for herbicide resistance, a fitness
advantage would occur only in an agricultural setting subjected to treatment with the herbicide
involved. The “escaped gene” imparting herbicide resistance would have no impact on the
fitness in a natural environment not treated with the herbicide.
Table 3. Examples of cultivated crops and sexually compatible weedy relatives.
Crop
Sexually compatible weedy relative
Canola (Brassica napus, B. rapa, B. juncea)
Sorghum (Sorghum bicolor)
Wheat (Triticum aestivum)
Rice (Oryza sativa)
Sunflower (Helianthus annus)
B. napus, B. rapa, B. nigra
S. sudanense, S. bicolor, S. almune, S. halepense
Aegilops cylindrica, Agropyron spp.
Oryza sativa
Helianthus annus
The consequence of a herbicide resistant transgene escaping to a weedy relative
would be realized by the producer and the developer of the transgene and herbicide in the
form of a loss in herbicide efficacy. Only the agricultural community would be impacted.
The concern about the increased risk of pest resistance to herbicides as a result of the
use of herbicide resistant crops is a result of the management practice utilized and not to the
transgene. Repeatedly using the same herbicide as the primary weed control measure exerts
a selection pressure on the weed population for herbicide tolerance or resistance. The
transgene imparting herbicide resistance in the crop has no direct involvement in the selection
for herbicide resistance in the weed population. It is possible that herbicide resistant crops
will tempt producers to use the same herbicide repeatedly. This repeated use of the same
herbicide is biologically unsound and will contribute to selection for herbicide resistant weeds
whether a transgenic crop is involved or not. The widespread occurrence of acetolactate
synthase (ALS) resistant weeds in the United States resulted following the wide use of ALS
inhibiting herbicides. Herbicide resistant crop cultivars were not involved.
A related issue sometimes confused with the escape of a herbicide resistant gene to
weeds is the escape of insect or disease resistance genes from crop plants to weeds. The
latter event could result in increased fitness of the weed in agricultural and native
environments.
The current controversy in the United States regarding the impact of Bt (Bacillus
thuringensis) corn pollen on the larvae of the monarch butterfly is an example of potential
adverse affect of a transgenic plant on a non-target species. The toxic protein encoded by
the Bt gene expressed in Bt corn maybe toxic to several Lepidoptera species. This is an
issue because the European Corn Borer (ECB), the target pest, is closely related to several
desirable species. In the case of an HRC, whether a transgene is involved or not, there is not
likely to be an adverse affect of the gene on non-target species. Misapplication of herbicides
could damage non-target organisms, however this is not specific to GMOs.
Gene trespass refers to the unintended movement of transgenes from a crop in one
field to an adjacent field, often via pollen movement. This results in the transgene being
present in the seed harvested from the “trespassed” field. This will be an issue of concern as
long as there is a market classification for non-GMO commodities. The intensity of this
problem will depend on the tolerance level set for GMO contamination of non-GMOs. Gene
trespass is an issue primarily with cross-pollinated crops.
Moral/philosophical concerns regarding transgenic crops have been expressed for a
variety of reasons including the premise that this represents an unnatural process. It should
be noted that cultivated wheat is a result of an interspecies cross that occurred in nature. A
soil bacteria exists that can transfer some of its genes to plant chromosomes. It is true that
genetic engineering techniques make it possible to transfer genetic material between
organisms quite different from each other i.e. bacteria and plants. An individual’s values will
determine whether they object to transfers of genetic material between very dissimilar
organisms.