Transgenic Crops and Issues in Weed Management

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11 Δεκ 2012 (πριν από 4 χρόνια και 7 μήνες)

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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 raind
rops 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
an
d 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 fert
ilizer 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 thr
ough 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 so
me countries.






Table 1. Herbicide resistant crops (HRC) and their date of registration.

Crop

Bromoxynil

IMI group

Glufosinate

Glyphosate

Sethoxydim

SU group

Alfalfa





2003



Canola



1999*


1999*


1999*



Canada

Corn



1992


1997


1998


1995/96


Cotton


1998




1997



Flax







Canada

Potato





2002



Rice





2003



Soybean




1998#


1996



1993

Sunflower



2001





Sugar beet



2002


2000**


2000**



Wheat




2001


2005


2003




*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 elimin
ating 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
producer
s 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 c
ontrolling
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 mo
dified
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 o
rganism, 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. Howe
ver, 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
contr
oversy. 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

Crop

Description

GMO

IMI
-
Clearfield

Several

Resistance to Imidazolinone Herbicides


N
o

Roundup Ready

Several

Resistance to Roundup (glyphosate)


Yes

SR
-
Poast Protected

Corn

Resistance to Poast (sethoxydim)


No

Liberty Link

Several

Resistance to Liberty (glufosinate)


Yes

STS

Soybean

Resistance to Sulfonylurea Herbicides


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 consolid
ation 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 i
s
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 cont
ain “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 t
raits 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 regar
ding 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
relat
ives, 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 te
chniques. 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
-
p
ollination, 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. Th
e “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 re
lative

Canola (
Brassica

napus
,
B
.
rapa
,
B
.
juncea
)

B
.
napus
,
B
.
rapa
,
B
. nigra

Sorghum (
Sorghum

bicolor
)

S
.
sudanense
,
S
.
bicolor
,
S
.
almune
,
S
.
halepense

Wheat (
Triticum

aestivum
)

Aegilops

cylindrica
,
Agropyron

spp.

Rice (
Oryza

sativa
)

Oryza

sativa

S
unflower (
Helianthus

annus)

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. Repe
atedly 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 selecti
on
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 cr
op 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 transgeni
c 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 sp
ecies. 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.


Ge
ne 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/p
hilosophical 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 n
ature. 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 plant
s. An individual’s values will
determine whether they object to transfers of genetic material between very dissimilar
organisms.