In the case of herbicide tolerant crops, the genetic or classical breeding
modifications required to make a crop resistance to a herbicide are not
designed and almost certainly never increase the varieties inherent yield
potential. No one claims that they do. In most cases, the genetic
transformation comes at some cost to the plant; i.e., it imposes some
degree of physiological yield drag because it requires the plant to not
just carry out normal growth processes, but also to do something that
detoxifies or otherwise makes the plant immune, or largely immune from the
effects of a herbicide. In general, there are still no free lunches,
although it is true that breeders are making progress, and will make
further progress, to isolate the specific genes that confer resistance and
more "efficiently" incorporate them in varieties. It is possible a no
yield drag Roundup Ready crop will yet be created, but I suspect it will be
the exception, not the rule.
Yield increases in herbicide tolerant varieties are often reported and
come from improved weed management and/or lessened injury from other
herbicides, especially the low-dose, very active and sometimes damaging
sulfonylureas and imidazolinone herbicides. Many of the herbicide products
in these two families of chemistry are very persistent in some
soils/systems and can cause carryover problems with subsequent crops.
The five major issues/concerns in need of monitoring over time in the case
of herbicide tolerant varieties are :
* these weed management systems are designed to be largely or fully
dependent on herbicides as weed management tools, and hence increase
reliance on them and the frequency and intensity of their use relative to
other systems that include some other practices in a multi-tactic, "many
little hammers" integrated weed management system
* cost/profitability consequences for farmers and the relative benefits
farmers receive versus seed/pesticide companies
* added sprays, and the higher rates possible with resistant varieties, and
needed for later season control when weeds are bigger and harder to
control, will trigger shifts in weed communities, requiring additional
herbicides or weed management practices (clearly happening with RR soybeans
in the U.S.)
* added sprays, higher rates can accelerate the emergence of resistant
weeds, a process that is also unfolding in the U.S.
* loss of flexibility in planning crop rotations and using cover crops
because of the need to avoid herbicide carryover damage
On the yield drag front, Kansas State University researchers have recently
posted a useful assessment of the impacts of different herbicide mixtures
on RR soybeans and conventional varieties (see
<http://www.ksu.edu/kscpt/98/98beans/kssbc.htm>. They conclude that
commonly applied pre-emergence or post-emergence products can be combined
with Roundup/RR beans without causing problems with RR soybean yields.
They also calculated the yield drag between RR varieties and closely
matched conventional varieties in a "weed free environment." This
experimental design is a good one to isolate yield drag from impacts on the
quality of weed management. Trials were conducted in three locations in
the state; the mean yield drag was 8.8% (5 bushels) in one; 2.2% in a
second location (not stat. significant); and 10.8% in a third location (3.8
bushels). These results are consistent with those found in the large
review of trials by Oplinger, which I further analyzed in my report on RR
bean yields accessible on Ag BioTech InfoNet at
<http://www.biotech-info.net/RR_yield_drag_98.pdf>
In a few months, results will start appearing on 1999 soybean yield trails
and we will see if progress has been made in narrowing the average yield
drag evident in the 1998 varietal trials. I would be interested in
"off-line" dialogue with anyone at land grant universities or allied
research institutions who is carrying out comparable analyses of GMO versus
conventional variety yields and performance, pesticide use, profitability,
etc. I welcome suggestions re how to resolve methodological issues and
carry out more accurate comparisons. It is a big job, perhaps we can make
more progress through some coordination of effort.
chuck
Charles Benbrook CU FQPA site www.ecologic-ipm.com
Benbrook Consulting Services Ag BioTech InfoNet www.biotech-info.net
5085 Upper Pack River Road IPM site www.pmac.net
Sandpoint, Idaho 83864
208-263-5236 (Voice) 208-263-7342 (Fax)
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