BILANAPHOS
VS GLUFOSINATE
About 2
years ago, I published an article on this same comparison, but from a different
angle (http://culturagriculture.blogspot.com.es/2016/01/66-plant-protection-4-on-boarders-of.html).
It seems
interesting to me to take it again from the angle of the opposition between
natural and synthetic. We have seen in previous articles that this
differentiation is sometimes at the limit of the reasonable. This example
demonstrates it once more.
This
time it is an herbicide. It is all the more interesting that the problem of the
control of harmful to crops plants is one of the great difficulties of organic
farming.
“How to Make a Natural Weed Killer
January 6, 2016 Posted by Andrew
Kniss*
Well over a year ago, I wrote about
a homemade herbicide containing salt, vinegar, and dish soap.
“Many of you have probably seen it
posted to Facebook or Twitter or Pinterest, or on your favorite home gardening
site. One of my favorite descriptions calls it a “magical, natural, weed
killing potion.”
That particular potion certainly
kills weeds, but it isn’t natural (and it certainly isn’t chemical-free). It
contains dish soap and vinegar, both of which are synthesized industrially, so
it isn’t natural by most definitions of the word. That’s disappointing, because
people really yearn for a natural weed-killer. They want to kill the weeds
around their homes and in their gardens, but they don’t like the idea of using
a synthetic pesticide. Most people (including me) would prefer to use something
natural, all else being equal. Unfortunately, there are very few truly natural
products that work as effective herbicides.
That being said, I’d like to
introduce you to a fascinating chemical named bilanaphos. In the early 1970’s,
bilanaphos was discovered independently by two different laboratories, one in
Germany and the other in Japan. Both groups isolated this chemical from
Streptomyces bacteria; S. viridochromogenes in Germany, and S. hygroscopicus by
the Japanese group. Bilanaphos is produced naturally by these naturally
occurring bacteria. So, by nearly any definition, bilanaphos is natural.
The scientists in Germany and Japan
both learned early-on that bilanaphos had strong weed-killing properties; when
it was applied to plants, the plants died. Upon further investigation,
scientists in the German group recognized that only part of the full bilanaphos
chemical was required for herbicidal activity. In fact, when bilanaphos enters
the plant, about half of the molecule is quickly chopped off, leaving behind a
smaller molecule – phosphinothricin. It is this smaller molecule that acts as an
herbicide in the plant.
When the naturally occurring
compound bilanaphos (left) enters the plant cell, the plant removes two alanine
residues leaving behind the chemical phosphinothricin (right). Phosphinothricin
exhibits herbicidal activity in most plants, by inhibiting the glutamine
synthetase enzyme.
So we have a natural compound
(bilanaphos) that is converted naturally by plants to another compound
(phosphinothricin) that works very effectively as an herbicide. And it turns
out that some Streptomyces species naturally produce a small amount of phosphinothricin
also. That sounds very much like a natural herbicide, right? Not so fast…
Phosphinothricin (better known in
the US as glufosinate) is widely used as an herbicide today. It is the active
ingredient in herbicides like Rely (mostly used in tree and vine crops), and
Liberty (most commonly used in conjunction with Liberty Link crops). But even
though the chemical occurs naturally, and was first discovered by extracting it
from naturally occurring bacteria, the commercial herbicide is produced
synthetically. So it is not considered a ‘natural’ herbicide.
The story of phosphinothricin,
while very interesting, is not unique. A huge number of scientists around the
world are searching nature to find new chemicals that have antibiotic,
pesticidal, or other useful properties. Between 1997 and 2010, USDA scientists
estimate that about 69% of all new pesticide active ingredients registered by
the EPA were either natural products, synthetic products derived from natural
sources (like phosphinothricin), or biological in nature. For example, another
commonly used corn herbicide was discovered after an initial observation that
few plants could grow underneath a red bottlebrush bush in a garden. But weed
killers are actually the smallest component (less than 7%) of these new
pesticides of natural origin; around 30% of new insecticide or fungicide active
ingredients are either natural products or natural product-derived.
Currently, the FDA is struggling to
define the word natural on food labels. It is an often-used marketing term with
no clear definition. It may be even more difficult to define when discussing
pesticides. As the phosphinothricin example shows, the lines between natural
and synthetic can get blurred quickly. Is it natural because it occurs in
nature? Or does it have to be physically extracted from nature to be considered
natural?
The ‘natural or not‘ distinction
can distract from what is really important when discussing pesticides. If the
compound is structurally the same, the naturally occurring and the
synthetically produced versions will share the same properties. The properties
of the compound are far more important, in my opinion, than the source of the
compound. Is the pesticide safe for applicators and the environment? Does it
break down quickly in the environment to non-toxic products? If so, then I’m
much less worried about whether it is natural or not, regardless of how we
define natural.
But there are questions related to
the source of the product that can be important. In particular, which has a
greater impact, synthesis in the lab? Or extraction from natural sources? I
rarely hear discussions related to this question, but this is among the most
important questions related to natural products (provided they are deemed
safe). If we can efficiently extract a renewable resource from nature, and
avoid the energy and fossil fuel requirements of synthetic production, then a
naturally produced product sounds pretty good to me. But if extracting
something from nature means we’ll have a greater negative impact on the
environment than we would producing it in a factory, then please give me the
synthetic version.
References:
Hoerlein (1994) Glufosinate
(Phosphinothricin), A Natural Amino Acid with Unexpected Herbicidal Properties.
p 73-145 in Reviews of Environmental
Contamination and Toxicology (Vol 138)
Dayan et al. (2011) Rationale for a
natural products approach to herbicide discovery. Pest Management Science.
68:519–528
Cantrell et al. (2012) Natural
Products as Sources for New Pesticides. Journal of Natural Products. 75:1231-1242. »
_______________________________________
* Andrew Kniss is Professor of
Ecology and weed management at the University of Wyoming. "
The
ideology of organic farming obliges us to use only pesticides of natural
origin. Yet exemptions exist, depending on the possibilities of certain molecules,
or on how to use them, or on the needs of farmers who sometimes justify
infringements to the rules, carefully kept under silent, at least towards
consumers.
It
is sometimes difficult to understand why some molecules, produced in a totally
industrial way (like deltamethrin which is a pyrethroid of synthesis) are
accepted in organic and others, quite comparable in their manufacturing
process, while being of simple copies of molecules of natural origin, are not, as
is the case of azadirachtin, naturally produced by the neem tree, and main
active ingredient of all organic pesticides based on neem oil.
By cons,
pheromones used in organic farming, as in conventional, are 100% synthetic
products, which are copies of pheromones, naturally emitted by insects. In this
case, there is no problem. It is true that these products are not sprayed on
crops. But they float in the air day and night for months, and are necessarily
deposited on products that will be food.
One
may also wonder for example, why spinosad, naturally produced by bacteria
(Saccharopolyspora spinosa), and manufactured on a large scale by a fully
industrialized process, is allowed in organic farming, while bilanophos also
naturally produced by bacteria (of the genus Streptomyces) is not?
After all,
what is missing to organic farming to be generalized?
Tools,
technical solutions to solve concrete problems, in particular at the level of
phytosanitary protection.
Most
of the other problems have coherent solutions (with nevertheless a weakness on
the nutritional aspects, which progress however quickly), and even often of a
very reasonable cost.
But
we can see that, while it is certain that conversions to organic farming are
becoming more numerous (rarely because of personal conviction, but more because
of societal pressure, or because of economic opportunism), there are also more
and more frequent steps backwards. The main cause mentioned by these farmers is
the unresolved phytosanitary problems that are accompanied by significant
reductions in yields or quality decline, and ultimately a serious problem of
income for the farmer.
Because the
increase in the supply of organic products and their democratization are
accompanied by a perverse effect, moreover quite predictable, which is the
decline in prices, not necessarily for consumption, but for the farmer.
Who says
lower prices, also says lower income, and obligation to improve the (visual)
quality, so to increase the real cost of production. While it is true that
organic production does not have the obligation to meet the same criteria for
standardizing food quality as conventional agriculture, the reality is slowly
changing and marketing circuits require more and more a product organic and
beautiful at the same time.
A kind of
backlash that could be largely avoided if the use in organic farming of ever
more numerous synthetic pesticides "copied from nature" was allowed.
Will
reason ever win the match against dogma?
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