Re: How does glyphosphate herbicide work?

Glyphosate is a herbicide that looks a little like an amino acid.  Remember 
this as it is important later.  I'm not sure of your educational background 
so I'll start at the begining.  Plants take carbon dioxide, water, and a 
few other nutrients and make more complex substances.  To do this takes a 
series of chemical reactions.  For the reactions to take place, energy must 
be expended.  Initially the sun provides the energy for the water and 
carbon dioxide to make sugar and other high energy compounds.  After that, 
plants make more complex substances using the energy in the sugars and 
other compounds (there are also other things like pH gradients and proton 
fluxes, but I will ignore these).  

Chemical reactions take a lot of energy.  Some very complex compounds, 
called enzymes help reduce the energy required to make the reaction work.  
They do this in a number of ways, but basically, one compound binds to the 
enzyme, the enzyme stretchs, bends, and/or reduces or increases charges, 
such that a second compound can now easily combine with the first, with 
much less energy required.  The new compound that is formed is released and 
then modified futher by another enzyme or used for something else by the 
plant.

Enzymes are made up of simpler compounds called amino acids.  There are 
twenty different kinds.  Three of them have an aromatic group which is made 
up of a ring of 6 carbons.  All three are made through a very similar chain 
of reactions.  Glyphosate, sort of looks like these at a stage of their 
synthesis (making).  Glyphosate binds to the enzyme (5-enolpyruvoyl 
shikimate phosphate synthase) which is one of the enzymes in the shikimic 
acid pathway.  Once there, it stuffs up the enzyme.  Glyphosate does not 
react with the second compound and it does not unbind.  The plant no longer 
can make the 3 aromatic amino acids and after a week or two the plant plant 
yellows up and dies.

Animals can't make these 3 amino acids and the shikimic acid pathway is not 
part of animal processes.  Therefor glyphosate does not cause the same 
thing to happen in animals or humans which gives us some safety.  Animals 
need those three amino acids however, and they get it by eating plants or 
eating animals which ate plants, or eating animals which ate animals who 
ate plants.

For this information I used an old weed science text "Weed Science: 
Principles and Practices" by Ashton and Monaco. 1991 Wiley and Sons, Inc.

There is a great web site for things weedy.  http://ext.agn.uiuc.edu/wssa/
It has info on weeds, weed management, even videos of plants dying after 
being sprayed.  It sound grim but it is great for determining what chemical 
killed your grapes when some farmer up the valley sprays and you think it 
was drift from is treatment that did it.  There are also lots of links for 
the inquisitive.