As rice is incredibly durable, and difficult to destroy with conventional explosives, and does not burn, the weapon of choice was herbicides. Agent Blue affects plants by causing them to dry out. As rice is highly dependent on water to live, using Agent Blue on these paddies can destroy an entire field and leave it unsuitable for further planting.
Today, large quantities of the chemical named Agent Blue are still used on lawns and crops throughout the USA. Taken from ZNet Ecology:
Arsenical herbicides containing cacodylic acid as an active ingredient are still used today as weed-killers. In the US they are used extensively, from golf courses to backyards. They are also sprayed on cotton fields, drying out the cotton plants before harvesting. So common -- and so profitable -- is the original commercial form of Agent Blue that it was among 10 toxic insecticides, fungicides and herbicides partially deregulated by the US Environmental Protection Agency (EPA) in February 2004. Specific limits on toxic residues in meat, milk, poultry, and eggs were removed.
Prominent military journalist and retired colonel David Hackworth died from bladder cancer in May, 2005. Associates believe that his cancer was caused by exposure to Agent Blue during his service in Vietnam, and are lobbying the United States government to have the substance acknowledged as a known carcinogen like the more infamous Agent Orange.