Wouldn't agent orange count as a chemical weapon? It affected plants and had at least in the medium to longterm, negative effects on humans.

I've never seen any documentation ever alluding to the release of a biological weapon by the United States during a time of war. Biological weapons on the Weapons of Mass Destruction scale are in heavy competition with nuclear weapons for the top slot.

The release of biological weapons is a decision that weighs even more heavily than that of the possible use of nuclear or chemical weapons. Chemical weapons, even persistent agents are a short term weapon and kill only within their area of saturation. It is the same for nuclear weapons, they only effect the area they're detonated in, sans radiation from fallout, assuming the weapon created any.

With a biological agent, it spreads based on contact with the target biological, be it human, livestock, crops, ect... fellow biologicals can contaminate other like biologicals, contaminated crops can contaminate biologicals, and so on and so forth. A biological agent can also mutate, either by design or out of a natural state to overcome obstacles.

The only instances I'v read on where biological weaponry was used during a time of war were; by Germany saboteur forces during World War One lacing crops and livestock with anti-agriculture bio-agents in the US and Europe and of course the infamous Japanese use of bio-weaponry against Allied POWs and Chinese forces in both Mainland China and Manchuria during the Second Sino-Japanese War.