Related Rule
Practice Relating to Rule 76. Herbicides
The Military Manual (1993) of the Netherlands states: “Opinion is divided over whether [the prohibition on the use of chemical weapons] applies to … defoliants.” Concerning defoliants, the manual states that Article 35 of the 1977 Additional Protocol I was drafted in the light of the large-scale use of defoliants in the Vietnam War. 
Netherlands, Toepassing Humanitair Oorlogsrecht, Voorschift No. 27-412/1, Koninklijke Landmacht, Ministerie van Defensie, 1993, pp. IV-8, § 14 and p. V-9, § 7.
The Military Manual (2005) of the Netherlands states:
0457. Defoliation agents
The Chemical Weapons Convention says nothing about herbicides, such as defoliation agents …
0458. Because of the military significance of modern chemical weapons, many States still allow for the possible use of such weapons. 
Netherlands, Humanitair Oorlogsrecht: Handleiding, Voorschift No. 27-412, Koninklijke Landmacht, Militair Juridische Dienst, 2005, §§ 0457–0458.
The Minister of Foreign Affairs of the Netherlands stated in a parliamentary debate in 1995:
Generally speaking, the use of herbicides as a means of warfare is prohibited according to international customary law, and also according to the ENMOD treaty and the Geneva Conventions (1949), if this use causes widespread, long-term and severe damage. Then, the prohibition is binding upon all states. 
Netherlands, Lower House of Parliament, Debate on Chemical Weapons, Tweede Kamer 68, 25 April 1995, pp. 68–4105.