Related Rule
Australia
Practice Relating to Rule 76. Herbicides
Australia’s Commanders’ Guide (1994) states:
It is prohibited to use methods or means of warfare which are intended or may be expected to cause widespread, long-term and severe damage to the natural environment and thereby prejudice the health or survival of the population. 
Australia, Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 909; see also § 930.
The Guide also states: “Weapons that cause widespread, long-term and severe damage to the environment are prohibited”. 
Australia, Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 310.
Australia’s Defence Force Manual (1994) states:
Any method or means of warfare which is planned, or expected, to cause widespread, long-term and severe damage to the natural environment and thereby jeopardise the survival or seriously prejudice the health of the population is prohibited … Means and methods which are not expected to cause such damage are permitted even if damage results. 
Australia, Manual on Law of Armed Conflict, Australian Defence Force Publication, Operations Series, ADFP 37 – Interim Edition, 1994, § 713; see also § 545(b).
Australia’s LOAC Manual (2006) states:
4.11 Environmental modification techniques having widespread, long lasting or severe effects are prohibited. An example of this is defoliant chemicals used by militaries to deprive the enemy of ground cover or kill food crops. The United States used Agent Orange during the Vietnam War for this purpose. These chemicals are not discriminating and are difficult to contain, often resulting in effects to water supplies and creation of toxins dangerous to humans.
7.14 Any method or means of warfare which is planned, or expected, to cause widespread, long-term and severe damage to the natural environment and thereby jeopardise the survival or seriously prejudice the health of the population is prohibited … Means or methods which are not expected to cause such damage are permitted even if damage results. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, §§ 4.11 and 7.14; see also § 5.50.
The LOAC Manual (2006) replaces both the Defence Force Manual (1994) and the Commanders’ Guide (1994).
In 1969, during a debate in the UN General Assembly on the question of chemical and bacteriological (biological) weapons, Australia stated:
The draft resolution [on chemical and bacteriological (biological) weapons under discussion] would declare as contrary to the [1925 Geneva Gas Protocol] “any chemical agent of warfare” with “direct toxic effects on man, animals and plants”. It is the view of the Australian Government that the use of non-lethal substances such as … herbicides and defoliants does not contravene the Geneva Protocol nor customary international law. 
Australia, Statement before the First Committee of the UN General Assembly, UN Doc. A/C.1/PV.1716, 9 December 1969, § 180.