When I first started my Ph.D. in Weed Science, I encountered a strange, new language that appeared to be composed, almost entirely, of acronyms. PRE. POST. PD. Layby. PPI. AI. Etc...
You see, I didn't grow up in, or even around, agriculture; I was born and raised in the "Coal Region" of Pennsylvania and I was infinitely more likely to see anthracite sliding down the chutes of coal trucks (into my neighbors' basements) than I was a John Deere tractor.
So I had to learn. This blog is meant to be a primer for the (similarly) uninitiated.
Firstly, a definition to get us started.
Herbicide: To be honest, there are lots of definitions out there, and they all say just about the same thing: Herbicides are materials that are used to control or kill plants.
Herbicide label: A legal document (recognized by courts of law) describing the brand name or trade name of the product; the name and address of the manufacturer; the amount of active and inert ingredients in the container; the net contents of the container; the EPA registration and establishment numbers; whether the product is for general or restricted use; directions for use, storage and disposal; re-entry, replanting, harvesting and grazing restrictions; environmental hazards and first aid treatments.
Herbicides can be further defined in several ways based on their general mode of action (contact vs systemic), their selectivity (selective vs non selective), their timing (e.g. pre-emergence vs post-emergence) and their application strategy or placement (e.g. broadcast vs banded, soil vs foliar), in addition to other characteristics. This next section will attempt to address the multiple classifications that people may expect to encounter.
Definitions describing the placement of applications.
Soil applied: Herbicides applied to the soil that come into contact with germinating or emerging weeds or into contact with the roots of emerged weeds.
Foliar applied: Herbicides that are applied directly to the plants.
Broadcast: The application of herbicides evenly across an entire area.
Banded: The application of herbicides over a portion of the total treatable area (for example, in strips on top of a seedling).
Directed: The application of herbicides that are targeted at a very specific area (for example, at the base of a crop plant). In certain situations, this might be referred to as a lay-by application.
Definitions describing the timing of herbicide applications.
Pre-plant (PP): Herbicides applied prior to planting. Often, this may refer to herbicides that are applied well in advance of crop planting in order to treat existing vegetation.
Pre-plant incorporated (PPI): Herbicides that are applied prior to planting and that are incorporated into the soil.
Pre-emergence (PRE): Herbicides that are applied prior to crop and/or weed emergence. The herbicides that are considered PRE may also be referred to as 'residual' herbicides meaning that they are applied to the soil where they provide 'extended' control of germinating or emerged weeds.
Post-emergence (POST): May also be referred to as 'topical' or 'over-the-top' herbicides. Herbicides that are applied after crop and weed emergence.
Definitions related to herbicide selectivity.
Non-selective: Synonymous with 'broad-spectrum'; a herbicide that controls many different types of plant species.
Selective: A herbicide that is effective at controlling some species but not others (for example, mostly broadleaves or mostly grasses).
Definitions related to activity.
Contact: Herbicides that affect only the plant tissues that they come into contact with.
Systemic: Herbicides that are translocated, or moved, throughout a plant.
Mechanism of action: The specific, biochemical site within a plant with which a herbicide directly interests. Often confused with 'mode of action', which references the entire sequence of events that results in plant death and injury. For more information about herbicide mechanisms of action, see the WSSA website.
Active ingredient (AI): The particular component of a herbicide that is responsible for its activity against the target plant.
The majority of this information was gleaned from training materials developed by the WSSA. The information was kept relatively simple on purpose. For the more learned, a detailed description of these, and other pesticide-related terms, can be found in a 2006 publication in Pure and Applied Chemistry (78:2075-2154), which is also available at the WSSA website.