Principles of Postemergence Herbicides

June 4, 1999
Postemergence herbicides are an integral part of an integrated weed management program. Applications made after crops and weeds have emerged allow for identification of the weed species present as well as the severity of infestation, so that herbicide selection can be tailored to a particular field. Postemergence herbicide applications minimize the interactions of the herbicide with factors associated with soil (such as soil texture and organic matter content), but often magnify interactions between herbicide and prevailing environmental conditions.

In order to achieve weed control with postemergence herbicides, the herbicide must come in contact with the target, be retained on the leaf surface prior to absorption into the plant, be able to reach the site of action within the plant, and finally induce some phytotoxic response. If for any reason one or more of these steps is restricted or limited, the level of weed control can be expected to decline.

The plant cuticle serves as an outer protective layer, or "barrier," that restricts the amount of water lost by the plant through transpiration. It also serves a variety of other functions, and the cuticle is often considered the primary barrier that limits herbicide absorption. The cuticle is composed primarily of waxes and cutin, sub
stances that effectively limit water movement out of (transpiration) or into (absorption) the plant. The type and amount of wax that comprises the cuticle influence the degree of wetting that can be achieved, and this composition can change with age and in response to changes in the environment. Older plants and plants under environmental stress generally have more wax or a different structure of the wax comprising their cuticles and are thus more difficult to wet. One of the main functions of certain spray additives is to enhance herbicide pene- tration through the cuticle.

Plant age and size, relative humidity, soil moisture, and temperature are other factors that influence absorption of postemergence herbicides. Younger, smaller plants usually absorb herbicide more rapidly than older, more mature plants. Many postemergence herbicide labels recommend that applications be made when target weeds are small and warn of reduced effectiveness if applications are made to larger plants. More and more postemergence herbicide labels are also cautioning users to delay applications if weeds are under "adverse environmental conditions." Examples of such adverse environmental conditions may include prolonged periods without significant precipitation (dry soil) or low air temperatures. On the other hand, high relative humidity, adequate soil moisture, and moderate to warm air temperatures all favor enhanced herbicide absorption. Remember that if conditions occur for enhanced absorption into weeds, conditions are also favorable for enhanced absorption into the crop, which may result in crop injury.

Postemergence herbicides vary in their mobility with the plant. Some demonstrate very limited movement following absorption and are commonly referred to as "contact" herbicides. Others can move extensively within the vascular elements of the plant and are referred to as "translocated" herbicides. Contact herbicides do show some limited movement following absorption, but not nearly as extensively as translocated herbicides. Thorough spray coverage of the plant foliage is very important with contact herbicides and somewhat less important with translocated herbicides.

Adjuvants are added to the spray mix to improve herbicide performance and minimize potential failures under adverse conditions. The most common adjuvants are nonionic surfactants (NIS), crop oil concentrates (COC), and ammonium fertilizer salts. These are used to increase the effect of the spray on the target site. There are numerous spray additives to choose from, and confusion as to which is the best to add is common. Take the time to read the herbicide label and determine what type of spray additive is recommended or which may actually be restricted from being used with a particular herbicide. Many times applicators will attempt to select an additive system to "heat up" the spray on the weeds, but remember that the spray solution may also "heat up" the crop if conditions are not favorable for rapid crop recovery.--Aaron Hager, Marshal McGlamery, Christy Sprague

Author: Aaron Hager Marshall McGlamery Christy Sprague