Foliar Active Herbicides

Foliar active herbicides are applied to the above ground portion of emerged plants (postemergence) and are absorbed by the exposed tissue, leaves, stems, etc. Foliar active herbicides that move, translocate, throughout the plant following  absorption are often referred to as 'translocated' or 'systemic' herbicides. Foliar active herbicides that do not move within the plant following initial absorption are often referred to as 'contact' herbicides.

Whether a herbicide is a translocated or contact type has implications for application requirements and performance characteristics. For contact herbicides to be effective the entire surface of a plant must be thoroughly covered for the plant to be controlled. Since contact herbicides do not move within the plant following absorption treating one portion of a plant with a contact herbicide will result in destruction of only the treated portion of the plant. Because translocated herbicides move within the plant following absorption treating one portion of a plant with the herbicide may still result in some effect in the entire plant (Figure 8).

The waxy cuticle on leaf surfaces is a barrier to absorption of foliar applied herbicides.

Environmental conditions and age of the tissue effect cuticle development. Cuticle development tends to be greatest on older mature tissue and least in young immature tissue. Cuticular wax characteristics are influenced by environmental conditions. Under high light and low moisture conditions, cuticular wax develops that is less permeable to water soluble herbicides. For this reason absorption of foliar active herbicides tends to be greatest in immature tissue normally located in the upper portion of a plant. For maximum effect with foliar applied herbicides the upper portion of the target plants should be treated.

Treating only the lower, mature, portion of otherwise susceptible plants may not result in control (Figure 9). Directing herbicide applications to the lower portions of crops is a common technique used to reduce crop injury from herbicides that have limited crop safety (Figure 10).

Because the waxy cuticle on the leaf surface is a barrier to absorption of foliar applied herbicides, surfactants or oils are often added to the spray mixture to improve absorption and herbicide activity. Herbicide mixtures applied in water do not readily 'wet' the waxy foliage because the high surface tension of water causes spray droplets to remain as droplets on the leaf surface. Surfactants reduce the surface tension of aqueous mixtures resulting in spray droplets spreading as a thin film on the leaf surface thereby increasing the area of contact between the herbicide and the leaf and thereby increasing penetration.

The use of  oil additives with aqueous mixtures of herbicides also results in the spray mixture spreading on the leaf in a thin film increasing the area of contact. Both oil additives and surfactants can act as solvents for the waxy cuticle and aqueous spray mixture resulting in increased penetration of the cuticle. Under conditions of a well developed cuticle, oils are often more effective than surfactants in increasing herbicide activity (Figure 11 & Table 5).

Plants growing under a limited moisture regime and low relative humidity tend to have a more highly developed cuticle which is a barrier to absorption of postemergence herbicides (Figure 12). As a result the activity of postemergence herbicides is reduced by dry conditions (Table 6).

The negative effect of dry conditions on postemergence herbicide activity can be offset in part by judicious use of spray additives (Table 5). The growth stage of a weed influences its susceptibility to postemergence herbicides. Other things being equal small weeds are more susceptible than large weeds for a number of reasons. The leaf cuticle of small plants is less well developed than on older plants hence herbicide absorption is greater in the younger plants (Figure 13).

The surface area to mass ratio is greater in small plants than large plants so the amount of herbicide absorbed per unit mass is greater in the small plant.

The life cycle (annual, biennial or perennial) of a plant interacts with growth stage (leaf number) in determining susceptibility to herbicides. Annual and biennial plants reproduce by seed only. Killing the top growth of these plants will result in their death. Annual and biennial weeds are most susceptible to foliar applied herbicides prior to bolting or flower stalk elongation (Table 7).

The smaller the weed at treatment the more susceptible it is to herbicides. Annual, biennial, and perennial weeds are all susceptible to herbicides in the seedling stage. Perennial weeds, after they become well established, reproduce by seed and vegetatively from roots, rhizomes, crowns and other vegetative structures. In order to kill a perennial weed the vegetative reproductive structures must be killed. In most cases the vegetative reproductive structures are located below ground and therefore would not be contacted by a foliar herbicide.

Therefore a foliar active herbicide must translocate to these reproductive tissues to kill them. Contact foliar applied herbicides have no effect on underground reproductive structures of perennial weeds. Translocation of foliar applied herbicides to vegetative reproductive tissues is greatest with treatments made to plants in the flower bud stage and in the Fall before a hard freeze.

It is common for different populations or biotypes of a weed species to differ in herbicide susceptibility. There is a great deal of genetic diversity within certain weed species. Some of this diversity relates to herbicide susceptibility. An extreme case of differential response within a species is illustrated by resistance wherein one biotype may be susceptible and another totally unaffected by the same herbicide (Figure 14). There are many examples of differential herbicide susceptibility within a species that do not involve resistance (Table 8).