Postemergence Herbicide Principles
Part 2, Section 1: Pest Management
Postemergence herbicide principles
A number of herbicides are applied after the weed emerges. These materials generally have foliar rather than soil activity, but some products have both. Foliar-applied herbicides run many of the same risks as soil-applied products, with applications being very dependent on climatic conditions before and after application.
Herbicide rates and timings are based on weed species, size, and growing conditions. Smaller weeds generally require less herbicide. Larger weeds or weeds growing under stress conditions are harder to control and may require higher rates plus the addition of specific adjuvants. Foliar-applied herbicide performance increases with temperature and humidity, while rainfall shortly after application (less than 6 hours) can reduce the effectiveness of many postemergence products.
Foliar-applied herbicides are classified as either systemic, which means they translocate in the plant, or contact, if little or no movement outside the treated leaf or structure occurs. Translocated herbicides are effective at lower spray volumes (5 to 20 gallons per acre), where the herbicide droplets are more concentrated. Contact materials require better coverage and should be applied at 20 to 40 gallons per acre (ground application). Flat-fan or hollow-cone nozzles work best for contact herbicides because they produce smaller droplets for better spray coverage. See the herbicide characteristics tables in each crop section of this guide for more information about systemic activity.
Adjuvants such as crop oil concentrate (COC), nonionic surfactant (NIS), and liquid fertilizer solution (UAN) often are added to the spray mixture to reduce surface tension and increase penetration. Spray additives generally improve herbicide performance, but may also increase crop injury. Check the herbicide label for specific information about adjuvant additions.
Foliar or postemergence control strategies are becoming more consistent as better products enter the marketplace. Although a "total postemergence" control program may not work in all situations, it is possible to manage many weeds with foliar-applied products in most agronomic crops.