Chemical Control

Part 2, Section 1: Pest Management

Pest Management

Weed Control

Chemical control

Develop a weed-management program to fit your farming operation. When choosing an herbicide program, base your decision on potential weed problems, crop and herbicide rotation, injury potential, tillage system and available application equipment, soil texture and organic matter, potential environmental hazards, and cost. Begin your selection by knowing what weeds are potential problems and using the herbicide effectiveness tables in this guide to help select appropriate products. Next, examine the herbicide choices for possible limitations that could prevent their use on your farm.

Crop rotation may influence the herbicide decision. For example, where atrazine or simazine are used in corn, spring-seeded small grains, small-seeded legumes and grasses, or vegetables should not be planted the following year for risk of carryover injury. A number of soybean herbicides also limit the planting of certain sensitive follow crops. Table 2.2-19 and Table 2.4-17 provide summaries of the recropping restrictions for corn and soybean herbicides. Refer to a current herbicide label for specific information about recropping intervals.

Herbicide rotation is an important management consideration. Rotating herbicides reduces the risk of developing herbicide-resistant weeds. Frequent use of atrazine or simazine in corn gave rise to the current problem with triazine-resistant weeds in Pennsylvania. To prevent herbicide resistance, avoid using products from the same herbicide class every year. For example, if Gemini, a sulfonylurea herbicide, was used in soybeans, avoid using additional sulfonylureas that same season and perhaps the following cropping season. This is especially true for the more persistent soil residual herbicides.

Other measures that help prevent resistant weeds from developing include: using herbicide mixtures that contain more than one herbicide class; using shorter soil residual materials, including nonchemical control measures; avoiding spreading resistant weed seed with machinery or in manure; and helping destroy weed-seed-infested forage by ensiling.

Crops occasionally are injured by herbicides registered for use on the crop. Crop injury ratings are provided in some of the effectiveness tables. In addition, cool, wet soils, delayed crop emergence, deep or shallow planting, seedling diseases, and poor-quality seed may contribute to crop stress and potential herbicide injury.

Reduced or conservation tillage systems may preclude the use of some herbicides and necessitate the need for others. Knockdown or burndown herbicides are especially important where tillage is not used to destroy vegetation prior to crop emergence. Some soil-applied herbicides are incorporated to minimize surface loss, reduce dependence on rainfall, and provide appropriate placement of the herbicides. Incorporated herbicides generally are not practical for no-till operations. Postemergence control is gaining in popularity, but it often requires critical timing and may not be the best choice for some producers. See the following section for more information on using postemergence control strategies.

Soil texture and organic matter are characteristics that help determine herbicide choices and herbicide rates. Some herbicides should not be used on coarse-textured soils and/or soils low in organic matter. The risk of crop injury from certain soil-active herbicides is much greater on these lighter-textured soils. In contrast, other herbicides interact strongly with clay and organic matter and may not perform adequately on finer-textured soils. Application rates for most soil-applied herbicides are based on soil texture and organic matter, and lower application rates generally may be used on coarse-textured, low-organic-matter soils. Know the soil texture and organic matter content for specific fields and check the herbicide label before selecting an herbicide or the appropriate rate.

Some herbicide labels carry Environmental Hazard Warnings on the label. The environmental hazard may specify a "water-quality advisory," which requires special precautions for coarse-textured soils (sandy), soils with a shallow water table, and soils with other potential water-contamination risks. Herbicides with water-quality advisories have been detected in small amounts in water supplies after normal agricultural use. Additional environmental dangers include toxicity to fish and wildlife and hazards to endangered species. Check the label or specific hazard warning information before using a product.