In conventionally managed brassica fields, weed control is achieved with preplant and/or preemergence herbicides. Fields are often cultivated 4-6 weeks after planting and then post-emergence herbicides are applied to control broadleaf and grass weeds. Hand hoeing is done as needed.
In organic production systems, growers manage weeds by encouraging weed emergence with irrigation, then killing these weeds with tillage, flaming, or mulches (straw, cardboard, etc.). There are OMRI approved organic herbicides that can assist in weed management in organic operations. These herbicides are non-selective, contact herbicides, and must be applied to green tissue. Most organic herbicides have limited residual activity and are used with a combination of controls like tillage, hoeing, and mulches. Be sure to consult your certifying agent prior to applying organic herbicides to ensure you stay in compliance.
Pesticide applicators should have a current chemical applicators license, have a copy of the label, and have read it carefully. Many herbicides are manufactured by different companies under different trade names. The use of chemical names (active ingredient) along with one representative trade name is listed in Table 4.5. Herbicide labels change so growers must always consult a current label to determine 1) if the crop is listed for herbicide use; 2) what precautions are required; and 3) what rates and application methods are allowed. It is a violation of federal law to use any herbicides for purposes other than those specified on the approved label. Off-label applications are hazardous to the environment, to people using the product, and can severely injure the crop.
Use the recommended amount of product and apply it as stated. Pay attention to re-entry intervals (REI) and pre-harvest intervals (PHI). Over-application wastes money and violates the law, and may damage the crop and make it unsafe for consumption. Don’t spray in high wind conditions. Work with your neighbors as many herbicides are toxic to other crops that are growing nearby. Finally, herbicides are just one tool available for weed control and their use should supplement other good weed-management practices.
Herbicides are applied in the following ways:
- Pre-plant incorporated: incorporated into the soil prior to seeding or transplanting the crop.
- Pre-emergence: applied to the soil after planting but before the crop or weeds emerge.
- Post-transplant: applied to the soil after the crop is transplanted either before weeds emerge or after clean cultivation. In some cases, sprays are directed to row middles and shielded from application to the crop.
- Post-emergence: applied to weeds after both weeds and the crop have emerged. In some cases, sprays are directed to row middles and shielded from application to the crop. When using a postemergence herbicide, the entire weed must be covered for maximum control.