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Weed Management


Weeds compete with sweet corn for sunlight, water, and nutrients. Historically, cultivation was used as the primary weed control option. This necessitated wider rows and limited production per acre. Herbicides are now the primary choice for weed management.

Many pre-and post-emergent herbicides are available that control the problematic grassy and broadleaf (lambs-quarter and various pigweed species) weeds. Researchers from Iowa State University reported that they achieved excellent season-long weed control using a pre-treatment of s-metachlor (Dual II Magnum) + mesotrione (Callisto) + atrazine (AAtrex 4L). They reported that corn could be seeded right after using the combination of the three pre-emergent herbicides, or that the three products could be applied soon after seeding. They concluded that weed control at the pre-plant and early post-plant times is imperative, and that later rescue treatments with herbicides controlled weeds far less effectively and greatly reduced yields.

Other combinations of herbicides and cultivation can be effective if timed correctly. Some common herbicides used in sweet corn include dimethenamid-p (Outlook, Frontier-P, others), S-metolachlor (Dual), and water-based 2,4-D and 2,4-D related products. Some of the newer varieties of sweet corn, however, are sensitive to selected herbicides, so ask about these limitations when purchasing sweet corn seed. There are also several new sweet corn varieties that have built-in crop safety to in-crop applications of Roundup herbicide. Refer to the sweet corn herbicide table (Table 10.2, at the end of the chapter) for further information on application timing and efficacy against certain weeds.

Most herbicides are manufactured by many companies under different trade names. Pesticide labels often change, so make sure to always consult the label to determine if sweet corn is listed on the label, what precautions are required, and what rates and application methods are allowed. It is critical that a copy of the label is obtained and read carefully before purchasing and applying any chemical. Comparing the costs of different brands that may have the same active ingredient and percent of active ingredient is also a good idea.

Important Considerations for Herbicide Use

  • Carefully read and follow all label directions and precautions.
  • Use herbicides only on crops for which they are approved and recommended on the label.
  • Use the recommended amount of product and apply it as stated. (Too much material may damage the crop (Fig. 10.5, 10.6) and make it unsafe for consumption.)
  • Apply herbicides only at times specified on the label and observe the recommended intervals of the time of planting and the time between treatments.
  • Follow re-entry intervals (REI) and pre-harvest intervals (PHI).
  • Don’t spray in high wind conditions.
  • It is a violation of the law to use herbicides other than as directed on the label. The EPA has the authority to seize any agricultural commodity that carries a pesticide residue in excess of the established tolerance levels. In addition, if residues of unlabeled chemicals are detected on fresh produce, they could be traced back to your farm.
    Figure 10.5Fig 10.5. Herbicide damage on corn

    Figure 10.6
    Fig 10.6. Herbicide damage on corn

Finally, herbicides are just one tool available for weed control and their use should supplement other good weed-management practices.

Herbicides for weed control are applied in the following ways (Table 10.2 at the end of this chapter):

  • Pre-plant incorporated: incorporated into the soil prior to seeding or transplanting onions
  • Pre-emergence: applied to the soil after planting but before onions or weeds emerge
  • Post-transplant: applied to the soil after crop is transplanted either before weeds have emerged or after clean cultivation
  • Post-emergence: applied to weeds after both weeds and onions have emerged
  • Directed post-emergence: applied as a directed or shielded spray post-emergence on small weeds in rows of taller crops or in row middles. When using a post-emergence herbicide, the entire weed must be covered for maximum control