Weed Control

NOTE:  For the herbicides listed below, one product trade name and formulation is provided for each active ingredient (common name) as an example of rates, days to harvest (dh), REI, and special instructions. In many cases there are other products available with the same active ingredient. Please see Table 17 and Alphabetical Listing by Trade Name for more information on products with the same active ingredients. 

Delayed Herbicide Applications

Growers using atrazine (Aatrex) for broadleaf weed control and either metolachlor (Dual) or alachlor (Lasso) for grass control in corn should consider delayed applications in their earliest plantings. Reasons for delaying herbicide applications in the earliest sweet corn plantings include:

1. Corn is most tolerant to both Dual and Lasso after emergence. Supersweets and some “se” sweet corn are easily injured by these herbicides when the soil is below 60°F. Both Dual and Lasso as well as atrazine can be applied at any time as long as the corn is less than 5” high. Bicep, a very commonly used prepack of atrazine and Dual, can also be used this way.
2. If the first few plantings of sweet corn are sprayed on the same day, the amount of cleaning and rinsing of the spray tank is reduced since it does not need to be cleaned between applications when the same herbicide is used.
3. Delaying herbicide applications on the earliest plantings will also extend the activity of the herbicide later into the season. This is important in maintaining good weed control throughout the season to minimize weed seed production. Growers often rely on sweet corn rotations to reduce weed seed populations in the soil.

Growers should consider waiting until their first planting of sweet corn is 3” to 5” high to apply herbicide. At this time all fields of sweet corn planted to date can be treated. Each field sprayed will be at a different stage of growth. After that, each field should be sprayed soon after it is planted, since soils will be warmer and grasses are more likely to emerge soon after seeding the crop.

The only possible problem with delayed applications involves the potential for poor activity on grasses. The Dual, Lasso, or Bicep (atrazine + Dual) must be applied before grasses emerge. If grasses are not controlled, yield reductions will likely occur. Also, as the soil warms up, grasses are likely to emerge soon after seeding. In all cases, be sure that the atrazine + Dual, atrazine + Lasso, or Bicep is applied before grasses emerge.

Finally, be sure to follow the herbicide rates recommended in this guide. Only 1 lb active ingredient of atrazine is recommended for sweet corn in New England. This is well below the rate on the label and constitutes best management practices for groundwater protection. This rate could be reduced further, although the grower should be prepared to make a second application of atrazine if any weeds escape. Rates for Dual and Lasso should be selected based on soil type. Follow the label to determine the correct rate. Reducing the rate of Dual or Lasso is dangerous since it is very difficult to control grasses in sweet corn once they emerge.

Special Atrazine Precautions

The Environmental Protection Agency and Ciba Geigy have revised the label uses of all atrazine products to reflect potential surface and groundwater contamination concerns. Be sure to read and follow all directions and restrictions listed on the label. The preceding section on Reduced Rate Weed Management reflects these concerns. All of the rates listed in this guide for atrazine are within the guidelines of the label. These include both single applications (applied before crop emergence or early postemergence) and sequential applications (applied both at planting and postemergence). Please read the label carefully.

Stale Bed Applications

The following herbicides are nonselective and are used to control weeds which are present in a field prior to planting the sweet corn or before the sweet corn emerges (see Stale Seedbed Technique). If a grower is using “no-till” or “minimum tillage,” these herbicides are also used to kill the cover crop which may be present in the field.

glyphosate (Roundup 4S): 1 to 5 qt/A (REI 12h, Group 9). Apply to emerged annual or perennial weeds prior to crop emergence. Do not feed crop residue to livestock for eight weeks following treatment. Consult the manufacturer’s label for specific weeds and rates. May be tank mixed with atrazine, simazine or alachlor.

paraquat (Gramoxone Inteon *2S): 2.0 to 4.0 pt/A (REI 12h, Group 22). Apply in 20 to 60 gallons spray mix to emerged annual weeds before crop emergence as a broadcast or band treatment. Field should be prepared several days ahead of planting and treatment to allow maximum weed emergence. Plant with a minimum of soil movement for best results. Use a nonionic surfactant at a rate of 16 to 32 oz per 100 gal spray mix. May be tank mixed with atrazine or simazine preemergence. Check label for directions.

pelargonic acid (Scythe 4.2): 3 to 10 gal/A (REI 24h, Group 27). See Stale Seedbed Technique.