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Corn and soybean growth is now accelerating because of the recent warm and wet weather. The Minnesota Dept of Agriculture Statistics Service reported that as of last week the average corn height in Minnesota was 12 inches compared with 13 inches last year and for the five-year average. Soybeans were 91% emerged versus 95% last year and 95%as the five-year average. Soybean height was 4 inches compared with 5 inches last year.

While the recent weather has been favorable to the growth of corn and soybeans it also has been just as favorable to the growth of weeds. The prolonged period of rain in May and June has prevented timely application of postemergence herbicides in many corn fields for the control of weeds. By the time conditions allow field work, corn may be beyond the optimum size for postemergence applications. There are several concerns with delayed applications, including: 1) label restrictions regarding crop or weed size, 2) increased sensitivity of large corn to herbicides, and 3) reduced efficacy due to large weed size and/or canopy interference with spray coverage.

The following tables list the maximum corn size specified on herbicide labels. As corn increases in size, the crop canopy intercepts a significant portion of the herbicide spray. This may reduce performance due to inadequate coverage of weeds or enhance the potential for crop injury. Drop nozzles will reduce these risks by directing the spray to the weeds, therefore improving coverage and reducing corn exposure.

Table 1. Application restrictions for postemergence grass control in conventional corn.

¹Use most restrictive measurement.

Table 2. Application restrictions for postemergence broadleaf control in conventional corn.

¹Use most restrictive measurement.

Table 3. Application restrictions for herbicide resistant corn hybrids.

¹Use most restrictive measurement.

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