Scouting Procedure

Scouting for weeds should begin when corn plants are 3 inches tall. If weeds are present at this time, either because the preemergence herbicide has failed or was not applied, cultivate or apply immediately overtop of the corn a labeled herbicide for broadleaf and/or grass control.
    Scout again when the corn is 12 to 15 inches tall to determine the need for a postdirected spray. A postdirected spray should be applied before the corn is 20 inches tall.
    Studies have shown that more than 32 fall panicum plants within 16 feet of a row will significantly reduce corn yields. In studies of broadleaf signalgrass, a postdirected application of a grass control herbicide, when two to three broadleaf signalgrass plants 1 to 3 inches tall per square foot are present at layby time (when corn is 15 to 20 inches tall), has been shown to significantly increase corn yields. If Texas panicum is present, a postdirected application of a grass control herbicide is almost always required because preemergence and preplant herbicides provide only partial control. If cornfields are maintained free of fall panicum, broadleaf signalgrass, and Texas panicurn for five to six weeks after corn emergence, yields will not be adversely affected.
Certain broadleaf weeds - for example, sicklepod, giant ragweed, and burcucumber - require a postdirected application for a successful weed management program. The only way to control triazine resistant pigweed and lambsquarter is with postemergence applications.
 

Economic Threshold

No economic thresholds for weeds in corn have been developed.