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Harrowing for Weed Control
Dry soil surfaces accompanied by warm dry days are optimum conditions to control weeds with a harrow.
Wheat and Barley into one-three leaf stage can be harrowed with minimal injury. Wheat can be harrowed twice, while barley should be harrowed only once. It is not recommended to harrow oats. Corn can be successfully harrowed starting at the second leaf stage.
Populations of shallow rooted seedlings of green and yellow foxtail (pigeongrass) and kochia can be severely reduced by timely harrowing.
Harrows should be set shallow 1/2 inch in depth and operating speeds monitored to avoid crop injury.
A light spring tooth harrow or rotary hoe can be used for weed control in soybean, corn and sunflower, but don't use these tools when the seedlings start to emerge as injury can be severe at this time. Soybeans with the first true leaves (unifoliate stage), sunflower in the 4-6 leaf stage and corn beyond the spike stage to 3 leaf stage can safely be harrowed. Tines should always be slanted back when harrowing for weed control. Trashy conditions which result in trash build-up in the harrow should also be avoided. Use of harrows and rotary hoes have been most effective at midday or on afternoons when temperatures are warmer and soil surfaces are dry, since these conditions promote weed drying and better kill. Also, less physical crop injury has been observed compared to early morning field operations.
Sunflower can be harrowed 2-4 times during the 4-6 leaf stage of development. If this type of weed control is planned, sunflower should be planted at rates 5-8% higher than normal.
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