Qn: Where did all the weeds come from in that corn field?

Farmers are usually well advised to “control weeds early and often” in corn fields. Although many of today’s herbicide tolerant crops and herbicides can control weeds at almost any stage, it is early season competition that reduces nitrogen use efficiency. University of Wisconsin researchers compared weed free corn to delayed weed control. Spraying when weeds were 4” and 12” tall, required an additional 20-60 and 65-160 lb nitrogen /ac, respectively, to produce corn yields equal to weed free plots.

In our 2014 Crop Diagnostic School lesson, corn fertilized with 100 lb nitrogen/ac yielded 145 bu/ac with season-long weed control, 15 bu/ac less when spraying 4” tall weeds and 60 bu/ac less when spraying 12” tall weeds (Figure 1). Unsprayed corn yielded 9 bu/ac. Conversely, with no added nitrogen but early and consistent weed control, the corn yielded 95 bu/ac.

John Heard in corn field

Figure 1. Corn to left in background with full, but late, control of weeds suffered extreme nitrogen deficiency.

However, there are warranted exceptions to maintaining all growing vegetation from growing in corn fields. Many corn fields in southern Manitoba are currently seeded with a wheat or oat companion crop (see Figures 2 & 3). These fields tend to be at high risk of wind erosion: sandy textured, following potato or other low residue crops. Cereals seeded at or prior to corn can then establish and produce early season ground cover to minimize soil erosion and sand blasting injury to young corn seedlings.

Figure 2

Figure 2. A nurse crop of wheat emerging in mid-May, ahead of the corn crop in Winkler area.

oats in corn field

Figure 3. A nurse crop of oats emerging in late May, in Carman area corn crop, prior to spraying for removal.

For more information on erosion and cover crops visit MAFRD’s website at: http://www.gov.mb.ca/agriculture/environment/soil-management/soil-management-guide/soil-erosion.html

Answer Submitted by John Heard, Crop Nutrition Specialist, MAFRD


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