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Managing Weeds in Cover Crops

Why control weeds in cover crops? Early planted cover crops allow annual weeds to flower and seed before frost, adding to the weed seed bank. Winter annuals easily establish in fall cover crops. Cover crops like pearl millet, Sudan-sorghum, or marigolds will suppress nematodes, but only where weed control is excellent, avoiding nematode buildup on weed roots.

Use a stale seedbed to reduce weeds: Tillage or burndown with glyphosate after harvest of the main crop destroys existing weeds. Planting directly into this stale seedbed avoids bringing weed seeds to the surface. Apply a second burndown just before the cover crop emerges to control many weeds.

For cereal cover crops: Use this opportunity to use hormone herbicides (eg. 2,4-D, dicamba) in the early fall to control broadleaves. Choose your day carefully to avoid wind and vapour drift.

Winter wheat or fall rye underseeded to clovers can be treated with Buctril or MCPB/MCPA (see p. 108, Publication 75).

Sorghum or millet cover crops can choose from these minor use registrations:

• IPCO 2,4-D Amine 600 at 0.5 to 1 L/ha (0.2 to 0.4 L/ac), for emerged broadleaf weeds. Apply when the cover crop is at the 4 to 6 leaf stage, but before the canopy is closed. Avoid hot (over 27°C), humid weather to avoid drift. 30 day PHI for forage crops.
  
• Basagran Forte for emerged broadleaf weeds and yellow nutsedge. Apply 1.75 to 2.25 L/ha (0.7 to 0.9 L/ac) when the crop is at 3- 6 leaf stage but before the canopy is closed. Leaf yellowing may be temporary in hot, humid weather. Best on small weeds. 30 day PHI for forage crops.
  
• PeakPlus, a co-pack containing Peak (prosulfuron) + Banvel (dicamba), can be applied at 13.3 g/ha (5.3 g/ac). Include a non-ionic surfactant like Agral 90 at 0.2% v/v. Apply this when the crop is between 3 to 5 leaves, and when broadleaf weeds are actively growing in the 1-6 leaf stage. Avoid hot, humid weather to avoid drift problems. Recropping restrictions after PeakPlus: 10 months for peas; field bioassay to detect soil residues for all other hort crops.

For broadleaf cover crops: A stale seedbed and preemergence burnoff, with a high seeding rate appears to give the best results. Postemergent graminicides like Venture and Poast Ultra can control grass escapes where registered.

Pearl Millet-2 months after planting, ready to mow

Marigold seeded into a stale seedbed

Marigold cover crop in full flower at the end of summer