Post-Emergence Grass Weed Control in Cereals

Post-emergence grass control in cereal crops is one of the most challenging aspects of crop protection in Southern Africa. Annual ryegrass (Lolium rigidum), wild oats (Avena fatua), brome grasses, and barnyard grass (Echinochloa crus-galli) are major yield robbers in wheat, barley, and maize.

The Challenge

Grass weeds are closely related to cereal crops, making selective control difficult. Repeated use of the same herbicide modes of action has led to widespread herbicide resistance, particularly in ryegrass populations.

Key Herbicide Groups for Grass Control

ACCase Inhibitors (HRAC Group A)

These are the most widely used post-emergence grass herbicides. They inhibit the enzyme acetyl-CoA carboxylase, disrupting fatty acid synthesis in grasses.

• •Clodinafop-propargyl: For wheat and barley; excellent on wild oats and ryegrass
• •Tralkoxydim: For wheat; good on wild oats
• •Fenoxaprop-p-ethyl: For wheat and vegetables; broad grass spectrum
• •Fluazifop-p-butyl: For broadleaf crops; excellent on annual and perennial grasses
• •Quizalofop: For broadleaf crops

ALS Inhibitors (HRAC Group B) — Sulfonylureas

• •Mesosulfuron-methyl + Iodosulfuron: For wheat; excellent on ryegrass, wild oats, brome
• •Florasulam: For cereals; broadleaf activity with some grass suppression

Application Timing

• •Apply at the 1–4 leaf stage of grass weeds
• •Earlier is generally better — smaller weeds are always more susceptible
• •Some products have crop growth stage restrictions — check the label

Adjuvants

ACCase inhibitors require methylated seed oil (MSO) or crop oil concentrate (COC) for optimal activity. Check the label — some products have registered tank-mix adjuvant requirements.

Resistance Management

Ryegrass resistance to ACCase inhibitors (Group A) is common in Southern Africa. Key resistance management strategies:

• •Rotate between HRAC Group A (ACCase inhibitors) and Group B (ALS inhibitors)
• •Include pre-emergence soil herbicides in the rotation
• •Use cultural practices: delayed planting, competitive cultivars, crop rotation
• •Never use the same mode of action more than twice in succession