Weed Management in Maize: A Valuable Guide for Farmers

by Caiphas Muyambo | 29 December 2024 | admin@seedlink.co.za

Effective weed management is one of the most critical components of maize production, as weeds compete with maize plants for nutrients, water, sunlight, and space. In South Africa, where diverse climates and soil type influence weed populations, it is essential to adopt a tailored approach to weed control. With rising production costs and narrowing profit margins, it is increasingly important for farmers to implement cost-effective and well-planned weed management programs. Additionally, responsible herbicide use is vital not only to maximize immediate effectiveness but also to ensure environmental sustainability and preserve the long-term benefits of these products. This guide focuses on the impact of weeds on crop production, the importance of reading and following product labels, and the need for farmers to critically evaluate spray programs recommended by contractors and advisors.

1. The Impact of Weeds on Maize Production

Weeds significantly reduce maize yields by:

• Competing for essential resources such as water and nutrients.
• Serving as hosts for pests and diseases.
• Interfering with harvesting operations and reducing grain quality.

Early control of weeds, especially in the first six weeks after maize emergence, is vital to maximize yield potential.

2.  Herbicides for Weed Management in Maize

Herbicides play a key role in weed control, but their effectiveness depends on correct application and selection. Every herbicide is designed to control specific weeds, and its effectiveness depends on proper usage. It is important for farmers to always pay attention to the active ingredients in each of the product in the spray program. Refer to CropLife herbicide mode of action guide on this link. Active ingredients are classified into groups based on mode of action. Some herbicides contain more than one active ingredient, offering broader weed control and reducing the risk of resistance development. For example, SureStart® is a Corteva Agriscience product that has 3 active ingredients (see label); acetochlor, flumetsulam, clopyralid which are in groups K3, O, and B giving it a 3-way mode of action to control weeds. Whereas, Roundup® PowerMAX has glyphosate as an active ingredient in Group G.

For effective results using herbicides, farmers must:

• Start by identifying Problematic Weeds: Know the dominant weed species in your fields. 
• Read Product Labels: Check the label to confirm the herbicide’s spectrum of weed control, recommended rates, and application timing.
• Understand Soil and Environmental Factors: Certain herbicides perform better in specific soil types and under particular climatic conditions.

Herbicides are broadly grouped by timing of application as pre-emergence and post-emergence herbicides: Pre-emegence herbicides are applied before weed emergence, targeting weed seeds and seedlings while post-emergence Herbicides are applied after weeds have emerged, targeting actively growing weeds.

Let’s look at examples:

Note: The examples provided are for illustrative purposes only and do not constitute official guidelines or recommended spray programs. For solutions tailored to your farm's specific needs, please consult your crop protection experts

Pre-Emergence Herbicides form a protective barrier in the soil that prevents weeds from germinating or establishing themselves before the crop emerges. These herbicides are particularly effective in the early growth stages of maize, where weed competition can cause the most significant yield loss. Here are a few examples:

• SureStart: A pre-emergence herbicide containing acetochlor, clopyralid, and flumetsulam (Group K3, O, and B herbicides). SureStart controls a broad spectrum of annual grasses and broadleaf weeds, offering residual activity to suppress weeds during early crop growth stages.
• S-Metolachlor: (e.g., Dual Gold, Group K3) Controls annual grasses and small-seeded broadleaf weeds. It provides prolonged residual activity and is often used in combination with other herbicides.
• Terbuthylazine: (e.g., Tyllanex 500 SC, Group C1) Effective against broadleaf weeds and certain grasses, often used in mixtures for enhanced efficacy.

Key Considerations:

• Soil type: Some pre-emergence herbicides, like acetochlor, perform better in clay-rich soils with good moisture retention
• Moisture Activation: Pre-emergence herbicides require rain or irrigation to activate. Applying them before expected rainfall is ideal.
• Weed Spectrum: Farmers must identify the dominant weed species and select a product effective against those specific weeds.

Why This Is Important: Pre-emergence herbicides prevent weeds from competing during the critical establishment phase of maize. Early competition can lead to significant yield losses, as weeds are more efficient than young maize plants at absorbing nutrients and water from the soil.

Post-Emergence Herbicides are used after both maize and weeds have emerged. They target actively growing weeds, ensuring that any escaped weeds from pre-emergence applications are effectively controlled. Some of the herbicides that are posrt in post-emergence programs include:

• Terbuthylazine: (e.g., Tyllanex 500 SC, Group C1) Effective against broadleaf weeds and certain grasses, often used in mixtures for enhanced efficacy.
• Glyphosate: (e.g., Roundup, Group G) Non-selective and highly effective for controlling a wide range of weeds. It is often used with caution in maize systems to avoid crop damage. Note that this herbicide is only applicable to Roundup ready maize
• Mesotrione: (e.g., Callisto, Group F2) Controls a wide spectrum of broadleaf weeds and some grasses. It is often tank-mixed with other herbicides for enhanced weed control.

Key Considerations:

• Weed Growth Stage: Apply when weeds are young and actively growing, typically at 2-4 leaves for grasses and small rosettes for broadleaf weeds. Do not apply glyphosate after V8.
• Crop Safety: Use selective herbicides that do not harm maize. Ensure correct application timing and application rates in tanks mixes. Environmental conditions such as wind and moisture must be ideal for spraying.
• Tank Mixing: Combine post-emergence herbicides with adjuvants to improve penetration and effectiveness.

Why This Is Important: Post-emergence herbicides allow farmers to control weeds that emerge later in the season, reducing competition and ensuring the crop can maximize its growth potential. Without effective post-emergence control, yield losses may still occur despite earlier interventions.

3. Tank Mixes at Pre- and Post-Emergence

Tank mixes, where multiple herbicides and sometimes other products are combined in a single application, are widely used in both pre- and post-emergence weed control. While these practices can be efficient, they require careful planning and consideration. 

Pre-Emergence Tank Mixes commonly combine residual herbicides like atrazine and acetochlor to control a broad spectrum of weeds before they emerge.

Key Considerations:

• Ensure compatibility of products to prevent reduced efficacy or chemical reactions.
• Follow label recommendations for soil type, pH and moisture conditions to avoid crop injury.

Post-Emergence Tank Mixes often include herbicides such as glyphosate and some selective herbicides along with adjuvants for improved performance. Other mixes may include some pesticides.

Key Considerations:

• Target weed growth stages: Apply when weeds are small and actively growing for maximum effectiveness.
• Avoid mixing herbicides and insecticides with antagonistic properties, as this can diminish control. For example, glyphosate is most stable at pH around 5.0 and therefore should not be mixed with herbicides that are require alkaline conditions.

Benefits of Tank Mixes

• Reduce the number of tractor field passes, saving time and fuel, and prevent soil compaction from tractor wheels.
• Broaden the spectrum of weed control by targeting multiple species at the same time.

Risks and Mitigation in Tank Mixes

• Phytotoxicity: Overlapping modes of action or incorrect rates can harm maize plants.
• Resistance Development: Over-reliance on certain herbicides in mixes can accelerate resistance. Rotate modes of action to minimize this risk.
• Sprayer Contamination: Clean equipment thoroughly between applications to avoid unintended crop damage.

4. Importance of Reading Product Labels

A herbicide product label is like a doctor’s prescription. It provides precise instructions to ensure effective results and avoid unintended consequences. Here’s why reading and following labels is critical:

1. Precision in Application: Labels specify the correct dosage, timing, and method of application, much like a prescription ensures the right treatment for a condition. Overapplication can harm crops and the environment, while underapplication can lead to poor weed control and herbicide resistance.
2. Tailored Recommendations: Labels are tailored to specific weed species, crop stages, and environmental conditions. Misusing a product outside its scope can result in failure or crop damage.
3. Preventing Resistance: Following label guidelines helps mitigate herbicide resistance by ensuring correct application rates and encouraging rotation with other modes of action.
4. Health and Safety: Labels include essential safety precautions, protecting applicators, crops, and ecosystems.
5. Legal Compliance: Product labels represent legal documents. Non-compliance can lead to penalties or loss of certifications, particularly for commercial operations.

Ignoring herbicide label is akin to ignoring a doctor’s prescription - it’s a recipe for disaster. Farmers must treat these labels as critical guides for successful weed management and sustainable farming.

5. The Role of Contractors and Farmer Oversight

Farmers often rely on contractors or advisors to recommend and implement spray programs. While these experts bring valuable knowledge and efficiency, it is crucial for farmers to remain actively involved to ensure the program is suitable for their specific circumstances. Key considerations include:

• Review Spray Programs Thoroughly: Always request a detailed breakdown of the recommended herbicides, including their active ingredients and the specific weeds they target. Avoid accepting abbreviated product names or vague descriptions that make it difficult to verify the program's suitability.
• Verify Herbicide Suitability: Cross-check the recommended herbicides against the problematic weeds in your fields by carefully reading the labels. This step ensures that the products are effective for the weeds in question.
• Account for Local Weed Pressures: Recognize that weed populations and pressures vary significantly by region. Ensure the advisor recommending the spray program has physically visited your farm to assess the weed types, soil conditions, and environmental factors.
• Check Tank Mix Compatibility: Before using tank mixes, confirm that all products are compatible. Improper combinations can reduce the efficacy of individual components or cause unintended damage to crops.

When farmers take an active role in reviewing and verifying spray programs, they can ensure the recommendations align with their farm's unique needs, leading to more effective weed control and higher crop yields.

Short Case Study:

A farmer in Limpopo noticed poor control of Cyperus esculentus (yellow nutsedge) despite following a contractor’s program. Upon reviewing the herbicide labels, it became clear that the products were not effective against this weed. Adjusting the program to include halosulfuron significantly improved control.

6. Say No to Blanket Solutions

Weed management must be site-specific. Blanket solutions, where the same herbicide program is applied across different regions, often fail to address local weed pressures. To avoid blanket solutions farmers must ensure:

• Weed Identification: Conduct regular scouting to identify dominant weed species.
• Herbicide Resistance: Rotate herbicides with different modes of action to prevent resistance.
• Tailored Programs: Work with agronomists to design a program suited only to your farm’s specific conditions.

7. Sustainable Weed Management Practices 

Long-term weed management requires integrating chemical, cultural, and mechanical practices to reduce reliance on herbicides. The following are some of the practices farmers can follow:

• Crop Rotation: Break weed cycles by planting different crops. Crop rotation disrupts weed cycles by alternating crops with different growth habits, planting times, and nutrient requirements. For example, rotating maize with legumes like soybeans or dry beans suppresses weeds that thrive with maize. Crop rotation reduces the buildup of weed species that are adapted to the management practices of a single crop
• Cover Crops: Suppress weed growth by shading the soil, reducing available light, and releasing allelopathic chemicals that inhibit weed germination. Examples include oats and rye that are excellent for winter cover and can be planted after the maize harvest. Incorporating cover crops into the rotation also improves soil health by reducing erosion, increasing organic matter, and fixing nitrogen in the case of legumes.
• Mechanical Control: Incorporate tillage or manual weeding where feasible. Before planting, ensure the seedbed is clean and free from weeds. Covering the soil with organic material such as straw or crop residues also suppresses weed germination and conserve moisture.
• Herbicide Stewardship: Use herbicides responsibly to minimize environmental impact and preserve their efficacy.

Conclusion

Effective weed management in maize requires a strategic and informed approach. By carefully selecting herbicides, adhering to label instructions, and critically evaluating spray programs, farmers can achieve effective weed control. Tailoring programs to local conditions is crucial for both immediate results and long-term sustainability.

Furthermore, meticulous record-keeping is paramount. Tracking weed species, herbicide applications (including active ingredients, application rates, tank mixes, and dates), and application outcomes allows farmers to:

• Identify effective management strategies
• Monitor weed resistance development
• Optimize resource use
• Proactively adjust future weed control plans

Continous learning on innovative weed control technologies, alongside ongoing farmer education and training programs, will be vital for ensuring the long-term sustainability and effective weed management.