Summer seedling corn is grown during a hot and rainy season, when weeds in the fields are widespread and diverse. According to the National Weed Census, there are 22 families, 38 genera, and 43 species of weeds found in corn fields. The most common weeds include crabgrass, madder, green foxtail, goosegrass, amaranth, purslane, iron amaranth, and fragrant grass, among others. These weeds can severely damage young corn plants, making it crucial to implement effective weed control strategies during the critical periods before and after seedling.
There are several types of herbicides commonly used for weed management in corn fields. First, single-agent herbicides:
1. **Amide Herbicides**: These are currently the most widely used class of herbicides in corn fields. They are absorbed by weed seeds and are effective in controlling annual grasses and some broadleaf weeds before they germinate. Common examples include acetochlor, alachlor, butachlor, metolachlor, and propisochlor. Studies show that acetochlor has the highest herbicidal activity, followed by metolachlor, propisochlor, butachlor, and alachlor. Their effectiveness depends heavily on soil moisture, with reduced performance in dry conditions.
2. **Triazine Herbicides**: These are effective against both annual grasses and broadleaf weeds. Atrazine is the most commonly used triazine herbicide due to its high safety and activity in corn. However, it is often mixed with acetochlor to reduce the dosage and improve crop safety.
3. **Phenoxycarboxylic Acid Herbicides**: These are mainly used for post-emergence control of broadleaf weeds and Cyperus bracteatum. Examples include 2,4-D and 2A4 sodium salts. These are particularly useful for controlling aconite without causing phytotoxicity if applied at the right time.
4. **Sulfonylurea Herbicides**: Compounds like nicosulfuron and rimsulfuron are effective against grassy weeds, sedges, and some broadleaf weeds. They are known for their low application rates and high selectivity.
5. **Other Herbicides**: Paraquat and glyphosate are non-selective herbicides used for total weed control. They are typically applied after the corn has reached 40 cm in height. Other options include bromoxynil and bentazon, which are effective against broadleaf weeds.
In addition to single agents, many herbicide mixtures are commonly used:
1. **Acetochlor + Atrazine (1:1)**: This combination is one of the earliest developed herbicide mixtures, such as Ethylene Mixture. It is safe for corn and provides good pre-emergence control of annual weeds and broadleaf weeds.
2. **Acetochlor + Atrazine (2:3)**: This mixture is suitable for early post-seeding periods and is generally safe for corn, though it may be less safe in drought conditions.
3. **Prometryn + Atrazine**: This combination is effective for controlling grassy and broadleaf weeds, but its performance can be affected by heavy rainfall or extreme weather.
4. **Nicosulfuron + Atrazine**: This is an ideal mixture for controlling a wide range of weeds, including perennial grasses and sedges, while maintaining corn safety.
5. **Acetochlor + Atrazine + Paraquat**: This combination offers both pre- and post-emergence control, and is effective throughout the growing season. Some formulations replace paraquat with glyphosate for broader use.
For application, the recommended dosages vary depending on the stage of corn growth. For example, at sowing, a 150–200 ml/mu E-A mixture is used, or 90 g/mu of 50% Ebel wet powder. At the early post-emergence stage (1–4 leaf), a mixture of 75–100 ml/mu of 38% atrazine suspension plus 75–100 ml/mu of 4% nicosulfuron suspension is effective. Later in the growing season, 200–300 ml/mu of 10% glyphosate or 100–150 ml/mu of 20% paraquat is used, with careful application under windless conditions.
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