Corn Weed Control Techniques (Part 2)

With the increase in no-till planting and the frequent occurrence of spring droughts, low temperatures, and strong winds, pre-emergence weed control has been less effective. Consequently, post-emergence weed control has become more widely used, as it adapts better to varying soil conditions and helps manage weeds after corn has emerged. Understanding how to select and apply post-emergence herbicides correctly is crucial.

Choosing the Right Post-Emergence Herbicide

Selecting the appropriate post-emergence herbicide depends on the corn variety, seedling age, weed species, and weed leaf stage.

1. Corn Seedling Stage
   The best time to apply post-emergence herbicides is when corn is in the 3-5 leaf stage. At this stage, corn has better resistance to herbicides, reducing the risk of damage. For corn at 5 leaves or younger, you can spray across the field. For corn with more than 6 leaves, lower the spray nozzle to avoid direct contact that could cause harm.
2. Weed Size
   There’s a common misconception that smaller weeds are easier to kill. However, very small weeds have less surface area for the herbicide to act on, which can reduce effectiveness. The ideal weed size for treatment is between the 2-leaf 1-heart and 4-leaf 1-heart stages. At this stage, weeds have enough surface area to absorb the herbicide effectively.
3. Weather Conditions
   Optimal conditions for spraying include temperatures between 15-25°C and relative humidity above 70%. Avoid spraying when temperatures are below 13°C or above 28°C, or when humidity is below 65%. Wind speeds should be below 4 meters per second. The best times to spray are before 9 AM or after 4 PM.
4. Proper Herbicide Dosage
   Follow these two principles for determining the correct dosage: first, use the amount recommended for the area being treated; second, adjust based on weed density, weed species, and weed leaf stage. For high weed density or advanced weed stages, use the higher end of the recommended range; for lower density or earlier stages, use the lower end.
5. Correct Mixing Method
   Use a two-step dilution method for mixing herbicides: first, mix the herbicide with a small amount of clean water, then add it to the spray tank. Add additional water as required, and stir for 5-10 minutes to ensure complete mixing. Avoid using cold water, which can reduce effectiveness.
6. Controlled Application Volume
   The amount of spray solution used affects weed control effectiveness. For post-emergence treatment, use 15-20 liters of solution per acre. Both too much and too little solution can impact results.
7. Pre- and Post-Spraying Considerations
   Avoid applying organophosphate insecticides within 7 days before or after herbicide application, as this can cause herbicide damage. Instead, consider using pyrethroid insecticides. Be cautious to avoid herbicide contact with corn leaves.

Rainy Weather Considerations

If it rains after spraying, you may need to reapply under the following conditions: corn is in the 3-5 leaf stage, and weeds are in the 2-6 leaf stage. Use herbicides such as Mesotrione, Tribenuron-Methyl, or mixtures of these to treat weeds. For grasses like barnyard grass and foxtail, use herbicides such as Bentazone or Bispyribac-Sodium. For broadleaf weeds like duckweed and amaranth, use herbicides like 2,4-D or Penoxsulam.

Challenges and Best Practices

Post-emergence weed control is increasingly important due to the challenges posed by extreme weather and larger farming operations. Delays in application or inconsistent standards in outsourced weed control can impact effectiveness. Adhering to the “pre-emergence and post-emergence” approach ensures comprehensive weed management and improves outcomes.

By implementing these strategies and staying informed about best practices, you can effectively manage weeds and support a successful corn crop.