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How to improve post-emergence herbicide efficacy of corn in hot weather
Spraying corn herbicides after emergence requires careful timing to ensure effectiveness and safety. The ideal window for application is within 2 to 6 hours after spraying, as this period allows the herbicide to be absorbed effectively by weeds. However, the success of the treatment heavily depends on environmental conditions—specifically temperature and humidity. During hot and dry periods, such as in the morning, noon, or afternoon, high temperatures cause rapid evaporation of the herbicide solution, reducing its ability to penetrate weed leaves. This leads to insufficient absorption, which compromises weed control. In addition, applying herbicides under such conditions can cause phytotoxicity in young corn plants. The best time to spray is after 6 PM, when temperatures are lower and humidity is higher. This allows the herbicide to remain on the weed leaves longer, increasing absorption and improving weed control. Spraying in the evening also enhances the safety of the corn crop, minimizing the risk of damage.
In terms of application method, it's important to use an adequate amount of water—between 15 to 30 kg per mu. Focus on spraying weeds directly, ensuring thorough coverage without wasting the solution on bare soil. This approach not only saves time but also maximizes results.
Another common misconception among farmers is that smaller weeds are easier to kill. In reality, very small weeds may not have enough leaf surface to absorb the herbicide effectively, resulting in poor control. The optimal time to spray is when weeds are between the 2-leaf-1-heart and 4-leaf-1-heart stages. At this stage, they have a sufficient surface area to absorb the herbicide, and their resistance is relatively low, leading to better control.
For corn seedlings, the best time to apply post-emergence herbicides is during the 2- to 5-leaf stage. At this point, the corn plant is more resilient and less likely to suffer from phytotoxicity. If spraying occurs after the 5-leaf stage, it's crucial to lower the nozzle and avoid spraying directly into the corn’s heart to prevent injury.
Some corn varieties, such as sweet corn, waxy corn, and Denghai series, are sensitive to nicosulfuron, a common active ingredient in many herbicides. Spraying these varieties may lead to phytotoxicity. Therefore, it's advisable to test new varieties before full-scale application.
When mixing pesticides, avoid using organophosphate insecticides 7 days before or after herbicide application, as this can increase the risk of phytotoxicity. However, pyrethroids and carbamates are generally safe. For example, if you're dealing with pests like thrips or aphids, you can use imidacloprid or acetamiprid, but it's best to apply them separately from herbicides. Spray herbicides first, then target the heart leaves with insecticides to avoid any potential interactions.
Lastly, due to the current hot and dry weather, weeds have developed stress resistance. They may appear small but are actually mature (often referred to as "old small weeds"). These weeds are covered in fine white hairs that reduce herbicide absorption. To counter this, increase the volume of spray solution to at least 40 kg per mu. This ensures proper coverage and maintains the herbicide’s effectiveness. Farmers should not hesitate to use the recommended dosage, as under-application can lead to poor results.