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How to improve post-emergence herbicide efficacy of corn in hot weather
When applying post-emergence herbicides to corn, timing is crucial for effectiveness and crop safety. The herbicide needs 2 to 6 hours to be absorbed by the weeds, and during this period, environmental conditions like temperature and humidity play a major role. In high-temperature and dry conditions—such as in the morning, noon, or afternoon—the spray solution evaporates quickly, reducing the amount of herbicide that actually reaches the weeds. This can lead to poor weed control and even cause phytotoxicity in corn seedlings. The best time to spray is after 6 PM, when temperatures are cooler and humidity is higher. This allows the herbicide to stay on the weed leaves longer, improving absorption and ensuring better weed control. Additionally, spraying in the evening reduces the risk of damage to corn plants.
The method of application also matters. Using 15 to 30 kg of water per mu ensures thorough coverage without waste. Focus on spraying only where weeds are present, and avoid unnecessary overspray. This not only saves time but also improves results.
Another common mistake is assuming smaller weeds are easier to kill. However, very small weeds may have limited leaf surface area, making it harder for the herbicide to take effect. The ideal stage for spraying is when weeds are between 2-leaf 1-heart and 4-leaf 1-heart. At this stage, they are large enough to absorb the herbicide effectively, and their resistance is lower, leading to better control.
For corn seedlings, the best time to apply herbicides is when they are in the 2- to 5-leaf stage. At this point, the corn is more resilient and less likely to suffer from herbicide damage. Spraying should be avoided once the corn has 6 leaves, as the plant becomes more sensitive. When spraying, lower the nozzle and target the base of the plants to avoid direct contact with the heart, which can cause injury.
It's also important to consider the type of corn being grown. Herbicides containing nicosulfuron may be harmful to certain varieties such as sweet corn, waxy corn, or Denghai series. Always check compatibility before use. For new or unfamiliar varieties, it’s wise to conduct a small-scale test first.
Pesticide mixing is another key factor. Avoid using organophosphate insecticides within 7 days before or after herbicide application, as this can increase the risk of phytotoxicity. However, pyrethroids and carbamates are generally safe. If you need to control pests like thrips or aphids, apply imidacloprid or acetamiprid separately, targeting the heart leaves rather than the foliage, to prevent chemical interactions.
Finally, due to this year’s hot and dry weather, many weeds have developed stress resistance. They appear small but are actually mature ("small old man" weeds) and covered in fine white hairs that repel the spray. This makes them harder to control. To ensure effectiveness, increase the volume of spray (at least 40 kg per mu), and don’t hesitate to apply the full recommended dose. Proper application is essential for successful weed management.