Can Post-Emergence Corn Herbicides Be Reapplied If They Don't Work? Causes and Remedial Measures

Generally, after corn emerges, farmers can start weeding it, but because corn varieties are diverse and the efficacy of herbicides varies, the problem of the herbicide showing no obvious effect after weeding is unavoidable. So can post-emergence corn herbicides be reapplied if they don't work?

Reasons affecting herbicide efficacy:

1. Weed influence. When weeds are young, their resistance to the herbicide is poor and efficacy is good; when weeds are older, resistance increases and efficacy is poorer. Under drought conditions weeds do not germinate; weeds that have already emerged look small but are actually older, especially with wettable powder products, which have poor adhesion, so the sprayed liquid dries quickly, the powder falls off, and the effect is poor.

2. Application method. Water volume, spray uniformity, missed spots, overlapping sprays, and the addition of surfactants all have a clear impact on efficacy.

3. Soil conditions. This has a greater impact on pre-emergence soil-applied herbicides, especially on plots treated with soil-applied agents: when organic matter content is high, the liquid is easily adsorbed by the soil, so the dosage needs to be increased. In sandy soil with low organic matter content, leakage and leaching occur easily, the chemical film forms poorly, and the pre-emergence weeding effect is poor.

4. Temperature. The efficacy of herbicides and the occurrence of herbicide damage are closely related to temperature. For example, nicosulfuron and mesotrione exert their effects slowly or poorly at lower temperatures. But under high-temperature conditions they easily cause crop damage. Therefore, applying the chemical around dusk is relatively safe for the crop.

5. Weather conditions. Under drought conditions efficacy often performs poorly; when applied after rain, soil moisture is high and efficacy performs better; when applied in windy weather, the liquid drifts, efficacy is poor, and drift easily causes damage to other crops.

6. Soil moisture. When moisture is good, less liquid is adsorbed by the soil, which favors weed absorption and gives good efficacy. Under drought conditions, the active ingredients of the herbicide are absorbed and fixed by the soil, making it hard for weeds to absorb them, so efficacy is poor.

Solutions:

1. Soil pre-emergence agents

Apply when soil moisture is good after irrigation or rain, use the dosage according to the instructions, do not arbitrarily increase or decrease it, and spray evenly on the soil surface to form a chemical film and improve the weeding effect.

2. Post-emergence treatment agents

Apply as much as possible after irrigation or rain, when weeds are in a growing state and their stomata are open, which favors absorption of the liquid; moreover, when plants are growing vigorously, the liquid conducts quickly within the plant.

Efficacy performs well and the weeding effect is good. During drought, be sure to increase the water volume, with no less than 30 kg of water per mu, spray evenly, and wet and penetrate the weed stems and leaves thoroughly to help absorption and conduction.

It is best to apply when weeds are at the 2-5 leaf stage, when the weeds are younger and the weeding effect is better. Silicone adjuvants and other surfactants can be added to the spray liquid to increase the spreading and wetting of the leaves, promote absorption of the liquid, and improve the weeding effect.

In addition, if post-emergence corn herbicides are used improperly, they may not only fail to thoroughly remove weeds but may also cause a certain amount of herbicide damage. So after the first spraying has no effect, wait 2-3 days before spraying again. More importantly, post-emergence corn herbicides must be used properly to avoid herbicide damage. Please read on for the specific operation methods!

Seven major precautions for herbicide use

1. Spraying time

Because post-emergence corn herbicides require an absorption process of 2-6 hours after spraying, during these 2-6 hours whether efficacy performs well (that is, whether the weeding effect is ideal) is very closely related to temperature and air humidity.

When spraying in the morning, noon, or afternoon of high-temperature, dry weather, because the temperature is high, light is strong, and the liquid volatilizes quickly, the liquid will evaporate shortly after spraying, limiting the amount of herbicide that enters the weeds and making absorption clearly insufficient, thus affecting the weeding effect; at the same time, spraying under high temperature and drought also makes corn seedlings prone to herbicide damage.

The best spraying time is after 6 p.m., because when spraying at this time the temperature after application is lower and humidity is higher, so the liquid stays on the weed leaf surface longer and the weeds can fully absorb the herbicide ingredients, ensuring the weeding effect. Evening application can also significantly improve the safety of corn seedlings and make herbicide damage less likely.

2. Spraying method

Mix the per-mu dosage with 15-30 kg of water, spray where there are weeds, spray carefully, and move on where there are no weeds, saving chemical and time with good results.

3. Look at weed size

When spraying post-emergence corn herbicides, many farmers have a misunderstanding: they think that the smaller the weeds, the lower their resistance and the easier they are to kill.

Actually this is not so, because if the weeds are too small there is no surface area for the chemical to land on, and the weeding effect is also not ideal. The best weed age is from the 2-leaf 1-heart stage to the 4-leaf 1-heart stage; at this time the weeds have a certain surface area for the chemical to land on and their resistance is not great, so the weeding effect is significant.

4. Corn seedling size

The best spraying time for post-emergence corn herbicides is the corn 2-5 leaf stage, when corn resistance is high and herbicide damage is unlikely.

Before the 5-leaf stage the whole field can be sprayed; when spraying after the 6-leaf stage, lower the nozzle and spray toward the corn plants to prevent the liquid from pouring into the whorl and causing herbicide damage (mainly for nicosulfuron without a safener; if it is a safened nicosulfuron, the whole field can still be sprayed.).

5. Corn variety

Because most post-emergence corn herbicides now contain nicosulfuron as the ingredient, and some corn varieties are sensitive to this ingredient and prone to herbicide damage, corn fields planted with sweet corn, waxy corn, Denghai series, and other varieties must not be sprayed, to prevent herbicide damage. For new corn varieties, please test first before promoting them.

6. Pesticide mixing issues

Within 7 days before and after spraying post-emergence herbicides, it is strictly forbidden to spray organophosphorus insecticides, otherwise herbicide damage easily occurs. However, they can be mixed and sprayed with pyrethroid and carbamate insecticides; when spraying, take care to avoid the heart leaves as much as possible to prevent the liquid from pouring into the whorl.

This year, Swedish fly and thrips occurred severely in some corn fields. To control these two small pests, imidacloprid or acetamiprid can be used to spray the heart leaves, but when spraying do not mix them with post-emergence herbicides either. Although imidacloprid or acetamiprid are not organophosphorus, controlling these two small pests requires spraying the heart leaves, and if mixed and sprayed into the heart leaves, herbicide damage easily occurs. They can be sprayed separately: spray the post-emergence herbicide in front, and immediately follow behind with imidacloprid or acetamiprid on the heart leaves.

7. The resistance of the weeds themselves

In recent years, weeds' own stress resistance has strengthened. To prevent excessive evaporation of water from their bodies, weeds do not grow so plump and sturdy, but instead grow pale and short, and their actual age is not small (the so-called “little old men”).

Most of these weeds are covered all over with small white hairs to reduce water evaporation. Thus when the pesticide is sprayed, the liquid is held by these small hairs above the weed stem and leaf surface, and the weeds themselves absorb very little, which naturally affects the performance of the chemical. Therefore, under high temperature and drought, no matter what chemical is applied, the amount of spray liquid should be increased so as not to affect efficacy. Farmer friends, when applying chemicals, please do not be stingy with water or with the chemical.