6.6 Timing and Methods of Application
Key Takeaways
- Application timing relative to the crop and pest life cycle includes preplant, preemergence, postemergence, dormant, and foliar windows; each matches a labeled product and pest stage.
- Treat when the pest is at its most susceptible life stage and the crop is at a labeled growth stage; off-timing is the most common cause of poor control besides resistance.
- Weather sets the spray window: wind between 3 and 10 mph, no temperature inversion, no rain within the label's rainfast period, and moderate temperatures to avoid volatilization.
- Application methods include broadcast, band, directed, spot, soil drench, and seed treatment; the method changes the rate calculation and the nozzle and pressure choice.
- Iowa-specific timing considerations include short preplant windows in wet springs, soybean aphid scouting from late June through R5, and tar spot fungicide windows at R1 to R3.
6.6 Timing and Methods of Application
Quick Answer: Match the application to the crop and pest stage. Preplant and preemergence treatments hit weeds or soil pests before the crop emerges; postemergence treatments hit actively growing pests after crop emergence; dormant applications hit overwintering pests on bare plants; foliar applications target the pest on the leaf. Apply only within the labeled window and when weather allows.
Application Timing Relative to the Crop
| Timing | When | Typical Targets | Example Products |
|---|---|---|---|
| Preplant (PPA) | Before planting, often incorporated | Soil insects, germinating weeds | Soil-applied herbicides, preplant insecticides |
| Preemergence (PRE) | After planting, before crop or weed emergence | Germinating weeds, soil pests | Atrazine, acetochlor, S-metolachlor |
| Postemergence (POST) | After crop and weed emergence | Actively growing weeds, insects, diseases | Glyphosate, glyphosate alternatives, foliar fungicides |
| Dormant | On dormant crop, early spring or late fall | Overwintering insects, pathogens | Dormant oils, some fungicides |
| Foliar | On leaves during the growing season | Leaf-feeding insects, leaf diseases | Pyrethroids, strobilurins, triazoles |
Some labels restrict application by crop growth stage (for example, "apply before V6 corn" or "apply between R1 and R3 soybean"). Applying outside the labeled window can injure the crop or be illegal.
Pest Life Cycle Timing
The right window is when the pest is most susceptible and the crop is most tolerant. Examples:
- Corn rootworm: soil insecticide or trait at planting targets larvae; adult beetle control is a separate window later in the season.
- Soybean aphid: foliar insecticide at the 250-aphid threshold from late June through R5; earlier or later applications either waste product or come too late.
- Waterhemp: POST herbicide when plants are less than 4 inches tall; larger plants are harder to control and produce seed.
- Tar spot in corn: fungicide at R1 to R3 if the disease is active; prophylactic application outside the disease window is rarely economic.
- Soybean cyst nematode: no rescue treatment; management is resistant variety, rotation, and seed treatment at planting.
Weather Conditions
Weather sets the actual spray window. Key limits:
- Wind: spray when wind is 3 to 10 mph. Below 3 mph, a temperature inversion may trap droplets; above 10 mph, drift increases sharply. Use a coarse droplet and lower boom height if wind is in the upper end of the range.
- Temperature inversion: warm air above cool surface air traps droplets near the ground and lets them move laterally. Inversions are most common near dawn and in low-lying fields; do not spray during an inversion.
- Rain: check the label's rainfast period (the time the spray must dry before rain). Rain before the rainfast period washes the product off and reduces control.
- Temperature: high temperatures increase volatilization of some herbicides (dicamba, 2,4-D) and stress the crop. Some labels set a maximum temperature cutoff.
- Humidity: low humidity increases evaporation and drift of fine droplets; high humidity slows drying and can improve uptake but raises the risk of off-target movement.
Application Methods
The label specifies the method. Each method changes the rate calculation and the equipment setup.
| Method | How It Works | Rate Calculation Note |
|---|---|---|
| Broadcast | Whole field treated uniformly | GPA formula uses nozzle spacing over the whole boom |
| Band | Strip over or beside the row; untreated row middle | Use band width in the GPA formula; product rate is per treated acre, not field acre |
| Directed | Spray aimed under the crop canopy | Use shields to avoid contact with crop leaves; common in cotton and some row crops |
| Spot | Handgun or single nozzle on individual plants | Rate per acre is approximate; record area treated for the report |
| Soil drench | Liquid poured or injected into soil around the plant | Used for soil insects, nematodes, some diseases; calibrate by drench volume per plant |
| Seed treatment | Pesticide coated on the seed before planting | Rate is per hundredweight of seed; not a field spray |
For banded applications, the formula GPA = (5940 x GPM) / (mph x W) uses the band width as W, not the nozzle spacing on the boom. The product rate per treated acre stays the same, but the amount of product per field acre is reduced by the ratio of band width to row spacing.
Worked Example: Band Application
- Row spacing: 30 inches; band width: 15 inches
- A 15-inch band treats 15/30 = 50% of the field area
- If the label rate is 1 qt/A on a treated-acre basis, the product needed per field acre is 0.5 qt
- For 40 acres, total product = 40 x 0.5 = 20 qt = 5 gallons
Iowa-Specific Timing Considerations
Iowa's short growing season and wet spring set the timing pressure.
- Preplant window: wet springs can compress the preplant window; soil-applied herbicides must go on before planting, and a missed window forces a POST-only program that is harder to control and increases resistance risk.
- PRE application: apply right after planting and before weed emergence; residuals work best when activated by rainfall within a week.
- Soybean aphid scouting: begin in late June and continue through R5; treat at the 250-aphid threshold, not on a calendar date.
- Tar spot: monitor from R1; fungicide at R1 to R3 if the disease is active in the area.
- Herbicide-resistant weeds: apply POST when waterhemp, marestail, and giant ragweed are less than 4 inches tall; larger plants are more likely to survive and set seed.
- Wind and inversion: Iowa's flat terrain and morning inversions make the 3-10 mph wind window critical; check Iowa State University's Drift Reduction Tool and morning conditions before spraying.
- Sensitive crops: Iowa has many non-target crops (vineyards, orchards, organic fields, gardens); check the DriftWatch and FieldWatch registries before applying growth-regulator herbicides like dicamba and 2,4-D.
Recordkeeping
For every application, record the date, time, field, crop stage, pest and density, product and rate, GPA, weather (wind speed and direction, temperature, sky), nozzle and pressure, and the applicator's name. Iowa records support both the label and any drift investigation.
A 15-inch band over 30-inch row spacing treats what fraction of the field area, and how much product is needed per field acre if the label rate is 1 qt/A on a treated-acre basis?
Which weather condition is most likely to cause off-target drift even when wind speed is below 3 mph?