Pasture Thistle Tank Mix Next to Flowering Soybean
You are a licensed agricultural weed control applicator. A 70-acre pasture has a heavy Canada thistle infestation at 40% cover and flowering stage. The producer wants a tank mix of 2,4-D amine and dicamba at labeled rates. Adjacent to the east is a 30-acre soybean field at flowering, 120 feet from the pasture edge. To the north is a rural subdivision with gardens and a pond 200 feet from the pasture. Forecast: 5 mph wind from the east, 31°C, RH 40%, clear, with a forecast low of 12°C overnight. The sprayer has a 500-gallon tank, 15 GPA, 20-inch nozzle spacing, and a 90-foot boom. The dicamba label has a downwind buffer for sensitive crops and a volatility advisory when temperatures exceed 30°C. The 2,4-D amine label has a grazing restriction. The producer wants to spray at 2 PM. Discuss product choice, timing, buffer, volatility, and records.
1. Evaluate the proposed 2 PM application. What would you change?
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Do not spray at 2 PM as proposed. At 31°C, dicamba volatility increases, and the label may prohibit or restrict application above a temperature threshold; check the label. The soybean field is east and the wind is from the east, so the soybean field is upwind, but volatility can move after the wind shifts or during the night. The subdivision is north with gardens and a pond 200 ft away; a 5-mph east wind would not move spray north, but the dicamba label's downwind buffer for sensitive crops must still be met. Canada thistle at flowering is a good target, but the tank mix of 2,4-D amine and dicamba may be antagonistic or increase injury risk; do a jar test. Change to early morning from 6 to 9 AM when temperature is below 25°C and wind is 3-6 mph, use coarse droplets, keep the required buffer from the soybean field, and consider 2,4-D amine alone if thistle control is acceptable. Also check the grazing restriction for the pasture.
2. How would you prevent and detect volatility-related injury to the adjacent soybean field?
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Prevent: choose a low-volatility dicamba formulation, avoid application when temperatures exceed the label limit, apply when the wind will not carry vapor toward soybeans, and maintain the required buffer. Do not tank-mix with volatile adjuvants. Detect: scout the soybean field at 1, 3, and 7 days after application for cupping, strapping, and epinasty on newest growth. Map where symptoms appear; volatility injury often appears in a pattern related to temperature and air movement, not just the spray boom. Communicate with the soybean grower before and after. If symptoms appear, collect photos and consider residue testing; do not apply another herbicide to the soybean field. The reasoning is that volatility can cause injury even when spray drift is zero, so the application decision must consider temperature and formulation, not only wind.
3. What records are especially important for this application?
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Record the formulation and EPA registration number, because volatility differs by formulation. Record temperature at application and the forecast overnight low, wind speed and direction, RH, time of application, rate, total product, buffer left to the soybean field and pond, nozzle type, and the jar-test result. Also record the label temperature restriction and why you chose the alternate timing. Keep the neighbor communication and any scouting notes. If a complaint occurs, these records show whether the application was inside the label and whether volatility was foreseeable. The key is that a record of wind alone is not enough for a volatile herbicide; temperature and formulation are central.