Ohio Christmas Tree Producers Manual

Bulletin 670

General Considerations When Using Herbicides

Regardless of what type of herbicide is being used, several important considerations should always be kept in mind:

Legal Responsibilities in the Use of Herbicides - The use of pesticides, including herbicides, is regulated by both federal (Federal Insecticide, Fungicide and Rodenticide Act) and state law. While a discussion of these regulations is beyond the scope of this publication, growers are encouraged to familiarize themselves with their responsibilities under these laws.

Label Specifications - The label on a pesticide is a legal document. Use of any pesticide, including herbicides, contrary to label specifications is illegal. Growers should read and understand the labels on herbicides and other pesticides they use.

Rate of Application - Stay within the label specifications. For some herbicides such as foliar-applied, a single rate such as one gallon per acre may be specified. Conversely, the label usually provides a range of application rates for many of the soil-applied herbicides. For example, the Princep 80W label indicates that it may be applied at rates from 2 1/2 to 5 pounds of the material in the bag (referred to as the bag formulation) per acre. This range allows the rate of application to be adjusted for differences in soil texture and organic matter content. Clay and organic matter reduce the effectiveness of many herbicides. As a result, on soils with sandy texture and relatively low organic matter content, lower rates of the herbicide are needed. As the amount of clay and/or organic matter increase, higher rates of the herbicide are required. Growers need to experiment to determine exactly what rate, within label limits, will most economically give the desired weed control.

Sprayer Calibration - Herbicides must be applied at the prescribed rate. If too little or too much herbicide is applied, it may result in unsatisfactory weed control, damage or death of trees.

Herbicides are generally prescribed in pounds or quarts per acre. Sprayer calibration involves determining how much herbicide and water (or other carrier) should be mixed in a sprayer to deliver the herbicide at the desired rate, whether the area to be sprayed is one acre or 1/1000 acre.

The first step in calibrating a sprayer is to determine the volume of spray delivered to a known area. This may be done in one of two ways: (1) Fill the sprayer with a known volume of water, spray a known area of ground as if you were applying herbicide and measure the amount of water needed to refill the sprayer. For example, a small compression tank sprayer might use 1 gallon (128 fluid ounces) to spray an area 4 ft. wide and 85 ft. long (340 sq. ft. or .0078/A).^b Similar determinations can be made using larger sprayers. (2) Fill the sprayer with a known amount of water, spray as if you were applying herbicide until the tank is empty and then measure the area sprayed.

Make these determinations several times and use the average value. The amount of herbicide to add to a particular volume of water can now be calculated as follows:

Using our example sprayer that sprays .0078/A with 1 gallon and a recommended herbicide application rate of 2 lbs./A (32 ounces/A), the calculation should be as follows:

To apply herbicide with our sprayer at a rate of 2 pounds acre, 1/4 ounce of herbicide should be added to each gallon of water. The amount of herbicide to mix with a full sprayer of water is then calculated as follows:

In our example, if the sprayer's total capacity was 2 1/2 gallons:

Herbicide recommendations are sometimes made in pounds of active ingredients per acre (lbs.a.i./A) instead of the usual pounds of total material per acre (lbs./A). When this is the case, the amount of herbicide needed is determined by dividing the "Weight of Herbicide Needed," as calculated above by the percent active ingredients in the herbicide.^c Using our example above, if the recommendation had been for 2 lbs. a.i./A instead of 2 lbs./A, and the herbicide had 80 percent active ingredient, we would have needed to add 0.31 ounces to each gallon of water or 0.78 ounces to our full sprayer to obtain the recommended rate of 2 lbs. a.i./A.

When adding herbicide to a sprayer, it is convenient to find a container that holds exactly the desired amount of herbicide, determine how many teaspoons or tablespoons of herbicide equals that amount or mark on a large container exactly how full it should be to contain the desired amount of herbicide.

Several factors significantly affect the spray rate of a sprayer.

Spraying Pressure - Spraying should be done at the same tank pressure as when the sprayer was calibrated. If a small compression tank sprayer is calibrated at 25-30 p.s.i. pressure, spraying should be done at that pressure. Spraying at higher or lower pressures delivers higher or lower than desirable rates of herbicide as the sprayer passes over the ground.

Nozzle Size - Sprayers must be calibrated for each nozzle size. The larger the nozzle, the more spray applied (at a given pressure) as the sprayer passes over the ground. To maintain a particular rate of application, a smaller amount of herbicide is needed when larger nozzles are used. Also, some herbicides, particularly the wettable powders, are abrasive and enlarge nozzle openings with use. This changes the calibration. Nozzles should be recalibrated or replaced often.

Speed of Travel - Spraying should be done at a uniform speed and at the same speed at which the sprayer was calibrated. Slower or faster speeds result in higher or lower rates of herbicide application. For this reason, hand sprayers such as small compression tank sprayers should be calibrated for use by each individual who uses them. Each person has a different speed of walking, and rates of application may vary. Self propelled sprayers should be operated at the speed they were calibrated. Examples of time required for herbicide applications using hand and tractor drawn equipment are presented in Table 6.

Method of Application - Most herbicides used to control herbaceous weeds in Christmas tree plantations are formulated to be mixed with water and applied by spraying. Most liquid and water soluble powder formulations mix easily with water at the recommended rates. However, some of the commonly used soil-applied herbicides are wettable powders (often indicated by the letter W after the name, as in Kerb 50W or Princep 80W). These disperse but do not dissolve in water, and at high application rates it may be difficult to keep them evenly dispersed. With uneven dispersal, the herbicide-water mix will not have a uniform concentration, and some areas may be sprayed at rates well above and others well below the desired rate. Care should be taken to be sure wettable powders are shaken periodically; large sprayers and tanks should have some type of agitator or mixing action.

Comments made earlier regarding selection of mowing equipment apply equally to selection of spraying equipment. Similar factors must be considered in determining what spray equipment is best and most economical for a particular operation (Figs. 35 and 36).

	Fig. 35: Hand spraying (L.) with herbicides to control weeds. Sprayers (R.) should have a 1) pressure regulator and 2) gauge so herbicides can be applied uniformly.
	Fig. 36: Weeds can be controlled with tractor-mounted sprayers if the proper herbicides and time of application are used.

A few herbicides such as Princep also come in granular form (Princep 4G) for dry spreading. Such formulations are often attractive to small growers because of the ease of application and the fact that they may be used with considerably less equipment investment. They can be spread by hand, hand spreader or machine drawn spreader. However, cost of herbicide for each area treated is usually higher.