20 gallons should be enough, but it's dependent on your speed. You need to calibrate your sprayer to know for sure. I'll try to attached what I used to calibrate mine.
Okay, that didn't work. I'll have to cut and paste:
Sprayer Calibration
When solving problems in mathematics, there are often several approaches to get the correct answer. This is true with sprayer calibration. The method presented here is described in many publications. There are other proven methods that also work. All methods provide the same answer a sprayer calibration value expressed in gallons per acre. This refers to the amount of spray mixture needed to spray one acre. A reasonable sprayer output is 15 to 30 gallons per acre. The formula used to calibrate a broadcast sprayer is:
GPA = (GPM*5940)/(speed*nozzle spacing)
The definitions of terms in this equation are:
GPA - Sprayer output in gallons/A.
GPM Average flow rate per nozzle in the spray boom in gallons/min.
5940 Mathematical constant that is the result of many unit conversion factors.
Speed Calculated ground speed of the ATV in miles/hour.
Spacing - Spacing of nozzle tips along the spray boom expressed in inches
Procedure
Step 1. A device to accurately measure water in fluid ounces (or milliliters) is needed. For safety reasons, dedicate this measuring device exclusively for sprayer use. Several containers are needed to collect spray output from each nozzle tip. A watch with a stopwatch feature is needed to accurately measure time in seconds. Finally, a handheld calculator is needed for the mathematical computations.
Step 2. Assemble and mount the sprayer to the ATV. Fill the spray tank with water alone (no herbicides). With the ATV stationary, turn on the sprayer and adjust the pressure to the desired level, if pressure adjustments are possible. Run the sprayer for a minute or so to make sure that spray is being discharged from all tips and there are no leaks in the lines and couplings. Use the cups to collect the spray from each nozzle tip along the sprayer boom for one minute. Measure the output volume from each nozzle tip and calculate the average output across all nozzle tips. Convert the average output to gallons/minute using either of the following conversion factors:
(fl. oz./min)/128 = gal./min.
or
(ml/min.)/3785 = gal./min.
Other useful conversion factors are listed in Table 2.
Step 3. Substitute the appropriate terms into the calibration equation. It is essential that each term in the equation be in the correct units. GPM must be in gallons/minute. Speed must be in miles/hour. Nozzle spacing must be inches. Using the wrong units will result in gross errors in calibration.
Step 4. Repeating the calibration procedure and all computations.
A sprayer is assembled and mounted on an ATV. A comfortable and repeatable throttle setting has been determined and with those settings, the ground speed of the ATV was found to be 3 miles/hour. The sprayer is configured with seven TeeJetｮ 8002 flat fan nozzle tips spaced 19 inches apart along the boom. The sprayer pump does not have adjustable pressure, so we use whatever pressure the pump generates.
With the spray tank filled with water and pump operational, we run the sprayer for one minute and collect the following volumes from each nozzle:
1st run: (fluid ounces of spray water collected from each nozzle for one minute)
Nozzle 1.......Nozzle 2.......Nozzle 3.......Nozzle 4.......Nozzle 5.......Nozzle 6.......Nozzle 7
27 fl. oz........25 fl. oz........26 fl. oz.......26 fl. oz........10 fl. oz.......25 fl. oz........26 fl. oz.
Notice the output collected from Nozzle 5. Compared to the other sprayer tips, something is obviously wrong. The nozzle assembly (nozzle tip, strainer, and nozzle body) is disassembled and a piece of metal filing is removed from the nozzle tip. The process is repeated and gives the following results
2nd run: (fluid ounces of spray water collected from each nozzle for one minute)
Nozzle 1........Nozzle 2........Nozzle 3........Nozzle 4.......Nozzle 5.......Nozzle 6.......Nozzle 7
26 fl. oz..........25 fl. oz..........25 fl. oz........27 fl. oz.........26 fl. oz......27 fl. oz......25 fl. oz.
The output from all nozzle tips is now similar, less than 10% variation among all seven tips. This is acceptable.
The next step is to calculate a simple average output per nozzle tip on the sprayer boom.
(26 + 25 + 25 + 27 + 26 + 27 + 25)/7 = 25.86 fl. oz./min.
Convert the output from fl. oz./min. to gal./min.
(25.86 fl. oz./min.)/(128 fl. oz./gal.) = 0.2020 gal./min.
(This answer is one of the critical terms in the calibration equation.)
Refer back to the original calibration equation and begin substituting.
GPA = (GPM*5940)/(speed*spacing)
GPA = (0.2020*5940)/(3*19)
GPA = 1200/57
GPA = 21 gal./A
The sprayer in this example is now calibrated. It will apply 21 gallons of mixed spray uniformly over one acre.
Sprayer Operations
Dilution and mixing. Carefully read the herbicide label, which in this context refers to the detailed instructions provided with the herbicide. The label also provides information on protective clothing when applying the herbicide. Protective clothing may be chemical resistant coveralls, boots, gloves, and safety glasses. Pay close attention to all safety statements, for your safety as well as the safety of others.
The first step is to estimate or measure the size of the area treated, which will determine the volume of spray mixture needed to treat the area. Unless you have a knack for estimating area, simply measure or step-off the rough dimensions and calculate the estimated area in ft2, which can then be easily converted to acres (1 acre = 43,560 ft2). As a reference, the area of a football field is 1.3 A. Once the size of the treated area is determined, use the sprayer calibration value (gal./A) calculated earlier. Use the calibration value and the area to be treated to determine the amount of spray mixture needed for the task.
(acres treated) * (gal./acre) = gal. of spray mixture
Depending on the calibration, area to be treated, and size of the spray tank, the task may require a partial tank or multiple filling to treat the site. Add the calculated amount of clean water to the spray tank.
Example: A food plot measures out to be approximately 0.8 A. You calibrated the ATV sprayer to spray 21 gal./A. and the tank has a capacity of 20 gal. The volume of spray water needed to treat the food plot is: (21 gal./A * 0.8 A = 16.8 gal.). Add 16.8 gal. of clean water to the spray tank.
(herbicide rate/acre) * (acres treated) = amount of herbicide to add to spray tank
Example (cont.): Vantageｮ is needed to control crabgrass in a planting of Whitetail Imperialｮ perennial clover. The label lists the Vantageｮ rate at 1.5 pt./A. The food plot is 0.8 A. The amount of Vantageｮ to add to the 16.8 gal of water in the spray tank is: (1.5 pt./A * 0.8 A = 1.2 pt.).
Useful conversions.
Liquid measurement
1 gal. = 4 qts.
1 lb. = 16 oz.
1 Acre = 43,560 ft.2
1 gal. = 8 pts.
Weight
1 lb. = 454 g
1 mile2 = 640 A
1 gal. = 128 fl. oz.
Area/length
1 mile = 5,280 ft.
1 gal. = 3785 ml
1 mile = 1,760 yds.
1 Tbs. = 2 fl. oz.
1 Tbs. = 3 tsp.
Times needed to travel test distances to achieve ground speeds in miles per hour.
Ground speed ...................Test distance of 50 ft..................Test distance of 100 ft.
2 mph........................................17 sec........................................34 sec.
3 mph........................................11 sec.........................................23 sec.
4 mph..........................................8 sec.........................................17 sec.
