From invasive species control to food plots, our sprayer is used nearly every day that I work on habitat management on our farm. The first few years that I was spraying food plots, I simply used the percentage of glyphosate (RoundUp) that I was told by a store manager would work. However, I learned that there was a better way, as proper sprayer calibration has made herbicide applications much easier and more accurate.
While spraying an area with a 2-5% mixture of glyphosate may work on most plants, it certainly won’t work on everything. Additionally, this mixture may be too heavy or too light of an application depending on how much water the particular sprayer uses. When applying herbicide, what matters is the amount of chemical applied per acre, not the percentage of herbicide in your sprayer. While being off a few ounces on a herbicide that is commonly used at a rate of a quart or two per acre may not make a difference, many selective herbicides (such as clethodim) are used at a rate of only a few ounces per acre. Applying too little herbicide in these situations can result in lower kill rates. To compound this issue, some herbicides can cause major issues if they are applied at too heavy of a rate. Having residual herbicide in the soil for a year or more can be a consequence of not knowing exact application rates.
In addition to using the proper amount of herbicide, sprayer calibration also makes your life much easier. Instead of guessing how much herbicide solution to make, I can simply add water and herbicide based on the acreage I am spraying. In the past I have had to refill a sprayer because I mixed up too little solution, and this is no longer an issue. This results in savings of both time and money, as I can be more efficient with my time and I no longer have extra chemical in the tank after I am done spraying.
Easy Sprayer Calibration
While there are several techniques for sprayer calibration, I use the technique printed on my herbicide measuring cup from the University of Tennessee Extension. This method can be found at https://extension.tennessee.edu/publications/Documents/W315.pdf , but I will describe it briefly here. First, the nozzle spacing on the boom must be measured. This measurement will provide a distance to lay out a “calibration course” based on the chart provided at the link. Following laying out this course, drive it at a comfortable speed and time yourself. It is important that the driving speed is consistent, and a speedometer phone app can be used to track this. Then, simply fill the sprayer up with water, turn it on and set the PSI on the sprayer at a desired pressure, and collect the water coming out of one nozzle for the amount of time it took to drive the course. I would recommend doing this on several nozzles and averaging it. The number of ounces of water collected from one nozzle during that length of time is equal to the number of gallons that sprayer will use per acre at that driving speed and PSI. While this may sound complicated, I can assure you it is extremely simple and will only take 10 minutes. When the time comes to spray a plot, now all you have to do is add enough water to cover that acreage, and add the amount of herbicide the label specifies for that acreage.
I cringe every time I see someone comment on habitat management Facebook pages about what percentage of herbicide to use on a food plot. Aside from spot applications, herbicides should be applied based on the amount of chemical per acre. Sprayer calibration is an easy step towards doing things the right way, and having confidence a herbicide application is accurate is well worth the few minutes of your time that calibration will take!