Speed doesn't kill: Slow down your spray rigs!

As we prepare for hull-split, I thought I would take a minute to discuss spray rig speeds. It is easy to hurry with the hustle of applying a hull-split spray on a large number of acres, but attention should be payed to the ground speed in which chemicals are applied. The University of California recommends applying your sprays at a ground speed of 2.0 MPH to ensure adequate height coverage and canopy penetration of the spray. This recommendation has been based on numerous studies using ground rigs at varying speeds applying a water spray to water sensitive paper placed within the trees canopy. Figure 1 shows data from a recent trial conducted by Joel Siegel (USDA-ARS) conducted in pistachio.

Figure 1: Spray coverage of a ground rig applying a water spray to water sensitive cards at 3 different ground speeds.

Spray coverage for almond trees is based on the height and density of the tree. For the most part, if the height of the tree is under 10 feet, adequate spray coverage can be attained at almost any reasonable speed. For taller orchards, however, this is not true. As shown in figure 1, the difference of a 1/2 mph (2.0 mph v/s 2.5 mph) can reduce the coverage by 30% for trees 18' or higher. Traveling at 3.25 MPH, less than 2% coverage was achieved at a height of 20 feet. This reduction makes the pesticide applied lose its efficacy due to low concentration and basically renders the application useless. It also increases the chance of resistance formation, loss of crop due to disease/insect damage, and tractor/rig damage.

Many people say that their rigs are better than the ones used in the study and therefore they can go faster. I would doubt that statement unless they can clearly demonstrate it with water sensitive cards on your farm. Spray trials testing applicator speed have been conducted for over 40 years by several generations of UC/USDA-ARS researchers with a variety of rigs and technologies. All of them come to the same conclusion: Apply your sprays at a ground speed of 2.0 MPH.

I guess, in this case, that speed doesn't kill.