By: Kirk Williams, Lecturer-Texas Tech University
While there are many methods to control weeds, weed control with herbicides remains a viable option due to its lower cost versus other options such as mechanical cultivation. When herbicide applications are done efficiently it can be a much faster process when compared to mechanical cultivation.
Application of herbicides through nozzles is important for correct herbicide distribution and dosage over the target. The target could be existing weeds in the case of post emergent herbicides or the soil surface with pre-emergent herbicides or a combination of both existing weeds and the soil surface in the case of tank mixing post emergent and pre-emergent herbicides. Coverage is critical for herbicides to work effectively. Hydraulic spray nozzles create a wide range of droplet sizes. These droplets are measured in microns with droplets from 200 to 400 microns considered the most appropriate size for herbicide applications. Larger droplet sizes may bounce or roll off leaves while very fine droplets are more prone to drift.
To provide satisfactory results, recommendations from Syngenta, one of the manufacturers of water sensitive cards, is to have 20 to 30 droplets per square centimeter for pre-emergent herbicides. For post emergent herbicide applications, it is recommended to have 30 to 40 droplets per square centimeter. A square centimeter is about 1/6 of a square inch.
One way to increase and improve coverage in herbicide applications is to use a double nozzle body. This allows two nozzles to occupy the same space as one nozzle. The double nozzle body allows you to easily increase the volume of coverage no matter what kind of nozzle you using. This is especially true when you have to use a relatively large spray nozzle to apply higher volumes. Larger sprayer nozzles tend to produce coarser droplets. Splitting the required flow into two nozzles can produce an efficiency gain by decreasing the number of coarse droplets. The double nozzle body also helps with increasing droplet coverage of target. (See the double nozzle body set up in Image 1).
It was found in a study with water sensitive cards that coverage was increased by 20% when double nozzle bodies were used when compared to single nozzle bodies. The study used regular flat fan off center 03 nozzles, calibrated to deliver 47 gallons per treated acre and the water sensitive cards were placed on the berm underneath the grape vines. While the double nozzle body had higher coverage the droplets per square centimeter went down due to the droplets running together versus the single nozzle body. The single body nozzle had 43 droplets per square centimeter while the double body nozzle had 17 droplets per square centimeter but the droplets are much bigger. (See the water sensitive cards in image 2). While the droplets per square centimeter are out of the recommended range for droplets per square centimeter for herbicides with the double nozzle body the better coverage should result in similar or better weed control with both pre-emergent herbicides and post emergent herbicides.
Herbicide application technology has improved with a wider selection of nozzles available. Adoption of these newer type of nozzles has been widely adopted in row crops but may be not as widely adopted for vineyard herbicide applications. These newer type nozzles reduce drift but still deliver good weed control.
Air Induction nozzles are a newer type of nozzle that are available in a wide range of spray tips. The air-induction nozzle is noted for producing large drops through the use of a venturi air aspirator. The venturi draws air into the nozzle through holes in the side of the nozzle and then the air is mixed with the solution to create larger spray droplets, which reduces drift potential. These larger droplets are filled with air bubbles and explode on impact with the target surface and produce coverage that is similar to other nozzle types.
Many sustainable grape growing standards include drift management as a component of their plans. The incorporation of air induction nozzles that reduce drift by reducing the number of fine, drift prone droplets could be a part of meeting the requirements of these standards. Drift reduction is important during the growing season to reduce phytotoxicity due to herbicides. When herbicide applications take place in the in dormant season, phytotoxicity to the grapevines is not an issue but with the wide spread adoption of cover crops, herbicide applications need to stay where they are intended to be, so they don’t impact the cover crops.
In a study, double nozzle bodies were equipped with air induction under banding nozzles (AIUB8503), calibrated to deliver 44 gallons per treated acre. Water sensitive cards were placed on the berm underneath the grape vines. This nozzle configuration delivered 56% coverage of the water sensitive cards and produced 26 droplets per square centimeter. As you can see, in the water sensitive cards in (image 3) the droplets produced are large with few small droplets. The droplets per square centimeter are in the recommended range for droplets per square centimeter for pre-emergent herbicides but outside of the recommended range for post emergent herbicides. The coverage produced should result in similar or better weed control with both pre-emergent herbicides and post emergent herbicides when compared to the regular nozzle set up. The air induction nozzles will also produce a minimum of drift prone fine droplets.
Adoption of double nozzles bodies into your herbicide application program will increase coverage for both pre-emergent and post emergent herbicides. Switching to air induction nozzles is one way to make sure the herbicides that you are using stay where they are intended to.
In addition to coverage and nozzle selection, don’t forget about integrated weed management principles. These principles include knowing and correctly identifying your weed problems so that appropriate herbicides can be chosen. Controlling weeds when they are small when they are more susceptible to herbicides and easier to have better spray coverage. Another integrated weed management principle is to keep annual weeds from going to seed which reduces the weed seed bank in the soil. Other principles include rotating among herbicides with different modes or action or tank mixing herbicides with different modes of action together which helps manage weed population shifts as well as herbicide resistance. Staying clean is easier than trying to clean up a mess, so pre-emergent herbicides may help to keep the vineyard berms clean.
Kirk Williams is a lecturer in Viticulture at Texas Tech University and teaches the Texas Tech Viticulture Certificate program. He is also a commercial grape grower on the Texas High Plains. He can be contacted at kirk.w.williams@ttu.edu
