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Are you maximizing control with your herbicide application?
Author: Tom Barber, Extension Weed Scientist

This is an interesting time in Arkansas agriculture as new technologies are on the horizon in many crops and resistant weeds are becoming more prevalent in all crops and across multiple modes of action.  We have screened multiple samples of barnyardgrass that contain 3-way resistance to Propanil, Facet, and Newpath (ALS) chemistry.  Also annual and yellow nutsedge populations have been documented as having resistance to ALS herbicides.  New technologies for glyphosate-resistant pigweed will be available in 2015 (pending approval) and already there has been resistance documented to these herbicides in the U.S.  Due to these issues, producers and commercial applicators need to place more focus on appropriate application methods to ensure that the proper herbicide rate is delivered to maximize control of these problem weeds.  Herbicide drift or off-target movement of herbicides is the main focus of most sprayer setups or configurations and calibrations.  Although drift is one of the main concerns, focus should also be on delivering the appropriate rate to control the target whether it is an insect, disease, or weed.  Ideal applications are those that deliver the appropriate rate of chemical to effectively control the pest while minimizing crop injury and drift.

There are several factors that affect spray coverage, drift, and effectiveness of pesticide applications.  Nozzle selection, spray volume, and pressure can all affect spray coverage, drift, and ultimately control of the pest in question.

Table 1. Nozzle classification with color coding according to droplet size (VMD) and measured in microns. Based on BCPC specifications in accordance with ASABE Standards

Table 1. Nozzle classification with color coding according to droplet size (VMD) and measured in microns. Based on BCPC specifications in accordance with ASABE Standards

Proper nozzle selection can vary based on the goals of the application, the pest targeted, and the chemical used.  Nozzles are classified by average droplet size.  The size of spray droplets has a direct effect on spray coverage, pest control, and drift potential.  In general, the larger the spray droplet, the less likely that it will drift, while small droplets (<150 microns) fall at slower rates and have highest potential to drift with wind currents. Spray droplet size can affect coverage and control especially with weeds.  On grass species, past research suggests that smaller droplet sizes provide increased control over larger droplet sizes, especially for contact herbicides because the larger droplets tended to bounce off of the leaf and droplet retention is decreased.  Small droplets will also provide increased surface area coverage than large droplets across multiple spray volumes.  On average, the recommended droplet range for maximum weed control across most herbicide labels is between 300 and 400 microns.  The British Crop Protection Council and the American Society of Agriculture and Biological Engineers (ASABE) developed a droplet size classification system with categories ranging from extra fine (XF) to ultra course (UC) based on volumetric mean diameter (VMD) measured in microns (Table 1).  Herbicides, insecticides, fungicides, and other additives such as surfactants, crop oils, drift retardants, buffers etc.  can also have an effect on spray droplet size.

Spray volume is a crucial component to successful weed control applications.  In regard to herbicide applications, spray volume can mean the difference between one, two, or possibly three applications needed for maximum weed control.  Recommended spray volume can vary greatly depending on the herbicide that is applied.  In fact, many issues with droplet size can be overcome with increased spray volume.  Figure 1 contains results from a nozzle study conducted by the University of Arkansas Division of Agriculture with Liberty herbicide.  In this study, Liberty was sprayed at 29 oz/A on 6 in and 12 in pigweed and barnyardgrass.  Once the spray volume was increased to 12 gallons per acre (GPA), there were no significant differences in nozzle performance in regard to 6 in pigweed control.  Pigweed that is 12 in tall is way too big to kill with Liberty or any other herbicide regardless of nozzle configuration or spray volume but differences were observed.  There was also a significant difference (70-92%) in percent control of barnyardgrass when GPA was doubled and AI nozzles were used.  The key point here is to understand the herbicide being used and match the nozzle and spray volume to allow for maximum control from the application.

Figure 1. Nozzle and spray volume effects on pigweed and barnyardgrass control with 29 oz/A liberty

Figure 1. Nozzle and spray volume effects on pigweed and barnyardgrass control with 29 oz/A liberty

Roundup Powermax (glyphosate) has one of the broadest spray volume recommendations of any labeled herbicide.  According to the label, Roundup should be applied from 3 to 15 GPA.  Several studies and published research have found that Roundup (glyophosate) actually provides better control when sprayed at a lower GPA with larger droplet size.  This has a lot to do with glyphosate’s systemic activity but also has much to do with glyphosate molecules and their interaction with water.  However, this is not true for most herbicides.    Contact herbicides such as Liberty or Flexstar all recommend a minimum of 10-15 GPA on their respective labels.   In fact, other systemic herbicide labels such as Newpath, Ricestar, and Clincher all recommend a minimum of 10 GPA on the label.  Some labels go as far as to recommend the droplet size required for maximum control.

The bottom line is, herbicide applications that do not deliver the appropriate rate needed for control will result in a means for that weed to become resistant to the herbicide.  One thing is for sure, there are no new herbicide modes of action that will be commercialized in the near future.  We must come together as an industry and do what is necessary to protect the herbicides and pesticides that work, and a large portion of this responsibility falls on the applicator.  Over the last 10 years, we have outfitted our spray equipment to deliver reduced spray volume, mostly around the Roundup Ready system or glyphosate herbicide.  We need to re-evaluate our spray configurations to match the current systems for pest control and in most cases this involves an increase in spray volume.

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