NDSU Crop and Pest Report
Weeds


ISSUE 11   July 15, 2004

APPLYING ADJUVANTS ON A %V/V OR AREA BASIS

Should adjuvants be applied at % v/v or by area? Labels of herbicides that require addition of an oil adjuvant have different rate recommendations for the oil. For example, Poast and Select require 1 qt/A of oil adjuvant while Assure II, Fusidale DX, and Fusion require 1% v/v oil. Also, labels of oil adjuvants can vary from 0.5 to 2 % v/v to 0.5 to 2 pt/A. Application on a % volume basis usually results in adding a lower amount of oil and may result in less weed control than adding the oil on an area basis (1.5 pt/A). This can make a difference with MSO type adjuvants where some labels suggest use at 1% and other labels suggest use at 1.5 to 2 pt/A Does it make a difference?

  • At 8.5 gpa, 1% v/v = 0.085 gal of oil adjuvant/A 0.085 gal = 0.68 pt/A
  • At 17 gpa, 1% v/v = 0.17 gal of oil adjuvant/A 0.2 gal = 1.4 pt/A
  • Since, most herbicides in ND are applied around 8 to 10 gallon/A, which do you think will give greater weed control? - 0.68 pt/A (equivalent to 1% v/v of an 8 gpa spray volume) or 1.5 pt/A. Data would suggest that under ideal conditions the 1% volume rate may sufficient but with adverse conditions, low herbicide rates, large weeds, and less susceptible weeds the 1.5 pt/A oil rate will enhance herbicide performance and give greater weed control.

     

    APPLYING GLYPHOSATE AT DIFFERENT SPRAY VOLUMES

    A common question and concept often misunderstood pertains to spray volume. Is weed control better at 20 gpa or 5 gpa? One might think that 20 gallons/A would give better coverage and result in better weed control but is that the case? Researchers at NDSU published experiments on applying glyphosate at 2.5, 5, 10, and 20 gpa of water. They used glyphosate rates of 1.3 to 5 oz ae/A to separate differences in control. These rates are not recommended for field use but do show what factors are important when herbicides are applied at different spray volumes.

    Table 1 shows increasing grass control with lower spray volumes. At the higher glyphosate rate, the spray volume did not affect control. The benefit of low spray volume includes a higher glyphosate concentration in the spray droplet, which increases herbicide uptake into the leaves, and reduces the antagonistic effects of hard water.

    Table 1.

     

    Oat and wheat control

    Spray volume

    1.3 oz ae/A Glyt

    5.3 oz ae/A Glyt

    (gpa)

    - % -

    - % -

    2.5

    82

    99

    5

    72

    99

    10

    58

    99

    20

    40

    99

    Table 2.  The effect of spray volume.

     

    Glyphosate concentration in spray solution

    Glyt rate

    5 gpa

    10 gpa

    20 gpa

    pt/A

    ---- grams/L ----

    0.5

    4.4

    2.2

    1.1

    16

    8.8

    4.4

    2.2

    2

    17.6

    8.8

    4.4

    This assumes a 3 lb ae/gal glyphosate concentration.

    How role does ammonium sulfate (AMS) play? The research found a greater benefit from adding AMS at high spray volumes than at low spray volumes probably because the less the water volume the less antagonistic ions in the water to react with glyphosate. Water in ND can vary any where from low (~150 to 300 ppm) to very hard water (~1600 to 2300 ppm). The water in the Devils Lake region can be high in Fe. In general, the hard water ion content increases as you move west with some of the worst water in the south central and south west regions.

    They also tested six different formulations of glyphosate and they only noted one major difference in control. All five preloaded formulations provided less control than the formulation that required the addition of 0.5% surfactant when applies at 2.7 oz ae/A and a spray volume of 10 gpa. In this case the preloaded formulations likely did not provide enough surfactant at the low rate. The preloaded glyphosate formulations provided control equal to glyphosate plus surfactant when applied in 2.5 gpa of water. Under this lower spray volume , the surfactant concentration of the preloaded formulations to provide equal control.

    The effect of spray volume on surfactant concentration can be shown by Table 3.

    Table 3.

     

    Surfactant concentration in spray solution

    Glyt rate

    5 gpa

    10 gpa

    20 gpa

    pt/A

    --- relative concentration ---

    0.5

    1x

    0.5x

    0.25x

    16

    2x

    1x

    0.5x

    2

    4x

    2x

    1x

    This assumes a 3 lb ae/gal glyphosate concentration.

    The application of this information is that glyphosate applied at lower spray volumes should give equal or better weed control than applications at high spray volumes. Using high spray volumes to get more thorough spray coverage of weeds may actually lower herbicide performance. This concept applies to other systemic herbicides where not every square centimeter of leaf tissue need be covered with spray droplets to control weeds. HOWEVER, THE LABEL IS THE LAW. Growers cannot applies pesticides at volumes lower than allowed on the label.

    Source: Ramsdale, B.K., C.G. Messersmith, and J.D. Nalewaja. 2003. Spray volume, formulation, ammonium sulfate, and nozzle effects on glyphosate efficacy. Weed Technology 17:589-598.

    Richard Zollinger
    NDSU Extension Weed Specialist
    r.zollinger@ndsu.nodak.edu


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