No. 4 — April 15, 2019

How Much to Apply?

Often I receive emails asking me how much of a product should be applied. Seems like a very straight-forward question, but it is not. Many of the pesticides used in agricultural production, which includes grape growing, come in various formulations. As an example, Rely herbicide initially was introduced formulated at 1.00 pound of glufosinate-ammonium per gallon. The application rate was 3 to 5 quarts per acre. Next, Rely was introduced as Rely 200 and formulated at 1.67 lbs of glufosinate-ammonum per gallon. The use rate for Rely 200 was 57.5 to 115 fluid ounces per acre. Finally, Rely was introduced as Rely 280 and formulated at 2.34 lbs of glufosinate-ammonium per gallon. The use rate for Rely 280 is 48-82 fluid ounces per acre. In addition there are the generic glufosinate-ammonium herbicides. Interline would be an example of generic and is formulated at 2.34 lbs of glufosinate-ammonium per gallon. The use rate for Interline is 48-82 fluid ounces per acre.

Asking the question of how much of a product should be applied per acre is dependent on the formulation. How many pounds of active ingredient per gallon or the percent active ingredient. It does make a difference.

An example of how formulations make a difference. When Rely 280 was introduced a grape grower applied the product at the Rely 200 use rate of 115 ounces per acre. This amounted to applying 0.6 more pounds of the active ingredient per acre. What happened next? The vineyard received over 8 inches of rain since the Rely application, which suggests that there was the potential for leaching. The vineyard is planted on course soil (sandy soil) which likely resulted in Rely infiltrating deep into the soil profile. Most all grape plants were showing visual injury symptoms of chlorotic shoots, and upward cupped leaves (Figure 1). The injury symptoms were not apparent on the older leaves, suggesting the material was translocated to the growing regions (meristematic tissues). There was no symptomology on the grapes leaves that would have suggested overspray or spray drift.

Glufosinate ammonium is very mobile in the soil and may travel though the soil unretarded. The half-life of glufosinate varies based on a number of factors, soil type, temperature, and soil moisture, with half lifes reported ranging from 3 to 70 days. Soils with higher organic matter content will degrade glufosinate ammonium at a faster rate compared to soils (sandy soils) with lower organic matter content. Glufosinate ammonium can potentially be leached into the growing root zone of grapes, but is there evidence that glufosinate can cause damage to plants by root uptake?

Photo of Brianna grapevine with phytotoxicity symptoms
Figure 1. Rely herbicide (glufosinate–ammonium) phytotoxicity to Brianna grapevines as a result of applying the wrong rate. Photo credit: D. S. Volenberg.

The herbicidal effects of glufosinate ammonium have been demonstrated after uptake in hydroponics (You and Barker 2002) and also uptake from the soil in potted plants (You and Barker, 2004). In these studies, besides noting visual herbicide injury symptoms, ammonium ion NH4 + accumulation was quantified in shoots. Glufosinate ammonium works by mimicking glutamate which results in the inhibiting Glutamine synthetase with the end result being an increase of ammonium ion in the plants. As ammonium ion increases in the plant, photosynthesis stops. Visual symptoms of glufosinate ammonium in plants are upward cupping of leaves, and leaf and shoot yellowing (chlorosis).

The take home message here is, read the label that came with the product!

References cited

You, W. and A. V. Barker. 2002. Herbicidal actions of root-applied glufosinate ammonium on tomato plants. Hort. Sci. 127:200-2004.
You, W. and A. V. Barker. 2004. Effects of soil-applied glufosinate-ammonium on tomato plant growth and ammonium accumulation. Commun. Soil Sci. Plan. 35:1945-1955.

For grape growers needing conversion for small amounts of pesticides, see the link below:
Mixing small spray quantities of pesticides

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