SA Grain April 2017

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Can adjuvants enhance
glyphosate efficacy?
Glyphosate is a phosphonate and is used as a broad-spec-
trum, non-selective, systemic herbicide. Glyphosate has
been on the market since 1974.
It acts by inhibiting the EPSPS enzyme, thus interfering with the
synthesis of three of the plant’s aromatic amino acids. Glyphosate is
one of the most widely used herbicides in the world.
When glyphosate formulations are developed, certain adjuvants
are added to the glyphosate product. Therefore, many glypho-
sate products contain all the necessary adjuvants when it is sold.
Some glyphosate products may contain no adjuvant system, while
other products contain only a limited amount of adjuvant. With
products like these, additional adjuvants need to be added in the
spray mixture.
In literature, glyphosate is known as a weak acid, meaning that
glyphosate will partially dissociate in the spray water. Some of these
dissociated herbicide molecules have a negative charge, which may
bind with positively charged cations (calcium, magnesium, sodium
or iron). When this happens, herbicide activity may be decreased.
Water containing high levels of these cations is known as hard
water. Water hardness, therefore, is a measure of how much salt is
present in the water. The harder the water, the higher the salt con-
centration. Many of the water sources used for herbicide application
in South Africa contain high amounts of cations.
The addition of ammonium sulphate (AMS) to the spray tank can
overcome many interactions with herbicides and cations. The am-
monium cation attaches to the glyphosate molecule and prevents
calcium, magnesium, sodium and/or iron from doing so. Calcium
within plants can also reduce the efficacy of glyphosate. Ammonium
sulphate can also assist in overcoming this interference by calcium.
Research
Ongoing research is being performed by ARC-Small Grain on the
effect of water quality on herbicide efficacy and the effect that
adjuvants have on this. Ammonium sulphate is classified as a salt
adjuvant. Many products are believed to work wonders with glypho-
sate, but this is seldom true.
An experiment was conducted to determine the effect of seven
ammonium sulphate formulations on glyphosate efficacy. The
ammonium sulphates were numbered from one to seven to be ob-
jective and non-biased towards the products. AMS1, AMS2 and
AMS7 are products that are already registered and available on
the market. The other AMS formulations are all formulations that
were included to determine the effect they have on the efficacy of
glyphosate. Three water types were used, i.e. distilled water, hard
water (CaCl2) and brackish water (NaHCO3).
The only treatment in distilled water that showed a significant in-
crease in percentage control, when compared to the control
(72,05%), was the AMS1 treatment (91,11%). The AMS4 treatment
showed severe antagonism (35,97%). The rest of the treatments did
not differ significantly from one another.
In the CaCl2 water, two treatments showed a significantly higher
percentage control than the control (53,29%). These treatments
were the AMS1 treatment (73,27%) and the AMS3 treatment
(73,35%). AMS6 showed antagonism (43,56%) in the hard water. The
rest of the treatments did not differ significantly from one another.
Five treatments showed significantly higher percentages than the
control (52,68%). These treatments were AMS1 (91,17%), AMS2
(94,19%), AMS5 (80,13%), AMS6 (76,98%) and AMS7 (95,01%).
No antagonism was observed in any treatment. The treatment
that showed the lowest percentage control was AMS4 (57,56%), al-
though this percentage was not significantly lower than the AMS3
treatment (68,36%).
Summary
According to the results, AMS1 is the best ammonium sulphate
adjuvant, followed by AMS2. AMS3 showed constant control, as
well as AMS5 and AMS7. According to the results, AMS6 showed
antagonism in CaCl 2 water, which could make the use of this
product risky. The ammonium sulphate adjuvant that showed the
lowest levels of control was AMS4. Usage of this adjuvant is strong-
ly discouraged.
The addition of the correct ammonium sulphate to the spray mix-
ture can improve glyphosate efficacy significantly. It is therefore
always important to read label instructions and follow the mixing
guidelines. Poor choices will lead to poor glyphosate efficacy.
For more information, please contact Ms Hestia
Nienaber at 058 307 3420 or
43
April 2017
ON FARM LEVEL Integrated pest control
Glyphosate / Efficacy / Herbicide
HESTIA NIENABER, ARC-Small Grain, Bethlehem
Graph 1: Comparing the effect of seven different AMS on the efficacy of glyphosate.
Grain SA/Sasol photo competition
– Riaan Potgieter 2014