Can adjuvants enhance glyphosate efficacy?

April 2017

HESTIA NIENABER, ARC-Small Grain, Bethlehem

Glyphosate is a phosphonate and is used as a broad-spectrum, 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 glyphosate 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 concentration. 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 ammonium 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 glyphosate, 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 objective 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 increase 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%), although 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 CaCl₂ 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 strongly discouraged.

The addition of the correct ammonium sulphate to the spray mixture 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 deweth@arc.agric.za.

Publication: April 2017

Section: On farm level