Spray Adjuvants – an important investment or just another input cost?

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Spray adjuvants have become a multi-million-dollar industry over 50 years. Has the investment delivered on the promise?

What are spray adjuvants?

Any material included in the preparation of spray mixes that in themselves do not have pesticidal activity, but modify the physical, chemical or biological properties of the spray can be classified as adjuvants. 

At the physical level, many spray adjuvants comprising surfactants, oils (petroleum or plant-based) affect the way in which droplets are formed by an atomiser by altering surface tension and thereafter the rate at which they evaporate once released.  While herbicides, because of their composition, can alter droplet characteristics relative to that of water-only, some added adjuvants can markedly alter spray properties to either increase or decrease the potential for drift.

However, a principal role for spray adjuvants is that of “activation” or the facilitation of uptake of the active constituent across the plant cuticle. This effect is not always well defined but does help explain why effectiveness of adjuvants modifying surface tension characteristics often exceeds that considered optimal for spreading and wetting.  It also may help to explain why products that are very effective as “wetters” do not always result in optimal activity of herbicides.

Ammonium sulphate

The use of ammonium sulphate as a “water conditioner” developed around uses with glyphosate particularly where water containing high concentrations of divalent metal ions including calcium and magnesium were present.  However, benefits to glyphosate performance were seen more widely, though not on all weed species, and also in situations where tank mix partners that reduced glyphosate activity were included in mixes. Ammonium sulphate was found to reduce the antagonism due to inclusion of simazine with glyphosate and with some Group I herbicides including 2,4-D amine and fluroxypyr.

Ammonium sulphate also has been found useful with “dim” herbicides and with clethodim, particularly where water containing high concentrations of bicarbonate ions were present. It would seem prudent to consider using ammonium sulphate more often where dim herbicides are used particularly in the light of weed resistance to the Group A mode of action.

New adjuvants and the need for evidence-based use

Opportunities to employ new spray adjuvant products are seemingly endless. Some new products are promoted widely based on a relatively narrow body of evidence and for this reason a measure of caution is needed. It is also clear, that use situations, environmental conditions and application methods vary widely and this points to the need to prove up our spray adjuvant use on a local basis. This provides the need for cooperative farm-based evaluation to supplement data generated remote from the intended place of use. There is a lot at stake in preserving or in enhancing the effectiveness of our farm chemicals as there is no guarantee there is a replacement around the corner.

– Andrew Somervaille, Jubilee Consulting

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