Glyphosate was first registered for use in Australia in 1976 with the introduction of a 360g/L isopropylamine salt (Roundup®). In broadacre uses, this product was superseded in 1984 by Roundup CT® (450g/L isopropylamine). At this time, the use of additional surfactants in conjunction with herbicides to ensure herbicide performance was normalised, with corresponding inclusion within label recommendations. Since this time, many new formulations have been introduced based on different glyphosate salts and surfactants.
The question of the use of various spray adjuvants including surfactants, oils and fertiliser salts has become more complicated with the introduction of various higher loaded formulations utilising different surfactant systems.
The purpose of this paper is to provide some clarity to the issues surrounding the use of different glyphosate formulations and adjuvants as well as to point to other performance limiting issues that face broadacre users of glyphosate products.
Glyphosate in its acid form is an insoluble material that cannot be readily utilised. Formulations have been developed around various salts with high solubility. Biological activity was found to be enhanced by the addition of various surfactants that not only reduce the surface tension of the droplets that are formed with spraying but also other properties; specifically, those which enhance the movement of a hydrophilic (water loving) active constituent across a lipophilic (fat loving) barrier being the leaf cuticle of the plant and ultimately to the sites of action.
Under favourable conditions, glyphosate is rapidly absorbed by plant foliage. Initial fast entry, followed by a longer phase of slower absorption has been widely reported (Franz, Mayo and Sikorski, 1997). Early screening of different surfactants identified tallowamine ethoxylate (MON 0818) as generally the most effective surfactant with isopropylamine salts of glyphosate to optimise activity on a wide range of plants. Tallowamine ethoxylates are produced by the reaction of fatty acid amines derived from tallow with ethylene oxide. Tallowamine ethyoxylates do vary in their properties according to hydrocarbon chain length and degree of ethoxylation while the concentration added by manufacturers can also vary below that formulated in original formulations.
Field studies undertaken in Australia determined that the optimum concentration of tallowamine ethoxylate for biological activity of IPA glyphosate was between 0.15 and 0.25% of total spray volume. This equated to the equivalent of 0.6 and 1 litres of formulated Roundup CT® in 50 litres carrier volume. At greater dilutions that are typical in broadacre applications, there was a good likelihood that performance could be enhanced by the addition of supplementary non-ionic surfactants including alkoxylated alcohols (e.g. BS1000®) and nonyl-phenol ethoxylates (e.g. Agral® 600) amongst others. For specific weeds including annual ryegrass and silvergrass, the octyl phenol ethoxylate (e.g. Wetter TX) was very effective even where there was sufficient tallowamine to optimise activity more generally.
The introduction of higher loaded (compared to Roundup CT®) liquid formulations occurred around 1996 with the launch of Roundup® CT Xtra (490 g/L IPA glyphosate). As was previously discovered, as the rate of active ingredient was increased there was less ‘room’ for surfactants and specifically for the tallowamine types previously found to be most effective. Roundup® CT Xtra was the first liquid formulation to utilise a different surfactant type (etheramine) and considerable effort was put toward optimising the concentration to minimise the need for additional surfactant.
Further development of high loaded formulations necessitated the use of alternate salts. In the first departure from the isopropylamine salts, a dry formulation based on the monoammonium salt was developed (Pacer Sol-Tech) which did not contain any surfactant and required the addition of a suitable in-tank surfactant. It was quickly determined that only tallowamine ethoxylates could consistently deliver comparable performance to existing liquid formulations and a suitable surfactant product was marketed. Later, ‘dry’ formulations based on mono-ammonium salts but including tallowamine surfactants were developed (e.g. Roundup® Dry, Roundup Ready® Herbicide) while other brands utilise alternate surfactants (most 700g/kg formulations).
Higher loaded liquid formulations initially utilised the mono-ethanolamine salt with a loading of 510g/L (e.g. Roundup® MAX) before potassium salts were introduced with a loading initially of 540g/L (Roundup PowerMAX®) and then 570g/L (Roundup ULTRA®MAX). Following this, various salt blend products were introduced including Weedmaster® DUO (isopropylamine plus mono-ammonium), Weedmaster® DST (potassium plus mono-ammonium) and Weedmaster® ARGO (potassium plus isopropylamine). High loaded isopropylamine products were not successful due to inferior performance and poor handling characteristics under cold conditions.
The development of high loaded formulations necessitated the development of alternative surfactant systems to replace tallowamine ethoxylate as the sole surfactant due to the impossibility of creating a stable high load formulation exclusively with this material. This has presented many challenges as many of the alternate surfactants available capable of being formulated in high load formulations have not delivered the high level of activity produced by original standards particularly on certain grass species. In addition, proprietary protection around surfactant systems used in some of the original high load formulations has forced competitors to utilise surfactants that have difficulty in maintaining the existing high standards of product performance. Generally, however the branded high load products have been extensively tested and can be relied upon to deliver comparable or slightly better performance to original standards.
One of the features of high load formulations that utilise chemicals other than a tallowamine ethoxylate surfactant as their sole component, is the lack of a generalised requirement for surfactant addition to optimise performance. Some of this has also occurred incidentally as use rates have increased significantly since the original introduction of Roundup® branded products in the 1970s and 1980s. However, this has also made the question of surfactant addition to high loaded products more complicated as it was noticed during development, that the addition of surfactant sometimes reduced product performance rather than enhanced it. Surfactant products routinely included with IPA glyphosate formulations based on tallowamine ethoxylate often produced negative responses when included with alternate salt formulations utilising different base surfactants.
The main message to take from this observation is that routine surfactant addition should be cautiously considered based on either data or personal experience, given the environment and weed types present.
Table 1. Salt formulations of glyphosate.