Factors to consider with the use of different glyphosate formulations and getting it right with the use of adjuvants

Take home messages

• Glyphosate formulations vary in performance but differences are often weed species specific.
• In-tank adjuvants may both enhance or detract performance of glyphosate formulations.
• Targeted use of adjuvants can assist in maintaining high level of weed control alongside other factors.

Background

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 In broadacre uses, this product was superseded in 1984 by Roundup CT® (450g/L isopropylamine).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.

Discussion

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.

Adjuvant choice to optimise glyphosate activity

Responses to adjuvants of any type are likely observed during three situations:

1. Where application rate of glyphosate becomes marginal for its performance due to certain factors including onset of marginal growing conditions, partial weed resistance (non-target site) and inclusion of some antagonistic herbicides.
2. High dilution rates where surfactant concentration is low. Where dilution rates of glyphosate are low as is typical in many situations for summer weed control, the likelihood of response to added surfactant is relatively low.
3. Where a formulation contains a surfactant type that is not optimal for control of the target weed. Addition of a surfactant may increase but not always increase performance when compared to an alternate formulation containing a more effective surfactant system.

In these situations, the decision to include surfactant should also be guided by the formulation used. For most 450g/L formulations, tallowamine surfactants are included. Responses to added non-ionic surfactants is well researched and generally predictable in that additions will not result in reduced performance. Control of some weeds such as sowthistle may be favoured by the addition of more tallowamine ethyoxylate where for others such as annual ryegrass, an alkoxylated alcohol or octylphenol ethoxylate may be more effective.

Where high levels of antagonistic cations are present, inclusion of adjuvants containing ammonium sulphate, citric acid or phosphoric acid derivatives may effectively sequester cations. Ammonium sulphate may enhance activity more generally. Acidifying surfactants including propionic acid are not effective in overcoming the potential adverse effects of calcium and magnesium ions.

Tank mixes to avoid in order to optimise glyphosate activity

While tank mixing of various herbicides is frequently recommended to ensure cost-effective broad-spectrum control of hard-to-kill weeds or to enhance speed of weed desiccation, some of these can detract from activity of glyphosate to the detriment of overall spray performance.

For this reason, careful consideration should be given to situations where the activity of glyphosate is critical for the control of the dominant weeds present. In many situations, this is the control of grass weeds and the tank mix partner in general does not contribute to the control of these species.

Widely used tank mix partners with glyphosate are summarised in Table 2. Of these, the phenoxy based herbicides including 2,4-D can be problematic with glyphosate for control of summer growing grasses but also for control of sowthistle (Sonchus oleraceus). Other auxin products that may be problematic include dicamba and fluroxypyr (e.g. Starane™ Advanced). On the other hand, clopyralid (e.g. Lontrel™) and fluroxypyr (e.g. Garlon™) are rarely antagonistic for grass weed control.

Herbicides that have the Group G (PPO inhibitor) mode of action including oxyfluorfen (e.g. Goal®), carfentrazone (e.g. Hammer®), flumioxazin (e.g. Valor®) and saflufenacil (Sharpen®) as a tank mix partner can cause significant reduction in glyphosate performance in summer months due to their rapid light activated activity which may effectively reduce uptake and translocation of glyphosate. This effect is most likely to be seen on grasses but also broadleaf weeds that may not be as susceptible to Group G herbicides and for which control is primarily dependent on glyphosate.

Other herbicides sometimes included with glyphosate include the Group C herbicides that inhibit Photosystem II. The more foliar active examples of these include atrazine, diuron, terbuthylazine and prometryn. Where grasses and certain broadleaf weeds have advanced beyond the seedling stage, glyphosate activity on these plants is reduced and separating applications to allow glyphosate to be applied first, will generally provide more reliable performance.

Table 2. Herbicide tank mixes with glyphosate and effect on performance.

Heat stress and the effect on glyphosate performance

In general, conditions that optimise plant growth are those that also optimise uptake and translocation of glyphosate in annual weeds. Exceptions exist where physiological growth stage does not favour sufficient translocation to roots, tubers or rhizomes where destruction of these plant organs is vital for the control of weeds.

High temperatures can adversely affect plant metabolism even where moisture may not be limiting for growth. For plants adapted to warm-season environments, day maximum temperatures above 35 can lead to shutdown of growth particularly if this regime is maintained over successive days. Under these circumstances, active plant growth may continue slowly or cease altogether even if night temperatures fall to moderate levels. Under this regime, poor glyphosate performance may occur irrespective of the use of spray adjuvants or scheduling of application at lower temperatures at the end of the day or early in the morning.

The best strategy for summer application where infrequent rainfalls are more normal is to target weeds before the onset of ‘heatwave’ conditions even if this requires a follow-up application after the resumption of periods of more favourable temperatures for plant growth. By timing applications earlier after emergence will also ensure reduced transpiration losses compared to applications delayed.

Conclusions

Glyphosate activity is affected by many factors including those that affect plant growth but also presence of other materials in spray solutions, including surfactants.

Surfactants vary widely in their ability to enhance glyphosate activity in plants and this effect may be specific to individual weeds.

A prescription to include adjuvants with glyphosate products should be, ideally, informed by local knowledge and is likely to vary with formulation type, adjuvant type and the dominant weeds targeted in application.

Indiscriminate inclusion of adjuvants may be counterproductive to herbicide activity.

Combination of certain herbicides with glyphosate (tank mixes) may be detrimental to weed control and ultimately contribute to accumulation of low-level resistant traits in treated fields.

Application of glyphosate should be avoided during periods of sustained high temperatures (day maxima>36 C).

Contact details

Andrew Somervaille
120 Post Office Rd Ravensbourne 4352 QLD
0407 021205
Andrew.somervaille@bigpond.com