Have we “got it”?

22 Feb 2013

Thirty years ago I was working as the ADAS liaison officer at the Weed Research Organisation (WRO).  This was one of the first research institutes to be closed; perhaps one reason for that may have been because by that point there was an effective range of selective herbicides in annual crops and, of course, glyphosate was the answer to perennial weeds.  

Prior to glyphosate there was a huge research effort on the cultural control of perennial weeds, notably common couch. So perhaps there may be good cause to revive this organisation if herbicide resistance continues to develop and product withdrawals carry on at the current rate?

One of my duties was to answer phone queries and there was a huge interest in the research being carried out on the use of adjuvants to reduce the dose of Roundup for perennial weeds.  At that time Roundup was priced at a level for the control of perennial weeds and it was accepted that annual weeds would be controlled by paraquat, with or without diquat.   

The price of Roundup in the early 1980s was £12.50/litre, which now equates to around £40.00/litre, so you can appreciate the interest in lowering the dose.  Mind you, the wheat price at the time was about £120/t which is around £375/t in today’s money.

The research at the WRO was partly taken out of context as the adjuvant additives were very significantly increasing the control of couch only at relatively low doses of glyphosate. As doses were increased in order to achieve an effective level of control, with or without the addition of adjuvants, the benefits of their addition became marginal.

I gave a paper at one of the much fabled BCPC Brighton Conferences saying as much and pointing out that in good growing conditions, the effective dose of Roundup could be much reduced even without the additional adjuvants.  Shortly afterwards it became clear that using a lower volume of application often increased the efficacy of low doses, probably due to increasing the concentration in the sprayed solution of the adjuvants that were already in the product formulation.

However, the phone calls kept coming. In those days recommending lower than label doses was viewed as heretical by the industry and I had a difficult path to follow. Nowadays, appropriate doses are commonly used.

There is still an interest in adding adjuvants despite the limited evidence of a significant economic advantage from their adoption and the current low price of glyphosate products. The two originally identified in WRO research were ammonium sulphate and a surfactant (surface acting agent) - ethoxylated amine surfactant. The original research suggested that both should be used with Roundup.

About 10 years ago there was a paper published on the subject by an INRA researcher in France. He confirmed that adjuvant additives were more effective in increasing performance when the dose of a glyphosate product, with or without the additional adjuvants, was so low as to give only about 50% control (of barley in this case). 

He added the same adjuvants as identified by the WRO and concluded that it was only worth considering using ammonium sulphate if the water was very hard; i.e. contained more than 200 parts per million (ppm or mg/l) calcium.

In less hard water he recorded an advantage from using ethoxylated tallow amine as an additive but it should be noted that he was applying the glyphosate product in a total volume of 200 l/ha. It may be that at lower volumes this advantage would not have been recorded. Finally, the WRO results that adding both adjuvants may give superior control was not confirmed, even where the water was very hard.

The reason for ammonium sulphate only increasing the performance of glyphosate in very hard water is clear.

The herbicidal part of any glyphosate preparation is glyphosate acid but the material is always formulated as a salt because the acid is practically insoluble in water. This is why the glyphosate content of a formulation is declared as acid equivalent (ae) rather than active ingredient (ai). Calcium and magnesium salts in hard water combine with the glyphosate salts and if present in sufficient quantities can effectively block some herbicidal activity. The addition of ammonium sulphate results in the calcium and magnesium binding instead with its sulphate ions, resulting in an effectively higher available dose of glyphosate.

There are few if any tap water sources that have a calcium content above 200 ppm (mg/l) but there may be some other water sources that could exceed this threshold. Your local water company will have a postcode search facility for the calcium content of tap water for your area. This also takes into account the calcium equivalent content of magnesium. Beware that I’m talking about calcium content and not calcium carbonate content, which is 2.5 times more.

So is there an argument to add ethoxylated tallow amines to some formulations? These surfactants are similar to those already in many glyphosate formulations. It is the old story, if there was a simple solution to adding these adjuvants to glyphosate products then we would have “got it” by now. More on this next week.