NIAB - National Institute of Agricultural Botany

Orson's Oracle

Making water wetter

Posted on 01/03/2013 by Jim Orson

When the wax finish on your car is still good, rain droplets will ‘bead-up’ and not really wet the surface as they are repelled by the wax. The impermeability of the wax on the car’s body and the surface tension in the rain droplets keep the water from ‘wetting’ the surface. Surface tension can be described as the contractive tendency of the surface of a liquid.

The same can be said for pesticides, particularly those that are water based such as glyphosate and the hormone herbicides.  Water has a higher water surface tension than the oily/waxy layers of the plant surface. Hence, it seems logical that any surfactant (surface acting agents) that reduces surface tension and so enables the spray droplet to spread will improve herbicide performance. Sadly it isn’t as simple as that.  

Reducing surface tension to as low as possible can result in droplets running off the leaf as the droplets coalesce (due to very low surface tension). Some surfactants may reduce surface tension to the point where the spray droplet is spread over such a large area that it dries too quickly, reducing uptake of an active ingredient that may need to be in the semi-liquid state to be absorbed.

So, there is a great danger in the assumption that the better the spread of droplets the better the activity of the pesticide. An additional issue is that having too much surfactant in the sprayed liquid can lead to damage to the plant surface, inhibiting herbicide uptake.

As I said last week (Have we got it?), in experiments the really big benefits of adding the surfactant ethoxylated tallow amine are only recorded at doses that are so low as to give unsatisfactory control, even after its addition. 

Research in France suggests that, even when sprayed in water that is typically marginally harder than anything we get from the tap, the benefits of adding this surfactant for the control of volunteer barley are not really apparent until the doses of the glyphosate 360 g/l formulations tested were reduced to 2.0 l/ha applied in a spray volume of 200 l/ha. The same concentration of surfactants from the formulation would be present in 1.0 l/ha product in a spray volume of 100 l/ha but crucially the actual dose/ha is still halved.

Other French research suggests that the addition of ethoxylated tallow amine surfactants to 1.0 l/ha of a 360 g/l product significantly increases spray retention when applied in a total volume of 150 l/ha.

The issue has moved on over recent years. There are some concerns over the use of ethoxylated tallow amines and many of the most recent formulations contain other surfactants. Alternative surfactants to ethoxylated tallow amine include the ethoxylated rapeseed oil surfactants.

However, some recent German, French and American research suggests that great care needs to be taken to get the right form of these surfactants. This research clearly demonstrates the danger of having a spray solution with too low a surface tension i.e. the droplets spreading too much.  

A German laboratory study identified the most biologically effective spray solution produced from mixing the unformulated glyphosate salt (the active substance) with individual ethoxylated rapeseed oil surfactants chosen from a range that differed in their impact on the surface tension of the spray droplet i.e. their level of ethoxylation. The most biologically effective surfactant was one that only marginally increased the spread of the droplet (when compared to the unformulated glyphosate salt) on easy to wet plants (e.g. chickweed and pansy) but did not increase the droplet spread on difficult to wet plants (e.g. fat-hen).   

rapeseed oil

The improved weed control resulted from minimising the glyphosate ‘footprint’ on the plant surface. This created very small but highly concentrated areas covered by glyphosate. The better biological activity may be attributed to a higher concentration gradient between the drying deposit on the outside of the plant and the xylem and phloem inside the plant.

In this German study, the droplets were applied individually and all were retained by the target plant. However, there have been field trials in France and the US which confirm that rapeseed oils with the same level of ethoxylation as those identified in the German study can be effective formulants or spray additives to some existing glyphosate formulations. The key is that the level of ethoxylation of these rapeseed oils has to be high.  The most effective ethoxylated tallow amine additives are also those with a high level of ethoxylation.

So, the assumption that the most effective pesticide formulations are those that result in the maximum spread or coverage of the plant surface is truly blown by this research on glyphosate. I’m sure that the same must apply to some other pesticides, but of course not all pesticides. It’s the same old message; no one adjuvant is a good partner to all pesticides and certainly, in some cases, at least it’s not worth making water much wetter.    

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Have we “got it”?

Posted on 22/02/2013 by Jim Orson

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.

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Choose trusty sentinels

Posted on 15/02/2013 by Jim Orson

“Use careful watch; choose trusty sentinels”

[Richard III to the Duke of Norfolk]

My father was a farmer, county councillor and history nut. The Leicestershire County Council offices were in the middle of the city and he used the designated car-park at Greyfriars. He would have been tickled pink if he’d known that he’d parked his car over the grave of Richard III. Richard IIII’m sure he must have read the biographies but I’m not sure how he viewed Richard; a bad king or a good king. Modern thinking suggests that he was a good king and Shakespeare, either intentionally or unintentionally, got it wrong. It’s interesting how views change over time.

The same could be said about the views on soils and plant nutrition. It’s easy to assume that soil is just there and a medium for various uses, such as growing crops and burying monarchs. However, as everyone now recognises, it is a living and an enormously complex medium; so complex that we still have only an incomplete understanding.

For instance, some soil scientists have held the view that the amount of soil mineral nitrogen measured in the spring has a dominant influence on the dose of nitrogen to apply to an individual wheat crop but examination of the trial databases doesn’t really support this view. The trials database assembled to help guide deliberations on the content of the most recent edition of RB209 (the Fertiliser Manual) shows that for any given value of measured soil mineral nitrogen, the optimum applied dose of nitrogen for feed wheat can vary by a factor of three. There is a long way to go in improving nitrogen recommendations.

The constancy of the presence of soil has also implied that some of the other guidelines for nutrition advice are also solid and unchallengeable. For instance, the advice in RB209 is to maintain the level of plant available phosphate at Olsen P Index 2.  This seems inviolable and so it comes as a shock to discover that this information was derived largely from only two soil types.

As a result there is an HGCA-funded project that is checking whether Index 2 is the correct soil index to maintain for cereals and oilseed rape over a range of soil types. The project has yet to be completed but the results so far generally suggest that this is correct. However, there have been higher yields on plots at Index 3 in trials on sites where poor soil structure may have inhibited the ability of the crops to scavenge for phosphate.

There is one soil type in the project that isn’t behaving like the others. It’s a Cotswold’s brash soil, where it hasn’t been possible to maintain the high soil indices that were created on the other soil types by using large initial doses of triple superphosphate fertiliser. Here, yields have been maintained by soil indices below 2 but there hasn’t been, so far, responses to the application of fresh phosphate at such low indices. This is probably due to the extremely high levels of calcium in the soil making the phosphate less available to plants as measured by the Olsen P technique used in the laboratories. So after some untangling of the results there may be some specific guidelines for calcareous soils in the next edition of RB209. 

The results of this project confirm that phosphate supply to the soil is not a straightforward process. Freshly applied phosphate remains available to plants for a few months but gradually much of it becomes, at least temporarily, locked up in ‘pools’ that have low availability to plants. However, in most UK well structured soils, at Index 2 the amount in the plant available ‘pools’ is sufficient for unrestricted crop growth even if fresh phosphate is not applied.

This is not the same the world over. In many, if not most Australian soils they have to apply some phosphate on an annual basis because that applied to previous crops is not sufficiently available.

This reminds me of some old correspondence I read when I was working in Australia last year. The letters were between a woman who had married and emigrated to farm in Western Australia in the 1920s and her mother in the UK.  It was really heart-breaking stuff about bringing up an increasing number of children whilst initially living in a tent and then in a very small house (hut).  Added to that was the pain of homesickness (no Skype in those days), the hardships resulting from what nature can throw at you in Australia and the land rights battles with neighbours. However, there was a happy ending of sorts. She came back for a prolonged visit to the UK in 1951 and her son wrote to her whilst she was here. He said that he had been to a local field day and a soil scientist had told him all about the benefits of annual applications of phosphate. This, according to his letter, would result in a golden future for the family. I think you have to be a true romantic to believe that!

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Damage limitation

Posted on 07/02/2013 by Jim Orson

I know the precise date when I first saw a television picture. It was 24th March 1951. How do you attribute this amazing display of memory? As is often the case, it is by association with a particular event. Our brand new 12 inch set was first switched on during the 1951 boat race and we witnessed Oxford sinking.

Another event from years ago that I remember seeing on TV was the great flood in Essex. This occurred precisely 60 years ago and there was a huge loss of life. It is surprising that those living in vulnerable properties were not warned to move out. I say this because I used to advise in North East Essex and a local farmer told me that he was in Harwich at low tide that fateful day and the water was already level with the top of the quay. However, I am experienced enough not to judge others on today’s standards. Hence, I shall not be apologising for recommending isoproturon for black-grass control in the past despite today’s knowledge about its possible impact on water biodiversity.

There were hundreds of acres of farmland inundated by the sea. In Essex, this was the old National Agricultural Advisory Service’s finest hour. They supervised the application of gypsum to overcome the impact of the salt on soil structure. They worked long hours and became skilled not only in getting the right quantity of gypsum to the right location but also in making sure that application was to the highest standard. Within a handful of years, soil structure was restored.

The muddy aftermath of the battle of Passchendaele’
The muddy aftermath of the battle of Passchendaele

Of course soil structure can be badly affected in other ways. A good example is sugar beet harvesting this winter. There are fields out there that look like the photos of the aftermath of the battle of Passchendaele; a muddy and pot-holed landscape. In this case, the pot-holes have been caused by tyres and the even deeper ones by digging out vehicles.

The debate is what to do with these fields. How do you repair the damage? Perhaps the right question is what do we have to do in order to assist nature in overcoming the problem as quickly as possible.

There have been a series of meetings recently, sponsored by HGCA and Catchment Sensitive Farming, on repairing soil structure. The talks featured more general drainage and machinery issues and not the specific issue of the extreme damage done to some fields because of late harvesting of root crops. However, I was able to chat about this with the speakers after one of the events.

The context of the discussion was of course to find the most cost effective way of doing this over the next couple of years. Naturally, the ideal solution is to wait for the soil to dry out (assuming that we are bound to have a long dry spell at some stage this summer) and then to move the compacted layers as little as possible to ensure that nature can take its course. The latter will entail establishing a cover crop.

However, this will mean no crop in 2013. Hence, in discussion, a more pragmatic solution was agreed upon; an approach that many farmers have adopted in previous years. The first thing to accept is that leaving such an uneven surface will mean the lowest points (the ruts) will take for ever to dry out. Hence, despite all the potential downsides of running even more tyres (or hopefully tracks?) across the field, the land should be cultivated, just deep enough to lift the shallow ruts, as soon as possible in order to level the soil surface. Once it becomes sufficiently dry, a crop should be established after using shallow cultivations to prepare a seedbed. However, in the most extreme cases of soil damage it may perhaps be more judicious to sow a cover crop (e.g. mustard, fast growing grass) instead, particularly if the soil cannot be moved until very late spring.

Obviously the resulting crop is not going to be great but at least there will be something to sell and its roots will start doing the job of restoring soil structure. The crop choice is probably limited to cereals in most cases. After harvest, provided the soil is sufficiently dry, then deeper cultivations or sub-soiling or moling may be required to help restore structure. The decision on the depth of these cultivations will depend on the observations made after digging a few holes during the summer or immediately after harvest.

Observations of the aftermath of the wet winter of 2000/2001 would suggest that some soils will take perhaps a good two crop years before they are back to where they were before the damage caused by the late harvesting of root crops this winter. This is part of the inevitable cost of root crops and one that should never be discounted when doing crop budgets. The salutary fact is that many experts are saying that we will have to expect more extreme weather events in the future. So it is worth observing and recording the results of trying to overcome the problems caused by the wet root crop harvest of 2012.

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I am not App.y

Posted on 30/01/2013 by Jim Orson

The relatively warmer weather tempted me out this morning to glyphosate the extension to my allotment. It is currently lawn and killing the grass before digging will save an awful lot of summer weeding. The App. on my phone suggested that I had seven hours before it rained but it badly let me down as it rained only four hours after spraying, so I didn’t get the desired six dry hours. On the other hand, typical of most gardeners, goodness knows what dose I used but it will have been way above that used by farmers.Cambridge weather app

Getting rain when it’s not expected is typical of a wet spell. The opposite is also true; the expected rain never arrives in a dry spell. During the very dry three months of spring 2011, my App. always promised rain in four days time! As they say in Norfolk “it never rains in a dry time”.

The continuous wet weather reminds me of the winter 2000/2001 when we also had huge amounts of rain.  I also recall the repercussions of that wet winter.

The feature of spring 2001 was widespread sulphur deficiency. This may have been due just as much to the dry spring that year as to the wet winter. It occurred in situations where it hadn’t been seen before. 

Luckily, or wisely, we took the decision to apply sulphur to all our trials. This was despite the fact that previously, we only treated the oilseed rape trials with this nutrient. The only trial we didn’t treat was one investigating plant nutrients in winter barley where the application of sulphur would have compromised a couple of treatments. However, we should’ve applied sulphur to it and accepted that we would not get all the information we originally intended. The sulphur deficiency was so bad that the yields from all the treatments were all over the place and we got no useful information at all.

The results of our nitrogen dose experiments in winter wheat were very interesting in 2001. The results suggested that higher optimum doses were not required despite the additional nitrogen leaching and denitrification that must have occurred over that winter. The levels of soil mineral nitrogen in the spring were on the low side but not exceptionally low. Again it shows that soil nitrogen supply is not a great guide to choosing what dose of nitrogen to apply.

The difference between the very wet winters of 2000/2001 and 2012/2013 is that the latter also followed a very wet summer. Waterlogging last May and June resulted in some areas losing a lot of nitrogen to denitrification.  Will this mean that we shouldn’t be comparing the experiences of 2001 with applying nitrogen this spring? I’m not sure and I suspect nobody knows but we will always be wiser once we see the trial results from this year!

No doubt there will be a lot of pressure to apply all kinds of trace element treatments to crops this coming season. The usual argument will be used, “why spoil the ship because of a halfpenny of tar?”. The experience of 2001 suggests that there will be no increased risk of micronutrient deficiency, so buyers beware!

Well, it has stopped raining. There wasn’t a great amount and my glyphosate application may not have been wasted after all. I say this in the knowledge that there was research at the Weed Research Organisation that concluded that the activity of glyphosate was actually increased by 0.5 mm of rain shortly after application. Perhaps I can forgive my App.?

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