NIAB - National Institute of Agricultural Botany

Orson's Oracle

The maths of black-grass control

Posted on 06/08/2015 by Jim Orson

This is my 150th blog and perhaps too many have been on black-grass, the nation’s most talked about weed. You have to believe me when I say that I was trying to avoid writing about this weed yet again but recent press results have driven me back to the keyboard.

The issue that I would like to raise is the too simplistic conclusions made from field data. These can over-estimate or under-estimate the role of cultural or chemical control of the weed. Good field data is not easily achieved and it is the industry’s responsibility to use it intelligently.

I have written before about the huge fuss made a few years ago about increased crop competition from wheat being able to reduce black-grass seed heads by half. Many took this as the answer to the black-grass problem but the maths of the weed’s dynamics told us differently. Models suggest we have to chemically control 97% of black-grass plants emerging in a continuous wheat crop grown in ‘normal circumstances’ in order to contain populations at current levels. This means that we can let only 3 out of 100 plants make it to seed shedding. Crop competition does not reduce black-grass plant numbers but reduces the ability of the plant to set viable tillers. Hence, reducing seed heads per plant by half through crop competition means that we can allow 6 rather than 3 out of 100 plants to make it to seed shedding. So a 50% reduction in seeding per plant reduces the need for chemical control to 94% rather than 97% control. Every reduction in the reliance on chemical control is useful but this is not the game-changer that was originally claimed. Obviously, greater reductions in seed return through competition would reduce the need for chemical control by a greater percentage. However, more than a 90% reduction in black-grass seed set from increased competition will be required to get down to a chemical control requirement of 70% of plants; a level that is now often achieved by pre- and/or early post-emergence herbicides.

This is an example of when a large percentage figure can over-estimate the value of a control technique. In contrast, recent quotes in press reports seem to be under-estimating the benefit of stacking herbicide products to control the weed. Some say that using more than two products in a mix adds little to the percentage control of plants. These statements can undermine the value of using mixtures of three or more products.

The best way to explain what I mean is to give an example. Let us again assume that there are potentially 100 black-grass plants/m2 that will emerge in a winter wheat crop. A two-way mix applied pre-emergence may control 60 of these plants (i.e. 60% control) and a three-way mix may control 70%. This is ‘only’ a 10% increase at face value. This does not sound a lot and some might say it would be better to apply this third component early post-emergence where potentially it may give a higher level of control. The maths of this situation are illuminating.

In this example, the two-way mix will reduce the numbers in the crop from 100 plants/m2 to 40/m2 and adding the third component of the mix will reduce the numbers from 40/m2 to 30/m2. Reducing survivor numbers from 40 to 30 is in fact 25% control, which is a lot higher than the headline figure of an additional 10% control. Put another way: using a two-way mix pre-emergence would let 40 plants/m2 survive and achieving 25% control from an early post-emergence would provide the same result of a final plant number count of 30 plants/m2 (more than 29 too many!).

I realise that this is a rather simplistic view and herbicide resistance will ‘nuance’ the situation. We know little about the impact a particular pre-emergence application will have on the susceptibility of weeds to a particular early post-emergence application. Are the percent controls of plants achieved by individual products simply additive or does an earlier application pre-dispose its survivors to be more or less difficult to control with an early-post emergence application?

What I am saying is that data need to be carefully interpreted. In practice, it may be that stacks of more than two products applied pre-emergence are particularly relevant to later sown winter wheat crops provided the seedbed and the weather are conducive to good control. Simpler mixes may be more suited to pre-emergence or peri-emergence applications to earlier sown winter wheat crops, particularly when the weather and/or seedbed are not suited to good control, with an expectation that a further application of a mix could well be necessary early post-emergence.


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Black-grass – an apology

Posted on 23/07/2015 by Jim Orson

A few weeks back I wrote a blog (Black-grass allelopathy nonsense - posted on 30th April) on my analysis of the very limited and indirect evidence as to whether or not multiple applications of glyphosate to seedling black-grass in ‘false seedbeds’ were beneficial, in terms of reducing numbers in succeeding crops. I came to the conclusion that there was no compelling evidence to support multiple applications.

However, I have now been supplied with evidence from one well conducted trial that specifically investigated this subject and which suggests that multiple applications may well be beneficial and so I have to retract what I said a few weeks back. However, I remain of the view that the possible explanation cannot be allelopathy. Allelopathic exudates from black-grass roots have never been identified.  The most likely explanation is that emerged black-grass plants shade the soil surface restricting further black-grass germination.

There are two further comments to add. Firstly, should a winter crop be sown if there are sufficient black-grass plants emerging in a ‘false seedbed’ to provide enough shade to reduce further germination? Numbers in the crop may well be too high to control adequately with herbicides. Secondly, is the threat of black-grass resistance to glyphosate sufficiently high to avoid multiple applications and to rely more on shallow cultivations to kill some or all of the emerged plants? Nobody knows the answer to that conundrum but black-grass has shown it is a worthy foe and has developed resistance to a range of modes of action.

One method of running down the seedbank of viable black-grass seed is to spring crop. However, in some areas this year, there are horrific numbers of the weed emerged in spring sown crops. I have noted that these instances tend to be in the areas that received the worst of the deluge in 2012 and the soils in that summer would have been waterlogged. This has led me to the conclusion that these fields are not infested with a new spring emerging strain of black-grass, as some suggest, but are continuing to suffer from the impact of exceptional levels of dormancy in the seed set in 2012. I realise the some may think that I am off my head and eventually will have to offer another apology for this observation! However, the HGCA Project Report 498 on dormancy shows that with typical levels of high dormancy, the majority of seed will germinate at least 12 months after shedding. It is only a small step to say that with exceptional levels of dormancy, the germination of a significant proportion of the seed could be delayed for a couple of years.

The other explanation is that there is always a small proportion of black-grass that will germinate in spring crops and so a high spring emergence is a reflection of an exceptionally high soil seedbank. I do not quite buy that one in the cases that I have experienced this year. I have only witnessed such high populations in spring sown crops since the summer of 2012 and I have been around a long time. The inevitable conclusion is that the weather and soil conditions during seed maturation that year had a profound impact on future germination patterns.

I initially got it in the neck from one colleague this spring because he tried a half-field comparison of cultivating/drilling or direct drilling a spring sown cereal. The experience in the industry is that ‘true’ direct drilling will reduce black-grass emergence in spring sown crops because of the lack of soil disturbance. In turns out that in this case there was no difference in black-grass emergence, which was extremely high, between the two cultivation approaches tested. However, where the land was direct drilled the soil soon cracked in the dry conditions and a very high proportion of the black-grass was emerging from the cracks. It shows why agronomists often have to mention a few caveats when they give advice! 

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2015 wheat yields

Posted on 09/07/2015 by Jim Orson

Since 2012, I have been attempting to predict wheat yields in a blog at about this time of year. Looking back, 2012 was perhaps the easiest year to attempt this task. It was relatively clear that the lack of sunshine and the saturated soils would result in lower than average yields. Last year it was also reasonable to think that overall there would be very good yields. Indeed, we had a record UK average of 8.6 t/ha with the East averaging 9.1 t/ha.

This year is much more complicated. Generally, crops over-wintered in good condition and the levels of radiation that establish potential yield have been above average for almost every month this year, including the critical month of June. Night time temperatures were below average in June and consequently the crop did not respire so much overnight, hence retaining more of the day’s gains than average.

However, there have been negatives that may result in much of the potential not being realised.  The heat of last week would have done wheat no favours. Luckily it occurred in perhaps the latter half of grain fill rather than earlier in this process. 200C seems about ideal for wheat, 250C is less than ideal, 300C is harmful and 350C even more harmful. Coupled with this we had high overnight temperatures where the wheat must have been respiring very actively. I cannot get a clear picture of what is the critical night temperature above which respiration is particularly harmful: the scientific literature quotes between 9 and 140C. In many areas last week, the night time temperature over two or three nights barely fell below 200C.

However, I think that the main determinant of how much of the good potential established by above average levels of radiation we harvest this year will be the availability of soil moisture. Much of the rainfall has been patchy and there are large areas of the country where rainfall in March, April and June was way below average. Looking back at the good yielding years of 1984, 2008 and 2014, it seems that May rainfall is very important to exploit wheat’s potential. This year rainfall during May was particularly patchy. The arable areas north of the Wash and also the West Midlands and parts of the South Coast tended to receive reasonable levels of rainfall in May.

This makes me think that in terms of yield we are in for a mixed UK wheat harvest, depending on the soil moisture availability in late May and June. Those areas north of the Wash which had some good May rains perhaps also missed the extremely high temperatures that were experienced further south. It seems to me, purely based on weather records, that those areas that received significantly below average rainfall in May will do well to achieve average wheat yields, except on the most moisture retentive soils. Those areas where the moisture deficits were closer to the average situation in mid-June may achieve more pleasing yields.

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Observing nature

Posted on 24/06/2015 by Jim Orson

We have just returned from a holiday in the sub-alpine region of France. There was a lot of nature to observe and this is an opportunity to share our experiences.

Let me start with the behaviour of that most interesting species, the human being. We stayed in one hotel for five nights and each evening the conduct of some of the species was the same. The dining room overlooked a beautiful lake and the surrounding mountains. There were a limited number of tables by the windows and there was much manoeuvring to try to grab one of those tables. So there were several couples circling close to the door of the dining room when it opened but all seemingly uninterested in entering it. However, once one couple made a move there was a slow motion stampede. To make matters worse, some of these same couples managed to get through a gorgeous four course meal in less than an hour. I assumed they were British because the French would never attempt such a feat.

We spent the days in the mountains. We were not very high, between 500 to 1,500 metres above sea level. Once we climbed the extreme paths behind the hotel, we arrived onto beautiful alpine pastures. There appeared to be no grazing livestock and the pastures were going to be made into hay; they had just started mowing as we returned to the UK. The number of wild flowers in the pastures was amazing and the air was filled with their scent and the buzzing of insects. I thought that this must be the ideal that many green groups would like to achieve in the UK.

But, but, but…… there were no birds flying over these pastures. In addition, there were only a few birds skulking in the many woods that lined the side of the steep climbs. You could hear them but rarely saw them. I kept on wondering why this was so? The local environment seemed ideal for farmland and woodland birds.

One explanation soon became apparent to us. There were a number of raptors gliding on the air currents above the wooded hills. Some seemed nasty pieces of work. I could see why even the birds in the woodland were keeping such a low profile.

The explanation as to why there were no birds over pastures that were buzzing with insect life was less clear to me. Raptors may be one explanation but I was unconvinced that this could be the full explanation. So when we returned to the UK I asked a couple of twitchers.

They immediately provided the explanation. The birds that would be expected to be flying over the pastures hoovering up insects are the migrating species. They, in turn, are now hoovered up by vast nets at the watering holes on their migrating routes through Africa. The twitchers said that they are amazed that any now survive the journey.

I briefly wondered if the French farmers in the region are getting it in the neck for the apparent absence of birds. It would be grossly unfair if they are but green groups seem to always like to conclude that it is the farmers’ fault. In the UK, despite the great efforts made by UK farmers to reverse the decline in farmland birds, the numbers have stubbornly plateaued. For some species there are reasonable explanations as to why, usually relating to the lack of a particular habitat or food source. However, it is important to note that the lack of farmland birds may not always be due solely to the actions of UK farmers: a more honest debate is necessary.


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Arbuscular mycorrhizae – is it all just hype?

Posted on 08/06/2015 by Jim Orson

Perusing the recent agricultural press would suggest that arbuscular mycorrhizae (AM; some say that the plural is spelt mycorrhizas) are the sole basis of soil health. In one article it is argued that oilseed rape is damaging soil structure because it does not form an association with these soil fungi.   

Let us start with a couple of definitions. Mycorrhizae are defined in the web version of the Oxford Dictionary as fungi which grow in association with the roots of a plant in a symbiotic or mildly pathogenic relationship. AM are those mycorrhizae that penetrate the outer layers of plant roots.  These have a symbiotic relationship with plants by helping with their nitrogen and phosphate uptake in exchange for the plant giving them some of the products of photosynthesis. However, under severe stress conditions, these fungi turn into self-survival mode and primarily look after themselves at the expense of the plant. 

The other benefit of AM is that they form a protein structure, described as glomalin, which helps to bind soil particles into aggregates that make the soil easier to cultivate and form resilient seedbeds. 

Some crops potentially have very good associations with AM, such as oats, barley and the legumes. Some crops do not form associations with AM, notably the brassicas including oilseed rape. Hence, the accusation that oilseed rape is damaging to soil structure. 

It is the latter comment, which appears to me to be overplaying the properties of AM and prompted me to contact Professor Penny Hirsch of Rothamsted Research. This was also partly because I remember being told at university that a high soil status of nitrates and/or phosphate restricts the abundance of AM.  

Prof. Hirsch confirmed that a high nitrogen and/or phosphate status of the soil restricts the abundance of AM and she made the intriguing comment that modern wheats have largely lost the ability to benefit from AM compared to wheats bred before the 1950s. Perhaps the ability to form an association with and potentially benefit from AM has been lost because modern breeding programmes have been carried out in a background of high levels of nitrogen use. 

So the wheat and oilseed rape rotations using high levels of nitrogen and with good phosphate supply are perhaps not the best for encouraging the abundance of AM. Despite this, a thirty year plus trial at NIAB TAG Morley is showing, in almost continuous wheat, that current rates of nitrogen produces more dry matter production of straw and presumably roots when compared to zero nitrogen application, leading to a slight increase in soil organic matter. However, despite a small shift in soil organic matter, there is a significant improvement in soil aggregate stability where current rates of nitrogen have been used. This perhaps demonstrates that other soil microflora, which also produce glomalin, have responded to the higher annual incorporation of straw and roots.  

It is widely reported in the scientific literature that high levels of AM are beneficial to plant nutrition only where the soil supply of nitrates and/or phosphate is limited. In our conventional systems the supply of these two nutrients is not limited and so the need for assistance from AM is less important.

It seems to me that whilst high levels of soil AM have theoretical advantages, the way we fertilise our crops in conventional agriculture largely or totally negates their ability to improve plant nutrition. The claimed advantage that they produce glomalin is also largely or totally neutralised by the fact that other microflora also form this protein structure that binds soil aggregates. I am an enthusiast for the regular application of organic material to the soil to increase soil microbial biomass but the current enthusiasm specifically to encourage AM seems to me to be overblown.

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