NIAB - National Institute of Agricultural Botany

Orson's Oracle

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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The cost of less reliable technology

Posted on 28/05/2015 by Jim Orson

Black-grass heads are now appearing above wheat crops. Perhaps it is too early to estimate the levels of control achieved this year but it is getting very close to the end of the window for patch spraying with glyphosate in order to prevent the setting of viable seed. In my opinion, based on field experience and limited experimentation, some of the final dates for this operation suggested in the press are far too late. It is also important to note the downside of this approach. The crop in the patches is also killed and so there is no green matter keeping the soil dry if there are meaningful summer rains. This can lead to complications in post harvest cultivations.

As the ability to control weeds with herbicides reduces, the need to be pro-active and adopt other means of control increases. No opportunity should be ignored. This spring, I noted a large field of oilseed rape where there was charlock in a small area only. It might have taken a couple of hours to hand pull the plants and put them in a bag but it would perhaps have prevented a costly problem in the future. Similarly, I have seen very small patches of black-grass which would take only a few minutes to hand pull but have been ignored. This type of activity goes against the grain for many who have become accustomed to large scale control using herbicides but the current reality means such opportunities have to be taken.

Whilst singing the praises of the opportunist hand-pulling of weeds, it is important to remember that although it is assumed that it is 100% effective, the reality is that the outcome can be disappointing unless it is done diligently. In the days of hand-roguing wild-oats, it was estimated that it was equivalent to reducing seed shed by 85% and was far more effective if the fields were re-walked a few days later.

SeptoriaThere is now the challenge of increased resistance of septoria to the triazoles. This results in all kinds of issues, including less reliance on the eradicant properties of fungicides and adopting programmes that delay resistance to the SDHIs for as long as possible. It also creates the dilemma of how much fungicide to use and also making sure that the time gap between applications is not too long. Currently, farmers are not holding back on fungicide use despite the low price of wheat. On the other hand, there are seed houses claiming that some varieties require less fungicide.

When the triazoles were providing effective control of septoria, there was a real opportunity to exploit the value of good varietal resistance through lower doses. The more resistant varieties only required two thirds of the dose normally applied to the most susceptible varieties. I suspect that in most cases this potential saving was ignored on the grounds of keeping it (too) simple or the “just in case” principle.

Nowadays, I suspect that the reason for using the more disease resistant varieties is largely to do with security of production in a difficult spraying season rather than a significant saving on inputs. There is a big financial risk to ‘getting it wrong’ when planning fungicide programmes. In high disease years, the yield penalties from using lower input programmes can be very significant compared with small potential savings in low disease years which are difficult to predict. I am sure NIAB TAG members are well aware of the options through the information they receive.

As always, it is well worth farmers and advisers keeping an eye out for all opportunities to save money in the short and/or medium term. This means being well informed and occasionally going back to practices that were commonplace a few decades ago.

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