Reinventing the wheel

What have we learnt about black-grass control in the last 100 years?

Over the last twenty years, no weed has been discussed as much as black-grass. But after countless articles, technical presentations and trials on the best control methods, it’s still here, why does it remain so difficult to control?

Lack of information is not the cause. Good knowledge of its agroecology and cultural control techniques has been available for at least 50 years. The guidance in the 1975 MAFF Advisory Leaflet 522 ‘Black-Grass’ hasn’t been superseded, just added to, for example:

“Black-grass infestations are generally worst on badly drained heavy soils, where autumn sown crops are grown frequently. But the weed is often a problem on heavy soils even when the drainage is good.”

And:

“Early drilling often leads to more severe infestations not only because seed preparation coincides with the main flush of black-grass germination but also because the black-grass in these crops will be fully tillered and better able to survive the winter.”

And:

“Black-grass is more frequently serious where there is a high proportion of winter crops and if the crop is suffering from waterlogging or other adverse growing conditions the black-grass can become dominant. In these circumstances yield restrictions of well over 1 tonne of grain per ha of winter wheat are not infrequent.”

Further back in time, there was less information about black-grass, but it wasn’t considered as much of a threat. Winnifred Brenchley’s 1920 book ‘Weeds of Farmland (1920)’ is considered one of the first scientific account of weeds in the UK, it says black-grass

“… is capable of causing much damage in corn crops, but happily does not usually occur in any quantity, its habit being that ordinary methods of farming and rotation keep it in check.”

Ordinary farming methods no longer easily keep it check. Of course, ordinary farming is very different in the 2020s compared to the 1920’s.

“Black-grass has the same basic biology, but it is incorrect to say that our knowledge

hasn’t moved on since the mid-20th century,” says James Clarke, Research Director at ADAS. “It is the most researched weed in the UK, and we have learnt how black-grass has responded to changes in the farming system.”

He points out that our understanding of short-term dormancy has greatly improved as has knowledge of genetics and resistance. But it is fundamentally the same weed, so it will respond to the same management techniques discussed several decades ago. Herbicide resistance hasn’t substantially changed its emergence patterns or its response to cultural interactions.

James Clarke

Wheat boom

“In the 1960s and 70s we already had a good knowledge base for weed control. The 3 Hs – horsepower, herbicides and hydrology (drainage) tilted the balance and allowed us more flexibility to stretch biology in our favour. It increased the ability to grow autumn-sown wheat despite weeds – they were controllable – hence in the 2023 BCPC survey, the 1980s was considered the ‘easiest’ decade for weed control.”

The effect of ‘easy’ weed control was that fewer spring crops were grown, and drilling dates moved into September to improve yield and simplify workloads. Given what we now know about black-grass and other weeds, it is not surprising that this system was not sustainable for the long-term, especially without herbicides being as available or effective.

During this time, the area of wheat nearly doubled, and replaced barley as the main cereal crop. Improvements in varietal potential saw a doubling of average wheat yields between 1975–95. However, since then average yields have plateaued.

“Economics encouraged farmers to grow large areas of wheat, first of all to maximise profit and in recent years it has been one of the few reliable bets for a good gross margin. Financial drivers haven’t gone away, in fact it they are probably stronger than ever as the cost of production has increased (fertiliser, fuel and labour).”

The boom in wheat area coincided with a boom in herbicides. The number of actives available grew in the 1980s and early 90s, plateaued in the 00s and is now slowly declining. In addition, there is resistance to some actives in some weed species. However, Mr Clarke points out that chemistry is still very important for weed management.

“Weeds like wild oats and couch grass used to greatly influence rotation, now, as long as current control options remain effective, in most instances they are a nuisance rather than a major threat.”

“But to underline the main point, it is not that we’ve forgotten what we knew about black-grass, but the technical and economic situation has changed. Grass-weed problems are a by-product of growing tight rotations with frequent winter cereals.”

Change the rotation?

If the underlying cause is tight rotations, then the solution appears to be simple, more diversity in the rotation.

During the 2010’s, many farms with black-grass problems grew more spring crops, alongside widespread use of delayed drilling of winter wheat and effective pre-em chemistry. This was helped by six consecutive ‘good’ autumns from 2013–2018. A sequence which was rudely brought to a halt by 2019. Although, even 2019 forced many farmers to spring crop or fallow which helped weed control. In general, there was a sense that black-grass can be controlled but it requires time, attention to detail and market conditions that make lower margin break crops viable.

This is borne out by the Rothamsted BGRI initiative which recorded a moderate decline in black-grass abundance from 2014–21 on the 160 farms being monitored.

But the last two years have seen frequent bad weather and difficult market conditions. This has have made it harder to employ the cultural control techniques which can reduce black-grass numbers. There has been a tendency to drill earlier and maximise wheat area in order to achieve the best gross margin. Neither help black-grass control.

Financial pressures and limited break crop options are not going away, so wheat will almost certainly retain its dominant place in the rotation. Black-grass prone land will continue to have grass-weed problems because heavier soils tend to be good for wheat yields, and crucially, less suitable for other crops. So, there isn’t a simple rotational fix for controlling black-grass.

Source: Rothamsted

Looking forward and back

Having worked in weed research since the 1970s, Stephen Moss has seen this story first-hand. On the brink of fully retiring, he remains very interested in the question of how black-grass became a problem and what to do about it.

In his 2017 article: ‘Black-grass: why has this weed become such a problem’ he identified more autumn sown crops, earlier sowing, minimum-tillage instead of ploughing and herbicide resistance as the underlying reasons for black-grass problems.

Stephen Moss

But, with the candour that retirement can bring he questions some of the decisions made over the last 50 years. “Was a rotation of alternate winter wheat and oilseed rape ever a good idea? Was it right to drill earlier, use lower seed rates, spray faster, use lower volumes, not apply fertiliser at drilling and move away from mixed farming? Or was it simply the right thing to do at the time?

“This might seem like being wise after the event, but the lesson is surely to scrutinise what farmers, agronomist and researchers are doing now and question everything more critically. For example, is rotational ploughing really a bad option now? It may be anathema in regenerative agriculture circles but I suspect it may well be considered ‘good practice’ again in the future.”

He points out that these changes came about fundamentally to increase yield. But after the leaps in yield of the 1970s and 80s. Wheat and oilseed rape yields have been more-or-less static this century.

“This is not a novel observation, but really does need addressing. Looking ahead, farmers may need to cope with a situation where mean yields are static but costs ever-increasing.”

Add herbicide resistance and the possible loss of actives into the equation too and it seems that grass-weed control will become harder in the next decade. More optimistically, the experience of the last decade has shown that integrated weed management strategies are effective in the field and not just in a research paper. But they require investment, in time and in money, to succeed.

“How you deploy the knowledge we have is the difficult bit,” says James Clarke. “You have to think spatially and temporally, both prior and future years when considering weed pressure. There are several ‘nuggets’ of information that have to be pulled together. But most important of all, the farmer and agronomist have to juggle a lot of decisions.”

Rethinking herbicide resistance

The theory about how herbicides select for resistance is well-understood, but Stephen Moss thinks the rapidity of field scale resistance appearing to herbicides such as mesosulfuron is worth looking at again. “Low level selection by herbicides which have marginal effects on specific weeds has not been investigated enough worldwide. For example, if a broad-leaved weed herbicide, such as metsulfuron, kills 20% of black-grass and is applied every year for ten years you may well be getting an appreciable selection for resistance with a herbicide that no one uses for grassweed control.”

“You may have increased the frequency of resistance from 0.000001% to 1% of the population, which would not be noticed. Use Atlantis and wham, selection pressure is much higher and resistance at a field scale appears within a couple of years.”

Notes on naming

In some older literature, different names have been used for black-grass. Alopecurus agrestis was an alternative botanical name that was superseded by Alopecurus myosuroides. Black bent was an alternative name for black-grass, nowadays, black bent is thought of as Agrostis gigantea which is a type of couch.

Source: UK Government Statistics

Source: UK Government Statistics