Co-product, waste, or valuable commodity? Straw bale availability for building houses, now and in the future

At this year’s Big Straw Bale Gathering, there was much talk about straw price, how difficult it was to get bales, and what it might be like in the future. In order to understand more about this, here I will summarise what the market looks like at the moment in terms of quantities of straw produced, end user demands, and how these might change.

How much do we grow?

The main cereal crops grown in the UK are wheat and barley, with some additional straw from oil seed rape, and some other more minor cereals (e.g. oats). Accurate annual data on hectares of arable crops are available from Defra here. Calculating yields based on those areas is a bit more complicated, but according to this report, UK straw production is around 12 million tonnes per year (NB, whilst this report is the most recent detailed assessment for the UK, it is relatively old, having been published in 2008).

12 million tonnes seems like a lot, but it’s important to note that straw is a highly regional commodity; there is very little arable agriculture in Wales and Scotland compared to England, and consequently very little straw; 2008 estimates for wheat alone were England (5.92 million tonnes), Wales (45,600 tonnes) and Scotland (359,600 tonnes). Even within England, straw production is highly variable; the East of England produces 1.6 million tonnes of wheat straw, with the North West producing just 105,000 tonnes.

What are the end uses?

The main end uses for straw are:

  • Animal feed and animal bedding
  • Burning for heat and electricity production
  • Soil incorporation
  • Minor uses (i.e. less than 100,000 tonnes/year) include straw bale building, mushroom farming.

Animal feed and bedding

The highly regional nature of the straw resource is compounded by almost opposite and highly regional demands; agriculture in areas with little cereal production is largely livestock based, and therefore has high demands for straw for animal bedding (largely cattle and sheep, but pigs are also bedded on straw). In addition to bedding, oat and barley straw is widely fed to cattle as a source of roughage. As much of this straw use is very locally based, national figures are both difficult to come by and potentially misleading. In the 2008 assessment referred to earlier, regional straw balances were calculated after taking livestock use into account (i.e. local production minus local use for livestock). These showed significant straw deficits in Wales, the South West and Scotland, with surpluses elsewhere.

straw balance
Net straw availability in regions of the UK, after deductions for use as animal bedding. From Copeland & Turley (2008).

As we can see, there is a large transfer of straw from East to West, and a smaller transfer from England to Scotland. We can also see that there is a lot of straw left over once animal bedding needs have been taken into account. This study estimated that a total of 6.3 million tonnes of straw was used for livestock purposes. Whilst not directly relevant to the current calculations, it is worth noting that increasing the amount of straw bedding under animals can dramatically reduce ammonia emissions; if straw price increases and farmers try to use it more sparingly as a result, the CO2 impact of livestock agriculture also increases.

Burning

Around 1 million tonnes of straw per year is now being burnt in straw-fired power stations. These burn bales to produce electricity, which goes into the National Grid. There are four of these, all in the east of England; if you are burning around 250,000 tonnes of straw per year, you are obviously going to pick a site that minimises transport distances and haulage costs. There are also an increasing number of small straw boilers on farms and industrial units across the UK (burning approximately 70,000 tonnes a year for local heat and electricity production according to an FoI request made by Crops for Energy). Again, the locations of these small boilers are closely related to straw availability. Whether or not burning straw for heat and/or electricity will expand a lot in the next few decades is difficult to know; it’s highly dependent on the availability or otherwise of low carbon incentive schemes, and for small scale applications there are relatively few types of boiler available than for woody biomass crops. It may well be a good option for arable farms that can use the heat and power on site.

straw burn
Large scale straw fired power stations in the UK. Data on tonnages and capacity from NNFCC (the National Non-Food Crops Centre).

Soil incorporation

Significant quantities of straw are ploughed back into soil by farmers. In practice, the decision whether to swath and then bale, versus chop and plough in, is made on the basis of weather conditions (which have a massive impact on farming logistics and whether the farmer is ahead or behind schedule with their seasonal activities) and logistics/pricing factors (a farmer will be more inclined to swath and bale if the straw price is high). The environmental benefits of soil incorporation have been widely studied, reviewed here. In essence, whilst there is much practical/anecdotal evidence that ploughing straw back into soil improves workability, maintains soil nutrients and decreases risk of compaction, and one would expect it also to increase soil carbon, the scientific evidence is less conclusive. Incorporating straw will improve soil Magnesium, Potassium and phosphorous balances, but if the soil is not deficient in these minerals, then there is no additional benefit from incorporating straw. However, despite the environmental benefits being relatively small in most circumstances, the choice of whether or not to incorporate is likely to continue to be dominated by the farming calendar, logistics and habit.

The quantities incorporated are difficult to determine accurately, but the numbers of farmers choosing this approach have been studied periodically via surveys. In 2013, whilst investigating the potential straw availability for turning into second generation biofuels, Glithero et al. asked 249 large arable farmers what their straw harvesting practice was, and the data is shown below.

glithero wheat
Results of a survey by Glithero (2013) on wheat straw management practices. On 36% of the land area surveyed, wheat straw was incorporated into the soil. Many farmers don’t bale straw themselves, relying instead on contractors (i.e. they sell the straw in the swath). N.B. the journal paper also looks at practices for barley straw and oil seed rape straw, which are markedly different.

Townsend (2018) in another survey of farmer practices found 54.6% of wheat straw was incorporated into soil, 8.7% was baled for on-farm use and 33.6% was baled and sold.

How much straw is left over after other uses have been taken into account?

Taking into account the production and uses discussed above, the current straw demands in the UK are as follows.

straw usage

From the above it is obvious that significant tonnages of straw are potentially available (perhaps 2 million tonnes per year if we assume that half of straw incorporated into soil is incorporated for nutrient management reasons). So there isn’t really a theoretical limit on straw bale availability.

Spare straw and the nature of surplus

An obvious question arises; what does surplus look like? Is there really any spare straw? Academics analyzing straw production and use in recent years have mostly done it in the context of the quantities that would potentially be available for renewable energy generation (e.g Glithero et al. 2013a, Glithero et al. 2013b, Whittaker et al. 2014, Townsend et al. 2018). Their assumption is that all (or most) straw that farmers plough into soil can be regarded as surplus or ‘waste’. But these surveys (e.g. Townsend et al. 2018) have also shown that many of the same farmers have little or no interest in selling excess straw to other industries; they are incorporating straw into soil by choice, with 43% (based on land area surveyed) saying they would continue to incorporate rather than sell for bioenergy. Quantifying what is genuinely surplus straw is not straightforward.

Bale type and availability – the practical limiting factor for straw bale builders

In practice the biggest limitation is not the total tonnages available, it’s the format that it’s baled in. Bales used by straw bale builders are around 45cm wide, 35cm high and a metre long (the conventional ‘small’ bale). However, the vast majority of straw is now baled in either large round bales or large square bales (approximately 500kg), with both approaches being massively faster for the farmer to harvest than conventional small bales (approximately 25 tonnes/hour for large bales, 6 tonnes/hour for small). Large bales also carry much lower labour costs; they’re easily moved around with tele-handlers which makes them easy to store and transport. Unfortunately I’ve not been able to find data on how the relative proportions of these baling approaches have changed over time; the biggest part of the future straw bale availability picture remains obscure.

 

Future uses for straw

Perhaps the biggest unknown in relation to future straw demand is the likely use for renewable energy generation. This could include bioethanol production (which in turn depends on policy decisions in relation to renewable transport fuels) and anaerobic digestion, neither of which are significant uses at present, and expansion in other uses such as burning and CHP. It’s not difficult to imagine a policy change in any of these industries resulting in an extra 1-2 million tonnes/year of demand. Large scale developments such as this will also affect the market via the types of contracts available. Whilst straw for bedding and small scale use is largely sold via auctions or private sales, ‘on the swath’ (to a contractor who will bale it), or as a standing crop, straw for power stations is likely to be secured via longer term contracts with farmers, in the same way that many other agricultural commodities are sold.

What does all of this mean for the future availability of straw bales for building?

The straw bale building market is small, and the quantities of straw used are a tiny fraction of annual production (e.g. we could assume 300 buildings per year, and 6 tonnes per building). So from a pure tonnage perspective, straw will always be available for those that are willing to pay. More of an issue is the availability of small bales. Again, this is probably a case of them being available for anyone willing to pay a premium. To my mind, there are two potential approaches to mitigate this. The first is to move away from the bale and use chopped straw in some kind of prefabricated panel. This is the approach taken by Ecococon, and others are working on similar types of prefabrication. The second approach is re-processing large bales into small building bales. This would allow the straw bale building industry to have bales made to order, and whilst the unit cost per bale would be higher, it would be a good way of guaranteeing bale availability given how few farmers are doing small bales straight off the field and might also be a good way of ensuring consistent quality. I discussed this briefly in a previous blog post here. A third approach that will remain relatively unusual is to build directly with big square bales; this is the approach taken by Tablehurst Farm, who used bales that were 240 x 90 x 85cm. If land availability isn’t a particular problem (you wouldn’t want walls this thick in an urban environment where space is at a premium), big bales have much to recommend them and are certainly much easier to come by.

Final thoughts

At a UK scale, there is plenty of straw available for the straw bale building industry, the problem is that relatively little is baled as small square bales and that it’s production is highly regional. However, there is a widespread network of straw wholesalers, who whilst mostly trading in large bales, are sufficiently well connected to be able to get small square bales. At a price.

Postscript. Was straw ever really a waste product? In defence of the early straw bale pioneers.

The idea of people thinking of straw as a waste might seem strange now given the prices paid for it, so it’s worth looking at the historical context that gave rise to this view. Until 1993, when straw and stubble burning in fields was banned in the UK, up to 41% of wheat straw in England and Wales was burnt in the field (Silgram & Chambers, 2002), so it is easy to see how straw bale builders in the 80’s and 90’s could be justified in referring to straw as a waste. At the time the ban was implemented, the farming industry was adamant that it would damage farmer’s incomes because they wouldn’t be able to get rid of surplus straw. But 25 years later, the prevailing view is very different. Straw is a commodity for most farmers. It’s certainly a precious commodity for straw bale builders who are paying £3/bale (around £170/tonne), which doesn’t always include delivery. However, given that most buildings are not going to need more than 5-8 tonnes, it’s still a very small proportion of a total build cost.

This post is based on work undertaken as part of the BEACON project, which is funded by the European Regional Development Fund via the Welsh Government. If you’re interested in building with straw, the SBUK website is a useful starting point.

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