According to the Irish Times (2019) the Minister of State for Natural Resources Seán Canney believes that ‘Transforming Ireland’s State-owned bogs into vast cannabis farms could create a lot of jobs.’ Considering growing hemp as a fuel and medicinal crop makes initial sense, due to its ability to grow quickly in difficult conditions (Kreuger et al, 2011) and the opportunity that medicinal cannabis “has the capacity to create a decent amount of jobs” (Irish Times, 2019). Bord na Móna have identified opportunities in herb and plant cultivation as part of their 2019 report ‘From Brown to Green’ for both electricity production and the herbal medicine and health food markets. They are currently trialling the cultivation of a number of indigenous herb varieties and naturally growing plants on the bogs of Co. Galway and Co. Offaly. In the selection and consideration of potential crops Bord na Móna have stressed both ‘indigenous’ and ‘naturally growing.’ Therefore, if hemp crops are under consideration, then crops that Ireland has a history of producing with great success such as rhubarb and barley should also be on the list. They are hardy, particularly rhubarb, and yield not only far more finished crop per hectare but can also produce greater yields of Biogas than hemp (AD UK, 2020) (Schrader, 2000).

This paper considers the opportunities of growing rhubarb (Rheum S.) on the peatlands. The Rheum species, not to be confused with Gunnera Tinctoria or Petasites Hybridus, are perennials, requiring little care and, additionally, have been shown that they can be used in human and animal feed, as a methane suppressant in dairy cattle, as an anti-inflammatory and for a number of other medicinal purposes (Neyrinck et al, 2017) (Bajic et al, 2016).

Current Status of the Market

Bord na Móna is going through a radical change as, by 2030, 70% of Irish electricity must come from renewable resources (Climate Action Plan, 2019). They have ambitious targets for the future. They aim to have 5,000 ha of their 80,000 ha converted to use for sustainable products by 2025 and to produce 3TWh of renewable electricity by 2030. They have over €10 million to spend on research and development in order to become Ireland’s leading renewable energy generator by 2030 and are seeking to sign commercial agreements with the ESB for the supply of biomass to the Lough Ree and West Offaly power stations. As they transition, they indicate in their report that ‘indigenous material will be supplemented by independently validated, sustainable, ethical, low-carbon at point-of-use biomass.’

The government in their ‘Statement of Strategy 2019 to 2021’ have committed to ‘support the transition of peat power plants to greater amounts of biomass, and to work with industry to develop a sustainable indigenous supply chain.’ The Departments for Agriculture and Energy have committed to ‘explore the potential of energy crops, which can be grown in the vicinity of Bord na Móna plants, through a national bioenergy strategy, recognising Bord na Móna’s strategy of moving away from peat.’  

It is clear both Bord na Móna and the government are committed to the growing of biomass on the peatlands as a future energy source. However, it is unclear what types of Biomass they intend to grow as both indicate that they are only at the exploratory stage.

 

Selected Growth Opportunity Area

In order to select the Biomass for peatland areas the relatively new practice of paludiculture in this instance should be considered in circumstances where the peatland is suitable for restoration (Wilson et al. 2012). This practice combines the reduction of greenhouse gas emissions from drained peatlands through rewetting with continued land use and biomass production under wet conditions (Grootjans, A.P., 2017). It is the practice of producing biomass for fuel production, food, product materials, and medicines and simultaneously sustaining biodiversity. Paludiculture uses 80% to 90% of the primary production that is not required for peat formation (Don, A. et al. 2012).  

If the degenerated peatland surfaces are converted with the correct type of crops into agriculture sites they may then constitute a habitat for rare species. This would also then be compatible with reducing the emission of CO2 from organic soils or, in this case, restoring the carbon sequestering capacity (Chapman et al, 2003). In relation to Ireland, in particular, Wilson et al. (2012) observed that ‘when stable hydrological conditions are achieved through rewetting of industrially extracted sites and where vegetation recolonisation is successful, it leads to short‐term reductions in CO2 emissions and could increase carbon savings by promoting new carbon sequestration.’ Perennial energy crops also provide significant moderation possibilities for N2O emissions due to decreased Nitrogen fertilisation demand and higher efficiency use (Don, A. et al. 2012).  

Trialling potential crops in a simulated paludiculture in order to measure greenhouse gas balances and to assess the suitability of potential biomass, Gunther et al. (2015) conducted measurements of carbon dioxide, methane, and nitrous oxide trade-offs in potential crops for two consecutive years. Their results showed that the Carbon dioxide exchange was almost neutral with Nitrous oxide emissions lying below a detectable limit and net greenhouse gas balances close to climate neutral. They also did not find any noteworthy short‐term effect of biomass harvesting on net greenhouse gas balances.  

Studies have shown that the viability of the future production of energy only crops depends on achieving sufficient improvements in productivity and yield from the selection, reproduction and control for growth on poor waterlogged land with low fertility soils (Don, A. et al. 2012). Therefore selecting a crop that is already known to grow well on peatland, that yields high amounts of biomass and methane and that can offer further value streams should potentially be of interest.

Bord na Móna have acknowledged in their report that the global herbal medicine industry is worth around €108 billion a year, with estimated annual growth of 8% year on year (Bord na Móna, 2019). Selecting herbs and plants that will grow on the peatlands and can be used for medicinal purposes is an area they are investing heavily in.    

Studies on various members of the Rheum family have found that one of the most abundant components, ‘Emodin, was capable of inhibiting cellular proliferation, induction of apoptosis, and prevention of metastasis.’ Another major component, Aloe-emodin was found to have anti-tumour properties and a third component, Rhein, was found to effectively inhibit the uptake of glucose in tumour cells leading to cell death. Several other components were found to possess promising anti‐cancer effects and could have wide-ranging therapeutic possibilities (Huang, Q et al., 2007). A number of studies on Rheum australe, for instance, has shown that it has widespread use as a herb in traditional medicine to cure illnesses of the circulatory, digestive, endocrine, respiratory and skeletal systems as well as to treat infectious diseases (Pandith, S.A. 2018). Rheum palmatum tannins have been shown to reduce levels of uremic toxins and to improve glomerular filtration and blood flow to the kidneys and in preventing and somewhat reversing kidney lesions. It was also found to reduce cholesterol, fatigue, cold aversion, anorexia, sexual dysfunction, and mental depression (Yarnell, E. 2002). Wang et al., (2010) also identified certain anti-microbial and anti-bacterial effects of rhubarb that could have future applications. Another more recent study found that some bacterial phytochemicals present in rhubarb extract could be involved in the modulation of the susceptibility to diseases linked to alcohol consumption (Neyrinck, A.M. et al. 2017).

In some interesting findings for the Irish beef and dairy farming sector, a further study found antimethanogenic activity on rumen fermentation and suggested that the Rheum species should be considered of interest in the development of feed additives to control ruminal methane production (Garcia-Gonzalez et al, 2010). In analysing further applications for rhubarb, a study from China has shown the possibilities of using rhubarb extract to extend the shelf life of beef (Cui, Y et al. 2013).

In a study on the structure of rhubarb, biological composite-like structures were discovered that suggest novel designs for manmade composites with improved properties. Further research could be undertaken to understand the functional and practical role (Graupner, N. et al. 2017).  

These studies simply underline the potential opportunities that could be exploited in the herbal medicine, manufacturing and food industries through further research and exploitation of the Rheum species.

Opinion & Critique

It has been suggested that the Biomass yield of crops is the key element buttressing their Greenhouse Gas efficiency and economic viability (Don, A. et al. 2012) but in the case of Ireland’s industrial peatlands consideration should also be given to the carbon sequestration potential through rewetting the peatlands, the financial and environmental costs of cultivation and the potential alternative revenue streams and employment potential for the selected energy crops.  

The amount of biogas that can be produced from any crop is a result of the methane that can be created from each kilo of volatile solids and the total yield grown per hectare, all of which are affected by the weather, growth conditions and the timing of harvest (Salter, A et al. 2008).

Drier Irish cutover bog can grow biomass rhubarb for fuel in woody fen peat with stalks growing up to two metres long and 15 cm thick (Feehan, J. 1996). Studies showing the ability of certain types of rhubarb to also grow in a paludiculture in Ireland are lacking, however, due to a paucity of field studies for cutover and rewetted peatland (Wilson, D. et al. 2013). Glenk et al. (2014) have found that the outcomes of rewetting to be exceptionally site specific, making valuation a challenge and Renou-Wilson, F (2019) has argued that a detailed evaluation is required if current and alternative land management uses are to be properly compared.

Other crops that have been examined appear to have some drawbacks. Reed canary grass, for example, is a possible alternative energy crop that has been adapted in Scandinavia to constricted growing seasons and low temperatures with resistance to drought and flooding and it grows well on most types of terrain with the greatest biomass yield being achieved on cutaway peatlands after peat extraction (Don, A. et al. 2012). However, the methane yield is still almost a third less than that of rhubarb (Frigon, J. & Guiot, S.R. 2010) & alternative revenue streams have not been identified (Sahramaa, M. 2008).  Perennial crops, with identified alternative revenue streams, such as hemp and sorghum, haven’t been grown at a significantly large enough scale on rewetted European peatlands and therefore their potential remains largely unknown (Don, A. et al. 2012). Additionally, hemp does have lower negative environmental impacts during cultivation, but produces less than half the methane energy yield per hectare as that of sugar beet (Gissen, C et al 2014). However, according to Frigon, J. & Guiot, S.R. (2010) cellulosic and lignocellulosic crops appear to be more ecologically sound for the creation of renewable energy than sugar and starch crops as they can be cultivated on poor quality land and generally do not displace food or feed crops.

The table below shows the methane potential for a range of different crops with rhubarb producing almost the highest yield;

 

Methane potential from lignocellulosic crops.
Crops	Operating Conditions	Yield (m3 STP CH4 / kg VS added)1
Ryegrass and clover	CSTR 2.5 kg TS/L.d	0.498±0.056
Rhubarb	BMP	0.490±0.030
Vetch ‐ oat	BMP	0.410±0.020
Lupine	BMP	0.360±0.040
Reed canary grass	BMP	0.34
Timothy	BMP	0.333–0.385
Cocksfoot	BMP	0.333–0.344
Tall fescue	BMP	0.332–0.340
Festlolium	BMP	0.328–0.359
Vetch	BMP	0.323
Oats, straw	BMP	0.320±0.020
Marrow kale	BMP	0.310±0.020
Red clover	BMP	0.300±0.060
Grass, lawn	BMP	0.300±0.040
Grass, mixed	BMP	0.298–0.315
Sugarbeet leaves	BMP	0.294
Clover	BMP	0.29–0.39
Straw, barley	CSTR 2.5 kg TS/L.d	0.285±0.054
Corn silage	BMP	0.270–0.298
Rapeseed	BMP	0.240±0.020
Alfalfa, ensiled	CSTR	0.24–0.26
Alfalfa	BMP	0.24
Grass, fresh	BMP	0.231±0.030
Hemp	BMP	0.230–0.409
Nettle	BMP	0.21
Napier grass	BMP	0.19–0.34
Straw, ryegrass	CSTR 2.5 kg TS/L.d	0.177±0.062
Sugarbeet leaves, alfalfa	CSTR	0.174–0.226
Giant knotweed	BMP	0.17
Grass, ensiled	BMP	0.128–0.392,

 

BMP: biochemical methane potential (batch assays).

CSTR: continuously stirred tank reactors (fed‐batch or continuous feeding).

Source: Frigon, J. & Guiot, S.R. 2010

 

Commercial yields of red rhubarb have produced up to 34-41 t/ha in ideal growing conditions but green rhubarbs, which aren’t grown commercially, can yield 50% more with perennial fields remaining productive for up to 15 years (Schrader, 2000). Perennial ryegrass yields approx. 33 t/ha with maize yielding approx. 40 t/ha and fodder beat approx. 80 t/ha.  

However, growing conditions have a substantial impact on yield with, for example, boreal conditions yielding significant lower amounts of rhubarb, 2-4 t/ha, than Jerusalem artichoke (9-16 t/ha), timothy-clover grass (8-11 t/ha) and reed canary grass (9-10 t/ha) (Lehtomäki, A., 2008).  Frigon, J. and Guiot, S.R. (2010) refer to the low solid content of rhubarb resulting in a much lower methane yield on a wet basis in an anaerobic digester. However, their findings were based on the same study of cultivation in boreal conditions and do not consider the different yields available under different growth and climactic conditions. Interestingly other work on crop yields and calculated methane yields for rhubarb have also appeared to use the same figures from the Lehtomäki, A. (2008) study for example, Murphy, J et al. (2011), and perhaps have had an impact on the consideration of rhubarb as a viable energy crop.  

In a newspaper article, a commercial grower in Yorkshire has suggested she grows approximately 800t of edible red rhubarb in 75ha per year, which gives a figure closer to 11t/ha and that in years with higher rainfall the yield is even greater (Guardian, 2008).  

In a Polish study (Salata, A. & Kozak, D. 2013) on the effect of the method of propagation of field-grown Rheum rhaponticum, much higher yields were observed and, additionally, the high early yield identified has previously been discussed as an important property for the consideration of rhubarb as a source of biomass (de Souza et al. 2011).

 

Propagation method	Petiole yield	Early petiole yield
	t/ha	t/ha
Micropropagation	47.3	11.5
Crown division	50	6.5
Generatively	34	2.5

 

Source: Salata, A. & Kozak, D. 2013

Identifying the right variety to generate energy from the biomass and which also offers the greatest potential for other products is a decision that should also be considered. In a study of 30 varieties of Rheum species there was considerable variation among the specimens in the concentration of components and antioxidative activities (Lizuka, A. 2004).  Autumn-cropping rhubarb are also available providing an extended harvesting season and varieties such as Livingstone don’t stop producing new shoots in summer and are ready to harvest from September onwards (Telegraph, 2014).

Harvesting techniques are a further consideration to add both financial and environmental costs when selecting a biomass. There have been mechanical harvesters in use on commercial farms for a number of years (Schrader, 2000) but in a paludiculture or other sensitive environment these would probably need to be redesigned for effective and sustainable utilisation (Pandith, S.A. 2018).

According to Don, A. et al. (2012), ‘the challenge for agricultural research is to optimise energy crop yields under the combined constraints of restricted or no fertiliser use and sub optimal soil and water conditions.’ But, as previously identified perennials, such as rhubarb, have low Nitrogen fertilisation demand due to their higher use efficiency.

Gissen, C. et al. (2014) have shown that it is important ‘to limit the transport of biomass to sites where it can be processed to products’ as the cost increases greatly when the distance is more than 20–30 km. Therefore having the land available for growing the biomass within a short distance of the power plant is an important further consideration.

Forced rhubarb is a highly regarded and protected culinary delicacy. It’s production is described in a newspaper article (Guardian, 2010) – ‘At two years, 8ha (20 acres) of the root (from which an outdoor crop has never been taken) are lifted from the ground and transferred into long, dark sheds where they remain for around 10 weeks. Such is the thickness and weight of the root, the strength of three grown men is required to manoeuvre the root from the field to the forcing shed. The warm temperature and dark, moist conditions manipulates the plant to grow quicker and the absence of light causes it to feed from the energy stored in their roots. The plant is then harvested by candlelight, the process of which further requires the manpower of 80 staff for any one given shed during the production process.’  

Previously used peat storage sheds could be heated from the thermal heat produced during the processing of the biomass to produce either further biomass or higher quality Rheum species for the food or medicinal industries. This would also require greater numbers of employees. The digestate and leachate from the process could potentially be used as fertiliser and water in these growing sheds (Lee S.A.R.L., 2012) (RTBF; 2020).

 

Recommendations

In order to properly evaluate the potential of rhubarb as a viable crop, further studies will be required to identify the yield potential in a paludiculture.  

Frigon, J. and Guiot, S.R. (2010) suggest that the technology to process the biomass should also be investigated and they recommend trials of a dry anaerobic digester, the use of high solids digesters or also dry fermentation technologies. These, they believe, would resolve problems such as the addition of supplementary water and the suspension of the crops on the surface of the digester.

Studies, that have used boreal conditions when considering using rhubarb as a biomass energy crop, could be considered again under the relevant climactic growing conditions present in Ireland.

Varietal selection for both growth and medicinal potential would also need to be carefully considered and working closely with food and medical research in making these decisions could offer greater future alternative value streams.  

 

Conclusion 

The Rheum species appears to offer excellent potential as an energy crop for growing on the peatlands of Ireland. This potential, combined, with the potential environmental benefits as well as medicinal uses, industrial uses and agricultural feed uses suggest that a crop that has a long history in this country should be considered even if many of the research papers to date appear to have discounted it as a viable feedstock in an Irish peatland perspective.

 

 

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