Biogas trials fuel maize growing interest

Official figures of on-farm energy crop anaerobic digesters are currently underestimating the bio-fuel generating potential by at least half, according to Richard Crowhurst, managing director of independent bioenergy consultancy, Enagri. Speaking to agronomists and growers during a visit to the Syngenta ‘Maize for biogas’ trials site, near Mildenhall in Suffolk, he reported current projects  would more than double the existing capacity of on-farm anaerobic digesters.

Hosted by the Shropshire Group, the trials have provided the chance to compare 12 Syngenta varieties, view agronomic programmes and to learn more about the current state of the UK biogas sector.

Mr Crowhurst warned that the recent cabinet reshuffle could signify uncertainty for those considering on-farm biogas. However, despite the reservations over government support, there was still a good appetite among investors for viable AD projects, he added. “In his first interview as Defra Secretary, Owen Patterson said that he didn’t want to see maize, which should be going to feed cattle, going into biogas plants. At last week’s Liberal Democrat conference David Heath made similar comments. It just shows that they do not understand how these crops are grown specifically as a biofuel source for efficient green energy production.”

Variety choice

The energy output of AD plants is largely dictated by the quality of the feedstock going in, advised Syngenta maize specialist Nigel Padbury.  “When selecting suitable maize varieties for biogas feedstock it’s all about the yield of digestible dry matter,” he said. “We don’t want to produce lignin which the biogas plant can’t digest.”

Mr Padbury pointed out that whilst growing maize for dairy cattle aims to produce starch and a dry matter of 32%, for biogas production this figure drops to 28% - 30%. “Furthermore, the anaerobic digestion plant also doesn’t care where in the plant that dry matter comes from. Around 50% of the energy will come from the cob, while the rest comes from the remaining part of the plant, or stover,” he explained. The starch in the cob gives a rapid energy boost, which makes maize work so well as a supplementary feedstock in biogas plants, but the energy from the rest of the plant makes all the difference and quality is key to get the best of the crop.”

He highlighted that the quantity and digestibility of the stover varies enormously with different maize varieties. “Quantity certainly doesn’t always equal quality in terms of biogas yield. What is driving yield is not the net weight, but the dry weight and digestibility. An energy rich maize variety generates more output from any given volume going into the digester.”

An example of this would be NK Bull, explained Mr Padbury. It is deceptively not the tallest in the field, but with a Metabolisable Energy (ME) of 11.68 it can produce 214,000 megajoules of energy per ha; ME provides a good indication of methane yield. In contrast, Paddy is higher yielding and has a higher dry matter, but lower ME. “Overall the two varieties produce a similar amount of energy per hectare,” he advised. “But as it doesn’t make sense to harvest, ensile and handle more material than necessary, gross yield is not the best way to compare varieties for biogas production. From a movement and storage point of view, I would go for the concentrated energy of NK Bull every time.”

However, he believes other varieties may still have a valuable role for biogas maize growers, many of whom are new to the crop. Avenir, for example, is incredibly consistent and has proven extremely popular with growers for many years. It is also one of the shortest season varieties available and has quite a rapid dry down period so well suited for growers looking to sow wheat after maize, or who have a large maize area and need to spread the harvest.

How the variety matures, and any environmental considerations, such as having bare soil following harvest in the autumn, will be other key considerations for growers. “With new biogas ventures having 800 hectares or more of maize to harvest, scheduling maturity to match harvesting capability is crucial.

“Furthermore, it is important to monitor the dry matter as harvest approaches and to know if you are dealing with a variety that is fast or slow to dry down,” he advised. “Avenir is quite quick to dry down but NK Bull, for example, takes longer to go from 28% to 32% dry matter, so it gives you more flexibility.”

Many new biogas maize growers are well versed in the agronomic demands of achieving high yields of arable crops, which will make them well suited to getting the best performance from maize crops, according to Syngenta Technical Manager, Stephen Williams. He pointed out that, with the relatively short growing season, maize can be especially vulnerable to weed competition, which makes herbicide selection and timing essential.

Herbicide trials

Reviewing the company’s weed control trials, he highlighted that the pre-emergence sprays of Dual Gold and Callisto had worked well on lighter soils near Benacre, but were less effective on rich organic soils in the fens at Shippey Hill. A post-emergence contact application of Callisto also provided good control, with the addition of Samson Extra giving control of annual meadow grass.

“There is clearly a difference in the residual activity of these pre-emergence applications when they are used on black soils,” commented Mr Williams. “With more maize being grown for biogas in traditional vegetable production areas, we aim to run further herbicide trials next year, to see what else is available to help growers deal with the issues of weed competition more effectively in the future.”

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