Crops are harvested 12 months of the year at Stracathro Estate on the east coast of Scotland to fuel a 2MW anaerobic digestion plant, which requires the equivalent feedstock of 2,000 dairy cows a year.

More than 60% of land at the 1,600ha estate is used to grow crops for energy: wholecrop rye, grass silage, winter wheat and, more recently, both sugar and fodder beet.

“We have no livestock on the farm, just one giant concrete cow that produces 4m cubic metres of gas to the National Grid each year. The yard is kitted out with five separate silage pits totalling 1ha in area,” says the estate’s farm manager, Iain Wilson.

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Harvest every month

The anaerobic digestion (AD) plant consumes about 110t of feed each day. It runs as a closed-loop system, which sees 100% of crop inputs grown on farm and 100% of digestate returned to the land.

A 140ha area of sugar and fodder beet is now grown to feed the plant from October until May to maintain an even supply of feedstock throughout the year.

This means the Stracathro team harvest a crop every single month.

“We’re constantly in ‘harvest mode’ and are busy year-round, so it’s important to have a good work-life balance for our staff.

“Beet lifting takes place from October to May, followed by grass silage, wholecrop and combinable cereal harvest, before the cycle begins again,” says Iain.

Beet harvest takes place every 10 to 14 days during the lifting period. Between 400-500t of beet is lifted each day, this is then washed and chopped to provide 35t of beet to the AD plant each day.

This is fed alongside 35t wholecrop rye and 40t of grass silage.

Fodder beet is predominantly grown due to its greater frost tolerance than sugar beet, something which is particularly important when located so far north.

A proportion of early lifted sugar beet offers increased sugar and greater energy content than fodder beet, which is harvested before the cold winter temperatures arrive.

“Before growing fodder and sugar beet, we would often see a drop in gas production hence the move to root crops to even-up crop supply.

“We did consider growing maize, but being so far north, the risk of a failed crop would mean it would not be worth it,” explains Iain.

Spring oats are drilled after beet crops lifted by the end of March. Any fields harvested after this are planted to a spring triticale and clover arable-silage mix to gain an extra crop in the rotation.

“It would be too late to plant a conventional spring cereal so the triticale and clover mix allows us to get another crop harvested into the silage clamps, before planting winter wheat the following autumn,” explains Iain.

The estate has now established a new beet storage shed that collects any beet run-off sugary juices, which is then fed back into the AD plant.

Why anaerobic digestion?

Stracathro Estate had historically been a mixed farming enterprise with sheep and cattle, but over the years, the arable operation was scaled-up and the livestock sold.

However, as the arable land intensified, the estate owners – the Campbell Adamson family – knew only too well that soil health and structure were starting to be at risk.

Looking for alternative business ventures that boasted both profitability and sustainable soils, the Campbell Adamsons decided to set up the AD plant, which has now been running for six years.

The site is leased to a third-party investment company, and the estate is contracted to grow, supply and feed the plant.

“We have a price framework in place to secure a market price, before the crops are even planted in the ground. It makes budgeting and cashflow much more straightforward,” says Iain.

This allowed the estate to continue with the arable enterprise while integrating grass back into the rotation, with a constant return of organic matter to the soil in the form of digestate.

Soil health benefits

The nutrient-dense product is an important soil conditioner, says Iain, providing a slow release of nutrients and range of trace elements.

“Digestate is certainly not a waste product and we use it to our full advantage. We’ve seen organic matter levels rise and earthworm numbers increase across the board.”

Solid digestate rich in phosphate and potassium is applied ahead of sowing and in-field applications of liquid nitrogen (N) is applied to wholecrop rye and grass via the 16m boom tanker.

“Crops stay greener for longer and we do not now have the same urgency to get going with fertiliser applications in the spring.

“They no longer experience that green flush from high N rates that makes them vulnerable to disease,” says Iain.

Digestate is analysed on site to ensure we always know what nutrients are being applied. This has seen significant reductions in purchased fertiliser use across the farm.

Importantly, Iain runs a variable-rate nitrogen programme to target and even-up crop N requirements as digestate can be more inconsistent in its nutrient profile compared with bagged products.

Iain tries, where possible, to run a controlled traffic farming with a 16m dribble bar, 32m tramlines, 8m drill and 16m rollers.

With such large tankers and trailers trafficking the farm, compaction can be an issue. A nurse tank system with separate field and road tankers equipped with different tyres and pressures are in place to help minimise compaction risks.