HARVEST is almost complete for Scotland’s newest arable crop – several thousand acres of hybrid winter rye has been cut as whole crop in recent days in East Perthshire, Angus and North Fife to provide feedstock for three new biodigesters.
Another is under construction in Angus and it seems likely that the area of the crop will increase in coming years. Sown last September, the rye might not have been as high as an elephant’s eye, but without a growth regulator it would have been – it’s a vigorous crop by any standards.
Rye’s appeal as a biogas crop in Scotland lies partly in the fact that it is ready for harvest three and a half months earlier than maize which is its nearest rival.
The early harvest, however, makes rye an ideal entry for oilseed rape and gas production from ensiled whole crop is good as long as it is cut while energy levels in the stems are still high.
The correct harvest timing silage is important to ensure optimal yield, quality and ensiling conditions. Crops should be between 35 – 40% DM, otherwise known as the ‘cheesy dough’ stage.  
Approaching harvest, the dry matter of the crop will increase by around 1% per day allowing for a narrow harvest window.  
In practice this will mean commencing harvest with a DM% that is slightly below optimum, say 32 – 33% and finishing harvest with DM% 37 – 38%.  
Harvesting too late  leads to compaction problems in the clamp and greater ensiling losses, as well as lower digestibility in the AD plant.  
A good stand of winter rye should produce between 40 and 50 tonnes per hectare and for efficient digestion is should be cut  to  a power sapping 6mm to 10mm chop length.
Crops are generally being grown within 10 miles of the biodigesters with a self-propelled forage harvester needing up to twelve 15- tonne trailers to keep it serviced.
Making quality silage is apparently every bit as important for  all-year-round biogas production as it is for feeding top yielding dairy cows.
The rye-fed biogas digesters, one at Coupar Angus and another in Fife, are producing methane  for pumping  directly into the grid after treatment.
Another at Monikie in Angus is using a gas powered marine engine to drive an electrical generator.  
A fourth plant near Dundee is using maize and pig manure as its main feedstocks.