Farmers could face more problems with blocked fuel filters when red diesel is replaced with ultra-low sulphur gas oil at the end of the year.

However, the problem is not caused by the reduced sulphur content, but by the increase in biodiesel content from the existing 5% to 7%.

This causes two separate, but related, problems.

Because biodiesel attracts moisture from the air, there is likely to be more water at the bottom of both tractor tanks and farm fuel stores and this could encourage the growth of microbes. These then form an emulsion that, in extreme cases, can block tractor filters.

Bob Hall from Shrewsbury firm Fuel Additive Science Technologies, which makes products to deal with so-called diesel bugs, says farmers seem to having increasing problems with fuel-clogged filters.

“We sell a product called Gasoil Extra which reduces the emulsification of diesel and improves its quality. Some 90% of it goes to farmers and the volume of it we’ve sold in the past 12 months has tripled compared to the year before.”

It’s not a problem with older tractors, he says, but the latest common rail diesel engines are less tolerant of impurities in diesel. And he expects things to get worse after the fuel change at the end of the year.

“When the biodiesel content of diesel starts to go up from 5% to 7% in November [ready for the end of the year deadline] there will be a lot more problems”.

The second problem is the fact that because biodiesel is a natural product rather than a synthetic one, it gradually breaks down and forms sludgy particles within the fuel that could potentially block filters. There is some debate about how long that might take, but most industry estimates suggest between three months and a year.

Not a problem if you turn your diesel around quickly, but for combines and self-propelled foragers where fuel can stay in the tank for 11 months, it could be an issue.

And while fuel could be theoretically be drained out at the end of the season, many owners would probably be loath to disturb the sediment that tends to sit at the bottom of the tank.

• We’ll be having a closer look at this whole area in the next couple of weeks.