The practice of growing crops for "renewable" energy is coming under increasing scrutiny by European governments and the EU. Is it right to use arable land to grow oilseed rape for bio-diesel, or wheat for ethanol to drive gas-guzzling cars when so many around the world go to bed hungry?

Others argue that before worrying about the use of land for non-food crops we should address the dreadful situation across Europe where half of all the food we currently produce is wasted as a result of poor harvesting, transportation and storage techniques. Then there is the widespread practice of supermarkets rejecting vegetables for animal feed because they are slightly blemished or not the right shape or colour. Even consumers could look at buying only what they really need instead of throwing out good food because it is near or past its sell-by-date.

Of course farmers across the EU are keen to have financial support from taxpayers to grow crops for fuel. Not only does such crops give them other income streams, it also reduces the quantity of crops for food coming onto the market and forces up prices.

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Speaking at a recent conference in Brussels that discussed a sustainable EU bio-energy policy post 2020, Chairman of Copa & Cogeca (organisations representing EU farmers' unions and farm co-ops) Bio-energy Working Party Dietrich Klein said: "Advanced bio-fuels have an additional and growing role to play by 2030. They improve the competitiveness of arable crops by creating additional market outlets for crops."

Most farmers would agree with that statement, but the more discerning taxpayer will probably be asking why they should subsidise non-food crops to make the food they buy more expensive. They can be forgiven for thinking that subsidies paid to farmers from the Common Agricultural Policy (CAP) were meant to help provide cheap food for EU consumers rather than make it more expensive.

To give an idea of the scale of ethanol production, the US produced 14.81bn gallons from maize last year, equivalent to 893,000 barrels of ethanol per day. Brazil is also a world leader in bio-ethanol production, but it uses sugar cane.

In the UK, ethanol production from wheat has had a chequered history due to the volatility of grain prices. Plants on Teeside and at Hull have the capacity of using up the exportable surplus from the UK's annual wheat harvest that varies between 14 and 16m tonnes.

Apart from growing crops to produce bio-fuels, there is also the recent practice of producing methane from food crops by anaerobic digestion (AD). AD is a natural process in which micro-organisms break down organic matter like whole crops of wheat and maize or animal slurry (a mixture of dung, urine and water), in the absence of oxygen, into bio-gas - a mixture of carbon dioxide and methane.

The bio-gas can be used directly in engines for Combined Heat and Power (CHP), burned to produce heat, or cleaned and used in the same way as natural gas, or as a vehicle fuel.

Now UK Ministers have placed AD and the feedstock needed for the process under the spotlight.

The National Non-Food Crops Centre's (NNFCC) third annual AD deployment in the UK says the dramatic growth of the UK bio-gas sector is unlikely to continue much beyond 2017. The industry faces rapidly diminishing support through tariff cuts to Renewable Heat Incentives (RHIs) and the halving of the Feed-in-Tariff scheme.

Thanks to favourable RHIs and FITs more than 130 AD plants came online in the past year, taking the total to 316. The rapid increase was caused not only by market maturity, but also fear that the main policy support mechanisms were facing imminent changes or potential closure.

Rather than using animal waste as a feedstock, maize grown for AD in 2014 was 19 per cent of the total maize area in England.

While I deplore growing crops for AD, for those who can afford the hefty initial capital cost of installing the technology, AD is a perfect environmental solution for handling slurry from large pig and dairy enterprises. It makes a lot of sense to use slurry in a holistic approach that improves fertiliser quality, reduces smells and generates electricity and heat.

Odour from the material left after the slurry has been "digested" is reduced by 90 per cent compared to untreated slurry, and that could be important to those farmers spreading it within close proximity to villages and towns.