Friday, May 16, 2008


I get involved in detailed calculations about these things, from time to time, because I wrote the carbon profiling software that, among other users, the UK government recommends for use by landowners and farmers (I make a point of never linking from here to anything commercial I do for third party clients; associating them with my personal political views would be inappropriate). This came out of a chat I had yesterday with a colleague in that project.

The use of biofuels on farms isn't new. In fact, at one time the only fuel used on farms was bio. A draft horse doing light work (which farm work would be classed as, taken across the whole year) eats about 2 tonnes of hay and 1.3 tonnes of oats per year, and what is that if it isn't biofuel?

At current yield levels, it would take the output of about an acre of land to feed each horse. It's hard to be sure of the next figure, but research carried out in Swavesy suggests that one horse is needed for every six acres under cultivation. Since this figure includes fallow land, you are probably looking at a ratio of less than 1:6 - perhaps 1:4. That is, of every four acres cultivated, about a quarter of the output is needed for "fuel" if you're farming with horse power.

So what if you use tractors, and grow biodiesel? Oilseed rape produces between 1.2 and 1.5 tonnes per acre of seed, and that yields 500-600 litres of oil. A tractor uses 40 to 50 litres of fuel per acre per annum, so the area farmed per acre of home-grown biodiesel is about 10 acres. This gives a ration of 1:10 - about one tenth of a farm's output is needed for fuel if you use tractors and biodiesel.

Of course, there are externalities* is associated fuel use outside the farm - production of fertilisers, transportation to and from farm gates, etc, but these apply in both cases, whether you use tractors or horses. Tractor manufacturing uses fuel, but so does the "manufacture" of replacement horses. I have no data to use for a comparison of those replacement costs.

This is just a bit of fun, but two conclusions can be drawn. Firstly, there is no doubt that energy use (fuel, fertilisers) increases food production. Secondly, it's "greener" to use tractors than horses.

UPDATE: Sorry, there's a third. Growing "biofuel" isn't new, although it did go into abeyance for a while. If all arable farms grew all their own fuel, then the net difference between that situation and the one that prevailed in the horse-drawn era would be an increase in food production per acre farmed, assuming constant yields (of course, yields have risen during that period). But the situation that creates the greatest level of food production per acre is, of course, the use of fossil fuels. And since that final conclusion is self-evident, it at least can be entered into the annals of the Journal of Obvious Research.

*Stupid misuse.


dearieme said...

Tractor dung isn't as useful, mind, so you ought to burden the tractor with the cost of the replacement fertiliser. Also, about oats - do your calculations allow for the fact that oat straw has good food value for cattle, unlike - for example - wheat straw or old tractor tyres?

Peter Risdon said...

I don't think you'd get the modern yields I'm using with horse dung. The roses would be lovely, though. You're right about oat straw, but these calculations get very complicated indeed when you start to factor in (as indeed you should) costs external to the farm. This is an external factor, for my arable farm, because cattle feed would be sold to a cattle farmer. Again, given the difficulty of coming up with a realistic ratio of livestock to arable on a mixed farm, it's easier to calculate for exclusively arable production.

Mind you, I extracted some data from a survey of 200 farms from my calculator and a group of academics are shortly to release their report comparing the profiles of different forms of production across the country. But that's real data, rather than a back of an envelope calculation, done for fun.

dearieme said...

Oxen? Why did horses replace oxen for ploughing? Would oxen be better than tractors?