2.13. Food and farming http://www.ck12.org
required to make Tiddles’ food is just shy of 2 kWh per day. A vegetarian cat would require less.
Figure 13.7:The power required for animal companions’ food.
Similarly if your dog Fido eats 200 g of meat per day, and carbohydrates amounting to 1 kWh per day, then the
power required to make his food is about 9 kWh per day.
Shadowfax the horse weighs about 400 kg and consumes 17 kWh per day.
Mythconceptions
I heard that the energy footprint of food is so big that “it’s better to drive than to walk.”
Whether this is true depends on your diet. It’s certainly possible to find food whose fossil-fuel energy footprint is
bigger than the energy delivered to the human. A bag of crisps, for example, has an embodied energy of 1.4 kWh of
fossil fuel per kWh of chemical energy eaten. The embodied energy of meat is higher. According to a study from
the University of Exeter, the typical diet has an embodied energy of roughly 6 kWh per kWh eaten. To figure out
whether driving a car or walking uses less energy, we need to know the transport efficiency of each mode. For the
typical car of Chapter Cars, the energy cost was 80 kWh per 100 km. Walking uses a net energy of 3.6 kWh per 100
km – 22 times less. So if you live entirely on food whose footprint is greater than 22 kWh per kWh then, yes, the
energy cost of getting you from A to B in a fossil-fuel-powered vehicle is less than if you go under your own steam.
But if you have a typical diet (6 kWh per kWh) then “it’s better to drive than to walk” is a myth. Walking uses one
quarter as much energy.
Notes and further reading
A typical dairy cow produces 16 litres of milk per day.There are 2.3 million dairy cows in the UK, each producing
around 5900 litres per year. Half of all milk produced by cows is sold as liquid milk. http://www.ukagriculture.com,
http://www.vegsoc.org/info/cattle.html
It takes about 1000 days of cow-time to create a steak.33 months from conception to slaughterhouse: 9 months’
gestation and 24 months’ rearing. http://www.shabdenparkfarm.com/farming/cattle.htm
Chicken.A full-grown (20-week old) layer weighs 1.5 or 1.6 kg. Its feed has an energy content of 2850 kcal per kg,
which is 3.3 kWh per kg, and its feed consumption rises to 340 g per week when 6 weeks old, and to 500 g per week
when aged 20 weeks. Once laying, the typical feed required is 110 g per day.