Dear Pablo: What is more sustainable, using a horse or a tractor?
The answer may seem simple at first. A tractor consumes diesel fuel and belches greenhouse gasses and air pollutants. The horse on the other hand does not. End of story? Not quite. The horse also requires fuel in the form of grains. These grains are probably harvested by a tractor that burns diesel. So the question really should be: Does a horse consume more fossil energy through its feed than a tractor consumes for the equivalent amount of work?
What Is The Impact Of A Tractor?According to a cost and return study on wheat by UC Davis Cooperative Extension 5.43 gallon of fossil fuels per acre are required to produce 3.25 tons. When combusted this fuel turns into 122 pounds of carbon dioxide (CO2). Although this number is not going to be the same for every crop we can assume that it is within the right range for all grain crops.
What Is The Impact Of Growing Grain?In addition to the 122 pounds of CO2 from fossil fuel combustion there are additional emission sources to consider from most agricultural activities. Wheat also requires 220 lbs on nitrogen-based fertilizer (anhydrous ammonia, urea and UN32). Depending on local temperature, moisture and other factors a portion of the nitrogen-based fertilizer will turn into nitrous oxide (N2O), a greenhouse gas that is over three hundred times more potent than CO2. According to one source it could be at high as 5.5%, or 12.1 pounds of nitrogen, which becomes 19 pounds of N2O, which is equivalent to over 5,700 pounds of CO2. So the impact of growing wheat, based on our data sources and assumptions, is 0.895 tons of carbon dioxide-equivalent (CO2e) greenhouse gas emissions per ton of wheat produced.
So What Is The Impact Of The HorseHorses doing heavy work are supposed to eat 2-3% of their body weight in feed each day, or up to 48 pounds. Growing this feed, according to to the calculations above, results in 43 pounds CO2e. According to a formula in the Almanac Of Rural Living, the speed of the horse (~2 miles per hour) time the width of the plow or other implement (~1 ft) gives you the acres per 10-hour day (2 acres). If we assume 2 acres per day for our horse that means that our horse is responsible for 21.5 pounds of greenhouse gas emissions per acre.
So What Is Better, The Tractor Or The Horse?The tractor, through its combustion of fossil fuels, emitted 122 pounds of CO2 per acre compared to the 21.5 pounds of indirect emissions from the horse. If the horse feed were also cultivated by horses this number would be even smaller and if the synthetic nitrogen fertilizer could be eliminated by rotating fields with nitrogen-fixing legumes (like soy beans) and fertilizing with biogenic chicken manure then the greenhouse gas emissions would be effectively zero.
Should We Switch Back To Draft HorsesThe reason why we switched from the horse to the tractor is the same reason why we still use it today: productivity. Seen purely from a cost perspective the biggest cost in plowing a field is the human labor. Since a tractor towing a wide implement can plow a field in minutes instead of hours it significantly cuts the cost. With today's expansive factory farms bringing in hundreds of mono-cropped acres at the peak of readiness is already a logistical challenge. Can you imagine trying to do this with horses? Still, for local farms that use organic practices and don't plant hundreds of acres with a single crop there can be many advantages to using draft horses. Horses have a lower upfront cost and their fuel (feed) also costs less, they tread lightly on the ground, and they even bring their own fertilizer to the fields.Pablo Päster is a weekly columnist for TreeHugger.com, an experienced greenhouse gas engineer and the Senior Environmental Program Manager at Hara Software. Send your questions to Pablo(at)TreeHugger.com or submit the via this form and connect to his RSS feed.
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