Cows wear a nozzle that collects methane and other gases discharged near the nose to be measured.
WILLEM DE ROOIJ/BUREAU TYPOGRAFIA BV
A new beef with role of cows in global warming
Statistics have been giving us a bum steer when they state how much cattle methane emissions contribute to global warming, a new study shows.
That's because mathematical equations used to predict cows' methane emissions are inaccurate and don't take into account factors such as dietary changes, said Jennifer Ellis, lead author of the study and a PhD student at the University of Guelph.
When cattle burp up their cud, they discharge methane with it, due to microbial fermentation occurring in their complex stomachs.
“Diet can change CH4 (methane) emissions quite a lot. For example, between two and 12 per cent of the energy a cow consumes will be lost as CH4,” she said.
“As a crude comparison, a typical dairy cow will produce as much in (carbon dioxide) equivalents per month as a mid-size car does travelling 800 kilometres per month. Changes (of) two to 12 per cent of the energy intake can be the difference between that cow being a compact car and an SUV or truck,” she added.
Methane is 25 times more potent than carbon dioxide among gases implicated in global warming.
When an equation calculates the quantity of methane emissions on one farm, it can't be used to accurately determine how much greenhouse gas is created worldwide because there is so much difference in cattle diet around the world and from farm to farm.
“CH4 is actually the biggest on-farm contributor to the greenhouse gas effect, so it is very important to get right,” Ellis said.
The study did not determine whether cattle produce more or less methane than had been believed previously.
The Food and Agriculture Organization of the United Nations estimates cattle are responsible for 18 per cent of the world's greenhouse gas emissions.Two methods are used to measure cattle methane production, 98 per cent of which is belched up.
In the most reliable method, “essentially you put the cow in a small room-size chamber and measure the exchange of gases in and out of the chamber for a period of 24 hours,” Ellis said.
In the other method a nozzle is placed right beside the animal's nostril and mouth that sucks up the gases discharged.
The amount of food eaten, the amount of forage relative to grain and the fat content of the diet are all factors in cattle methane production. More hay and grass tends to increase emissions while some dietary supplements and high-fat diets tend to reduce it.
As for the math, more complex equations with a better description of the animal and diet would help, she said.
The study was co-authored by livestock researchers at the University of Manitoba and in the Netherlands. It appears in the December issue of the journal Global Change Biology