Sunday, August 12, 2012

Cattle "Emissions"

Ancestral Health Symposium 2012 is now history. The symposium took place at the Harvard Law School August 9th-11th. Not a bad gig for a forage agronomist! The title of my presentation was The Reality of Ruminants and Liebeg’s Barrel: Examining the New ‘Conventional Wisdom'. All of the presentations were video taped, and will be freely available.
My title slide. Cattle mob grazing a pasture under center pivot irrigation.
Photograph by Dawn Gerrish.
One of the themes of this year’s symposium was sustainability. I don’t see that term in the simplistic way many do, in part because I remember being told by leaders of the sustainable agriculture movement that “animals have no place in sustainable ag!”

The truth is that the production of animal products from perennial forages is the sustainable agricultural system. Many, however, are concerned about the carbon dioxide and methane “emissions” from livestock in general, and cattle in particular. Some folks don’t seem to understand that a cow grazing grass can only emit carbon that was originally in grass, and that the carbon in grass has to have come from the atmosphere. So it’s a cycle not an “enrichment.” But there’s more to it than that!

Under the following assumptions:

Carbon:Nitrogen ratio = 17 and 3.4% N, giving 57.8 % C
3% of body weight daily dry matter (DM) intake; 1,000 lb cow = 30 lb DM per day
70% utilization - 30 lb DM eaten / 0.70 = 42.8 lb DM offered
Equal above and below ground DM distribution
90% of C consumed is “emitted” 

We’d see:

42.8 lb DM above ground, 42.8 lb DM below ground – 85.6 lb DM total
57.8% C in DM – 49.5 lb C total
17.3 lb C consumed
15.6 lb C emitted

Thus, for every pound of carbon “emitted” by a cow on grass, 3.2 pounds of carbon are fixed in plant roots, uneaten plant debris, or the cow herself or her calf. Even if we say that 100% of carbon she ingested is emitted (a biological impossibility!), there’d be 2.9 pounds of carbon fixed for every pound emitted! Beef cattle are carbon negative!

Someone (perhaps Todd Becker?) asked me what happens to methane in the atmosphere. A good question, and one I wasn't completely sure about. So I went looking and found the following at this site:
"In the lower part of the atmosphere, below about 10-12 km (the troposphere), the key cycles are mediated above all by the presence of what are called OH radicals — colloquially known as the atmospheric detergent. All hydrocarbon chemical species that are emitted can be eventually broken down (or oxidized) by these radicals to CO2 and H2O, and methane is no exception. An average molecule of CH4 lasts around eight to nine years before it gets oxidized. This is a long time compared to most atmospheric chemicals but is fast enough so that there can be significant year-to-year variability."