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Greenhouse Gases: Are Dairy Cows to Blame?

Published: May 2, 2013
By: Alvaro Garcia (Dairy Science Department, South Dakota State University)
According to the Intergovernmental Panel on Climate Change the main greenhouse gases carbon dioxide, methane, and nitrous oxide started to increase around the early nineteenth century (IPCC, 2006). From then on, and particularly during the twentieth century, their increase was exponential. Methane concentrations have more than doubled since preindustrial times whereas carbon dioxide and nitrous oxide have increased by roughly 25 and 10%, respectively (Figure 1).
Figure 1. Atmospheric concentration of carbon dioxide, methane, and nitrous oxide since 1,000 A.D. (Source: IPCC, 2001)
Greenhouse Gases: Are Dairy Cows to Blame? - Image 1
Cows produce methane from feed fermentation in the rumen. The more they eat the more gas is produced part of which is absorbed by the animal and part is lost to the environment. Then again, the higher the production level of the animal (i.e. beef or dairy), the more gases are used-up as energy sources and less are eliminated to the atmosphere. Nitrous oxide is generated from soil nitrogen by microbial action under warm, waterlogged conditions. In addition, it can also arise from cattle urine, legumes and nitrogen fertilizer. With nearly 100 million head in the U.S. between beef and dairy cattle, it is clear how the general public could perceive livestock production as significant when compared to other greenhouse gas sources. When assessing gas emissions from ruminants it is very important to take into account how much gas is emitted per unit of product yield. Cows have fixed costs of energy maintenance regardless of how much they produce (i.e. beef or milk), and they will eat to fulfill those requirements with methane and carbon dioxide being produced as a result. This is the reason why two grazed dairy cows that produce 50 pounds of milk each are more taxing to the environment than one cow producing 100 pounds. Allowing cows to manifest their genetic potential through additional feeding reduces gas emissions as the energy in them is deposited in animal products instead of being inefficiently eliminated to the atmosphere. Decades of permanently evolving best management practices have reduced these inefficiencies and allowed for a reduction in the number of animals needed to supply food for an ever growing world population.
Historic and present day greenhouse gases contribution
During pre-colonial times there were no domestic cattle (beef or dairy) but only wild ruminants whose methane emission can be predicted from their feed intake and compared to today’s livestock. A recent article published in the Journal of Animal Science (Hristov, 2011) compared pre-colonial enteric methane emissions from all wild ruminants in the U.S. with present-day emissions from farmed ruminants (US EPA, 2011). The most significant methane-emitting group of ruminants was the buffalo population. Estimates of the pre-colonial buffalo herd are somewhere between 30 (low) and 75 million (high). Methane emissions measured in teragrams per year were 4.89 for buffalo in pre-colonial times whereas currently 4.74 and 1.58 for beef and dairy, respectively (Figure 2). Carbon dioxide equivalent emissions measured in teragrams (one teragram equals one million metric tons) were 102.7 for pre-colonial buffalo, compared to 99.6 for beef, and 33.2 for dairy during present times (Figure 3). When the average figure of 50 million buffalo was used enteric methane emissions from buffalo, elk, and deer precolonial times were around 86% of U.S. current beef and dairy emissions. According to the author when the high figure of 75 million buffalo was used instead, wild ruminants in precolonial times emitted around 23% more methane than current domestic ruminants in the U.S.
Figure 2. Methane emissions of pre-colonial buffalo herd estimated at three population sizes and contemporary beef and dairy herds
Greenhouse Gases: Are Dairy Cows to Blame? - Image 2
Figure 3. Carbon dioxide emissions of the pre-colonial buffalo herd estimated at three population sizes and contemporary beef and dairy herds
Greenhouse Gases: Are Dairy Cows to Blame? - Image 3
South Dakota State University research (Garcia and Linn, 2008) reported that in 1924 there were 21.4 million cows in the U.S., weighing 1,000 pounds and producing 4,162 pounds of milk yearly. Conversely, during 2007 there were 9.1 million cows, weighing 1,400 pounds and producing 20,053 pounds of milk. When methane emissions were calculated for both set of data, the total cow population in 1924 produced 40% more methane than in 2007. These figures were even more contrasting per pound of milk produced, with nearly tripling the methane produced in 1924 vs. 2007. Capper et al. (2009) also reported dairy cows in 1944 produced 2.7 times as much carbon dioxide per pound of milk than in 2007. 
Efficiency of carbon dioxide sequestration by plants
We often think about ruminant livestock in terms of greenhouse gas emissions only. This approach fails to acknowledge that ruminants and the plants on which they feed are part of an interphase that has been in equilibrium throughout evolution. Photosynthesis is the daytime process by which green plants synthesize sugars using carbon dioxide, water, and solar energy (captured by chlorophyll), and release oxygen to the atmosphere in the process. During the night photosynthesis gives place to respiration which consumes both oxygen and stored plant carbohydrates. Photosynthetic rates (carbon dioxide uptake) in growing plants are higher than respiration rates (oxygen use) which results in a net increase in oxygen production and an uptake of carbon dioxide. For this pathway to begin, plants need to bind carbon dioxide to another molecule. Plants can be classified in C3 or C4 depending on the pathway used to capture this carbon dioxide (Sehtiya and Goyal, 2003). In the first group carbon dioxide (one carbon) is bound to a five-carbon compound forming a six-carbon molecule. Plants from cold and temperate regions of the world such as legumes and many grasses are members of the C3 group, which are not as efficient at capturing carbon dioxide as the C4 group. The C4 group binds carbon dioxide to a three-carbon compound instead (forming a four carbon molecule) in a much more efficient carbon-sequestration reaction. These plants originate in tropical regions of the world and include corn, sugar cane, and other tropical grasses.
Ruminant diets and carbon dioxide sequestration
Wild ruminants in the U.S. feed on legumes, grasses, and forbs depending on their foraging strategy. Buffalo in particular were and still are almost strict grazers feeding mostly on grasses of the Great Plains. None of the forages of the wild ruminants in the U.S. included C3-type plants and thus their efficiency of carbon dioxide sequestration was limited when compared to C4. Of 595 dairies surveyed in 1919, a total of 62% of the farms grazed with the remainder 38% fed in confinement (Garcia and Linn, 2008).  In addition, dairy cows in confinement early in the 20th century were fed mostly hay, green-chopped forages, and very limited amounts of grain or corn silage. Modern dairy cows diets on the other hand usually include large amounts of corn-derived feeds like corn silage, corn grain, high moisture corn, corn distillers grains, etc. Contemporary dairy cows eat approximately 50 pounds of dry feed daily of which 15-20 pounds could be corn silage and similar amounts of corn grain. The remainder of the diet will usually come from legume hay and/or silage, feed industry byproducts, as well as other feeds. Based on these calculations contemporary dairy cows during 305-day lactations can eat approximately 2 tons of dry corn silage and 2 tons of dried shelled corn.  A conversion of these figures to wet corn plants for silage and bushels of corn for grain results in 6 tons of corn plants for silage and 36 bushels of corn grain.
According to the National Agricultural Statistical Service, the 2010 U.S. corn silage and corn grain yields per acre were 19 tons and 152 bushels, respectively. If the modern dairy cow needs 6 tons of corn plants and 36 bushels of corn grain, that would represent 0.3 acres for silage and 0.2 acres for grain. If corn is planted at 32,000 plants per acre, one cow needs 9,600 corn plants for silage and 6,400 for grain for a total of 16,000 corn plants. Recent data collected by South Dakota State University (Carlson, Personal communication. 2012) suggests that one acre of corn yielding 150 bushels captures 32,427 pounds of carbon dioxide. If corn plant density is 32,000 seeds to an acre then a corn plant absorbs one pound of CO2 per growing season. If the modern dairy cow needs 16,000 corn plants to sustain itself then it is also responsible for the removal 16,000 pounds of carbon dioxide from the atmosphere yearly. The total sequestration of carbon dioxide of 16,000 pounds is equal to 3.63 tons of carbon dioxide equivalents per cow yearly. If this figure is multiplied by the 9.15 million dairy cows in the U.S. it equals a removal of 33,232,800 tons or 33.2 teragrams of carbon dioxide equivalents. Current total carbon dioxide emissions estimated for U.S. dairy cows are exactly 33.2 teragrams (EPA). It can therefore be hypothesized that carbon sequestration by the corn plants biomass can neutralize emissions if modern dairy cows are fed to match requirements for milk production and corn-derived feedstuffs are maximized in their diet.
Greenhouse Gases: Are Dairy Cows to Blame? - Image 4
Current research has shown that buffalo in pre-colonial times produced more methane than either the modern U.S. beef or dairy herds. In addition, research does not support the contention that modern dairy cows emit more methane and carbon dioxide than cows early in the 20th century. Modern dairy milk production needs to be viewed as an animal-plant interphase where forages are required for feeding purposes and can act as a sink for carbon dioxide emitted by cows. Maximizing corn-derived feedstuffs in diets of dairy cows fed to their genetic potential will increase carbon dioxide sequestration to the point where it more than likely cancel-out the total cow’s emissions for this gas.
Literature cited
1. Capper, J. L., R. A. Cady and D. E. Bauman. 2009. The environmental impact of dairy production: 1944 compared with 2007.J. Anim. Sci. vol. 87 no. 6 2160-2167.
2. Garcia, A.D. and J. G. Linn.Year. Dairy cows and the environment: Were we better off 83 years ago? J. Dairy Sci. Vol. 91, E-Suppl. 1: page #?
3. Hristov, A. N. 2012. Enteric methane emissions in the United States: Historic, preEuropean settlement, and present-day contribution of wild ruminants to enteric methane emissions in the United States. J. Anim. Sci. vol. 90 no. 4 1370-1375.
4. IPCC (Intergovernmental Panel on Climate Change). 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group 1 to the Third IPCC Scientific Assessment. [Houghton, J.T., et al. (eds.)]. Cambridge University Press , Cambridge, United Kingdom, 881 pp. 
5. IPCC. 2006. Chapter 10. Emissions from Livestock and Manure Management. Guidelines for National Greenhouse Inventories. Vol. 4. Agriculture, Forestry and Other Land Use. 10.1-10.87.
6. Sehtiya, H.L. and Goyal, S. S. 2003. Comparative uptake and assimilation of nitrate by excised roots and leaves from seedlings of C3 (barley) and C4 (corn) plants: effect of light, ambient CO2 and exogenously supplied sucrose. Ind. J. Plant Physiol. 7: 203-210.
7. US EPA. 2011. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2009, US Environ. Protection Agency, Washington, DC. http://epa.gov/climatechange/emissions/downloads11/US-GHG-Inventory-2011-Complete_Report.pdf. Accessed April, 2012.
Authors:
Alvaro Garcia
South Dakota State University
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Andrew Simmons
30 de abril de 2014
with the loss of 99.8% of the Bison in the years between 1850 and 1880 would not the IPCC graph on methane not have shown this reduction?
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