The livestock sector has been reported to be responsible for 18 percent of the total anthropogenic greenhouse gas (GHG) emissions on a worldwide basis. The GHGs considered in this publication are carbon dioxide, methane and nitrous oxide.
It is important to remember that this is a global estimate and does not differentiate between animal types, production systems or animal productivity. A recent report from the same organization used a life cycle assessment approach to estimate GHG emissions from the dairy sector. In this report, the global dairy sector contributed 2.7 to 4 percent of the total anthropogenic GHG emissions.
The lower figure includes emissions associated with milk production, processing and the transportation of milk and milk products. The higher figure adds the emissions related to meat production from culled or fattened animals. This report also provides information on regional emissions.
Regional emissions range from 1.3 to 7.5 kilogram CO2 equivalent per kilogram of fat- and protein-corrected milk (FPCM). North America and Europe had the lowest GHG emissions per kilogram FPCM. The highest value was in sub-Saharan Africa. In this report, the U.S. provided about 16 percent of the total world milk production but about 8 percent of the total GHG emissions associated with the dairy sector.
Total GHG emissions in the U.S. increased by 13.5 percent from 1990 to 2008. However, there was a 2.9 percent decrease between 2007 and 2008, due primarily to changes in fuel used in the transportation sector. The agricultural sector accounted for 6 percent of the total U.S. GHG emissions in 2008. During this same time period, methane emissions decreased by 7.5 percent and nitrous oxide emissions declined by 1.3 percent. Enteric fermentation from ruminants accounted for 25 percent of the total methane emissions in 2008. This is a 6.4 percent increase since 1990. Dairy cattle were responsible for about 23.5 percent of enteric methane emissions while beef cattle accounted for 71.5 percent of these emissions.
Even though it is not a GHG, EPA also has an inventory of ammonia emissions from animal operations. In this report, they estimated ammonia emissions from 2002 through 2030. Total ammonia emissions from animal operations were reported as 2.42 million tons in 2002. This was projected to increase to 2.67 million tons in 2030. Dairy and beef cattle accounted for 23 and 27 percent of these total emissions in 2002. The total N lost as NH3 from dairy operations was estimated to be 38 percent.
Nitrous oxide (N2O) is a concern since it has a global warming potential (GWP) of 310 times that of carbon dioxide. The majority of the N2O emitted on dairy farms is from soil and manure. The quantity of nitrous oxide emitted directly from dairy cattle is very small. A recent study from Japan estimated that the daily emission of N2ON from cattle was 5.2 (range 2 to 9.9) milligrams per day.
A paper from California reported a N2O emission rate of about 0.02 grams per cow per hour for cows housed in environmental chambers. A simulation model approach has also been used to examine total N2O emissions from a dairy farm. This model used a 100-cow dairy herd housed in a freestall barn with milk production of 19,800 pounds per cow per year. Manure was stored as slurry and spread on the cropland twice per year.
The base run had a total yearly N2O emission of 681 kilograms. This was divided into 485 kilograms from crop production and 197 kilograms from manure storage. The total yearly N2O emissions could be reduced to 421 kilograms per year by making more efficient use of the nitrogen fertility program and using a cover crop on the corn land.
Agriculture is not identified as a major source of carbon dioxide (CO2) emissions. However, CO2 emissions do occur on farms, due mainly to animal respiration and decomposition of soil organic matter. These same authors conclude that CO2 emissions from animal respiration accounts for about 90 percent of the total carbon dioxide emissions on a dairy farm.
The average daily CO2 emission for dairy cows producing 63 pounds of milk per day was 6,137 liters. The range was 5,042 to 7,427 liters per day over a six-month monitoring period. A recent paper reported the CO2 emissions using data summarized from the USDA Energy Metabolism Unit. This is a large dataset obtained from animals using the indirect respiration chambers.
This includes more than 1,200 individual lactating cow trials with an average daily milk production of 51 pounds per day. The range in milk production was from 11 to 125 pounds per cow per day. Key points from this paper are:
• The average daily CO2 emission was 5,309 liters per day (range = 2,035 to 8,682).
• Daily CO2 emission was highly related to milk production and dry matter intake. Higher-producing cows had higher CO2 emissions.
• The average CO2 emission was 0.14 grams per kilogram of milk per day. The range reported was 0.96 to 0.54 grams per kilogram milk.
• Higher-producing cows had lower emission factor per unit of milk produced.
Total farm CO2 emissions were determined on the model farm described above using a simulation model. The net yearly CO2 emission was 150,479 kilograms. This increased by 22 percent if the farm increased alfalfa acres and decreased corn acres. However, if corn replaced all of the non-permanent grassland, yearly CO2 emissions were lowered to 35,198 kilograms.
Ammonia emissions from agriculture are receiving attention due to air quality concerns. More importantly, ammonia emissions represent losses of N from the farm and usually indicate a lower efficiency of N use. The primary means of ammonia emissions by ruminants is as a result of conversion of urea-N in urine to ammonia. The following points summarize this process:
• 30 to 70 percent of the total manure N excreted by dairy cattle is in the urine.
• 50 to 90 percent of the total N in the urine is present as urea.
• The fecal portion of the manure contains an enzyme called urease.
• The urease enzyme rapidly converts the urinary urea-N to ammonia.
• This enzymatic conversion is affected by both pH and temperature. The enzyme exhibits more activity at higher temperatures and a pH of 6.8 to 7.6. Enzyme activity is reduced when pH is either lower or higher than this range.
A key factor in reducing ammonia losses on a dairy farm is to balance rations decrease N excretion in manure.
Figure 1 contains an example of the relationships between N intake and manure N excretion. Note that manure N excretion increases with the higher CP rations.
Figure 2 contains the N excreted in the milk, urine and feces from this same trial. It is interesting to note that daily N excretion via milk and feces is relatively constant across this range of ration CP levels.
Milk production of the cows in this short-term trial was about 80 pounds per day and was not significantly different between ration CP levels. The main route of excreting N as ration CP increased via the urine. The urea-N proportion of the total urinary N increased from 55.4 to 81.8 percent as ration CP increased. This would indicate a higher potential for ammonia volatilization with higher ration CP levels.
How do these higher urinary urea-N (UUN) levels correspond to ammonia emissions? Growing dairy heifers were fed total mixed rations with 9.6 or 11 percent CP. Heifers fed the lower CP ration consumed 14 percent less N per day and had a 28.1 percent decrease in ammonia emissions. There was a linear relationship between N intake (grams per day) and UUN excretion (grams per day).
The relationship between daily N intake and NH3 emissions was also linear. Both UUN excretion (grams per day) and ammonia emissions (grams N per cow per day) increased as ration CP levels for lactating dairy cows increased from 15 to 21 percent CP.
The relationship between milk urea nitrogen (MUN) and ammonia emissions have also been reported. A number of papers have reported positive relationships between MUN and ammonia emissions. The results from these papers indicate that equations to predict ammonia emissions could be included in models using either UUN or MUN as the predictive function.
A major nutritional variable that influences N use efficiency and potential ammonia emissions is the rumen balance of RDP relative to requirements. One paper examined this question using diets with 12.9, 13.4 and 15.4 percent CP for dairy cows producing about 66 pounds of milk per day. Urinary N excretion and cumulative manure NH3 emissions were reduced on the lower CP diets.
Another paper used a large number of diets varying in alfalfa (as percent of the forage fed), starch and MP levels to evaluate N metabolism in dairy cows. Alfalfa comprised between 25 and 75 percent of the total forage fed. Diet starch levels ranged from 22 to 30 percent. These diets contained 10.7 percent RDP but MP ranged between 8.8 to 12 percent of diet DM. Ammonia produced per gram of manure increased as diet MP increased. As higher levels of alfalfa were fed, the ammonia produced per gram of manure decreased. Ammonia produced per unit of manure was lowest and milk protein yields were highest for a diet containing 75 percent of the total forage as alfalfa, 11 percent MP and 30 percent starch.
The carbohydrate balance of the ration can interact with ration N to alter ammonia emissions. One trial reported that replacing ground corn with steam-flaked corn lowered manure ammonia emissions. A trial was done using high CP diets (22 percent) for late-lactation cows that contained different carbohydrate sources. They found that providing a rapidly fermentable carbohydrate source lowered rumen ammonia concentration and shifted some of the excreted N from the urine to the feces.
A number of papers have reported estimated annual ammonia emissions for dairy cattle. A yearly emission factor of 84 pounds of NH3 per dairy cow per year was reported by EPA (2004). An annual emission of 88 pounds NH3 per dairy cow per year was reported from on-farm research on a 185-cow dairy in Washington. Three dairy herds in Wisconsin were monitored and annual NH3 emissions of 41.8 to 44 pounds per cow were determined.
A paper from Idaho reported an annual emission estimate of 125 pounds per cow. An annual NH3 emission factor of 20.7 pounds has been reported for an open lot dairy in Texas. These papers indicate some variation in these emission estimates. These differences are probably related to a number of factors including measurement technique, ration fed and environmental conditions.
When EPA moves ahead with ammonia emission regulations for dairy and livestock farms, it will be critical to have a uniform and accepted method of estimating ammonia emission factors, since on-farm monitoring is probably not realistic. A process-based model will most likely be needed to determine ammonia emission factors.
Methane is the GHG that has recently been receiving the most attention in popular press articles. Enteric methane emissions are produced by ruminants as a result of microbial breakdown of carbohydrates in the rumen. It is important to point out the changes that have taken place in methane emissions by dairy cattle over time. Table 1 contains data on methane emissions from dairy cattle in the U.S. in 1944 and 2007. The data in this table is only for milking cows and does not include replacement heifers and dry cows.
Methane production was calculated using the CNCPS 6.1 model. A 2009 study reported a 43 percent reduction in methane emissions when comparing dairy production systems of 1944 with 2007. The study included both dry cows and heifers in addition to milking cows. They also made adjustments for breeds and forage feeding systems.
The main sources of methane emissions on dairy farms are enteric emissions and manure. Enteric methane accounts for about 75 percent of the total on-farm methane emissions. Mean daily methane production was reported as 587 liters per day for cows averaging 63 pounds per day. Other studies have reported daily methane emissions ranging from 420 to 763 liters per cow per day.
A second approach is to express methane production as a percent of the gross energy intake. A 2008 study indicated that 6 to 10 percent of the total energy intake was emitted as methane. The average methane production for lactating dairy cows was 5.49 percent of gross energy (GE) intake using the USDA Energy Metabolism Unit database. The range was 2.53 to 7.82 percent of GE. These same workers reported an average methane production of 7.89 percent of GE for dry cows, with a range of 3.47 to 10 percent. It is not clear how low this value could be and still maintain rumen function and milk production.
A simulation approach was used to examine methane emissions from dairy farms. The same model dairy farm as described in the nitrous oxide portion of this paper was used in this simulation. This model indicated that the annual methane emission factors for the milking cows, dry cows and replacement heifers were 233, 127.6 and 169.4 pounds per animal unit. The weighted average value for the whole herd was 312 pounds per cow per year.
Total yearly whole-farm methane emission was 46,624 pounds with 68 percent of this from the animals and 32 percent from manure storage. These base runs used a ration that was 50 percent forage. Annual methane emissions increased by 15.6 percent if a ration containing 60 percent forage was fed. The annual methane emission was reduced by 8 percent if a high-forage diet and seasonal grazing was used. The enteric methane emissions were increased by 2.3 percent when the high-forage diet plus grazing was used. However, there was a 71 percent reduction in methane emissions from manure when this management change was implemented.
There are a number of strategies that can be used to alter methane emissions on dairy farms. These include using higher-quality forages, feeding higher grain diets, using ionophores and the addition of various fats or oilseeds to rations. However, there has been some variation in the amount of methane reduction associated with these in reported research studies.
These diet alterations are already being used in many herds. In addition, there are a large number of proposed additives that could be added to diets to lower methane emissions. These include yeasts, tannin extracts, essential oils, fiber-degrading enzymes, saponins and compounds such as garlic or oregano. While all of these may have potential to reduce methane emissions, additional research is needed before they will be routinely added to dairy rations.
The dairy cow does emit considerable quantities of carbon dioxide, ammonia and methane. However, minimal quantities of nitrous oxide are emitted by the animal. There will be continuing pressure on the industry to further reduce ammonia and GHG emissions. There are a number of basic nutritional principles that can be used to approach this situation.
The use of simulation models will also be important as part of this evaluation process to “estimate” the potential shifts in emissions due to alterations in variables such as level of milk production, forage quality, level of forage in the ration, rumen nitrogen and carbohydrate balance and feeding system (TMR, grazing, etc.). It will be important to evaluate the whole-farm response in addition to the animal component.
The whole-farm approach permits an evaluation of changes in emissions due to shifts in cropping programs, manure storage and manure application procedures. A key consideration in this process must be farm profitability and sustainability. It will also be critical to estimate the number of dairy cows and milk production levels in future years. Many of the larger-scale emission reductions are looking ahead 10 to 30 years. Thus, any future reductions will be a combination of changes in emissions per animal and the total number of animals in the population. PD
References omitted due to space but are available upon request.
—Excerpts from Cornell Nutrition Conference proceedings: http://www.ansci.cornell.edu/cnconf/2010proceedings/index.html
Department of Animal Science