Dr. Chanhee Lee, Assistant Professor, Department of Animal Sciences, The Ohio State University
One goal of dairy producers is to obtain high milk yields and good profitability. Most producers also want to be good environmental stewards and having a good nutrient management plan is necessary to mitigate nutrient losses and environmental (air, soil, and water) impacts. Among the livestock sector in Ohio, dairy operations produce the greatest amount of manure (51 million lb/day, Figure 1). Inappropriate management of feed and manure causes significant nutrient losses, which possibly can influence local, regional, and global environments.
Figure 1. Proportions (%) of daily manure production by
livestock in Ohio (calculated based on Ohio livestock
populations on January 2014 with typical livestock manure
production; USDA, 2015; OSU Extension, 2006).
Nitrogen (N) is a nutrient excreted from dairy cows in great amounts and is a pollutant that can contaminate air, soil, and water when manure is not properly managed. For example, a dairy cow producing 90 lb/day of milk excretes 1 lb of N in manure while capturing only about 0.4 lb of N as milk protein. In Ohio, total N excreted from dairy cows is about 300,000 lb/ day. Once urine and feces are excreted and mixed, i.e., fresh manure, ammonia begins volatilizing immediately. During manure handling and storage before field application, considerable N is lost through ammonia emissions. Depending on environmental conditions, storage time, and type of manure storage, the loss can be 20 to 80% of the total N in fresh manure. Farm animals may contribute up to 50% of total anthropogenic ammonia emitted in the US (NRC, 2003). Although ammonia in fresh manure (mostly originating from urine) is a readily available N source for crops, when manure is applied to the field, most of the N is not available in manure at time of application due to rapid ammonia volatilization during storage. Nitrogen excreted in manure contributes to surface and ground water pollution through N runoff and nitrate leaching during field application of manure. In addition, ammonia volatilized during manure handling, storage, and land application can be re-deposited on surface water (and land), which increases N loading in lakes. After field application, N is also lost through nitrous oxide (N2O) emissions during the process of soil microbial nitrification and denitrification. Nitrous oxide emissions are another loss of valuable manure N for crops. Furthermore, nitrous oxide is one of the powerful greenhouse gases and is 298 times stronger in global warming potential compared with CO2. Dairy manure may contribute 33% of total N2O emitted from animal manure in the US (US EPA, 2015). Lastly, ammonia emitted from manure reacts with nitric and sulfuric acids in the atmosphere to form fine particulate matter with a diameter ≤ 2.5 μm (PM2.5; ammonium nitrate and ammonium sulfate). Atmospheric PM2.5 is one of the greatest environmental risks that directly affect human health (respiratory diseases; WHO, 2005). Farm animals in the North Central region might contribute up to 20% of the total PM2.5 in cool weather (Hristov, 2011).
Phosphorus (P) is an essential nutrient for lactating dairy cows. If dietary P supply does not meet the P requirement, production (e.g., milk yield) can decrease and health problems can increase. The P requirement for lactating dairy cows has been estimated to be 0.32 to 0.42% in dietary DM. When a dairy cow producing 90 lb of milk is fed a 0.35% P diet, about 0.07 lb of P is secreted in milk and 0.11 lb of P is excreted in manrue. If P concentration in diets increases, fecal P excretion increases linearly (Figure 2). P losses are negligible during manure handling and storage. Therefore, manure can be a good P source for crops. Nonetheless, high P concentrations in manure from feeding excessive dietary P can result in an oversupply of manure P to the field due to an imbalance between N and P concentrations in manure (low N and high P in manure relative to crop needs). When oversupplied to the field, P is lost from the field through runoff, which increases the risk of manure P causing water quality problems, such as eutrophication and harmful algae blooms. For example, Ohio EPA (2010) estimated that 89% of total P loads into the western basin of Lake Erie is from non-point sources (mostly agriculture), among which animal manure contributed 27% and commercial fertilizers contributed 66% (biosolids contribute 7%). For Grand Lake St. Mary’s (OH), farm animal operations are responsible for most P loads causing eutrophication and harmful algae blooms (Tetra Tech, 2010).
Figure 2. Relationship between fecal P output and
P intake (Alvarez-Fuentes et al., 2016).
Methane (CH4) is produced from dairy operations as an end-product of feed (in the rumen; enteric) or manure fermentation by microbes. Once CH4 is released from the rumen by belching or from manure, the greenhouse effect potential of CH4 is 25 times greater than that of CO2 in the atmosphere. The formation and release of CH4 from the rumen are considered a feed energy loss that accounts for 2 to 12% of total feed energy consumed. Theoretically, a decrease in enteric CH4 emission can increase the feed energy supply to cows. A cow producing 90 lb of milk and fed a typical North American diet (about 50 to 60% forage diet) releases 400 to 500 g/day of CH4. In 2013, dairy cows produced an estimated 1,660 kilotones of enteric CH4, which accounted for 7% of total anthropogenic CH4 emitted in the US (estimated by US EPA, 2015). The most important factor influencing enteric CH4 emissions in dairy cows is feed intake (Figure 3). A cow eating more feed produces more methane. However, milk production is also closely associated with feed intake (i.e., the more cows eat, the more milk is produced). When methane emission is expressed as a unit of product (milk, energy-corrected milk, or fat-corrected milk; intensity), cows eating more feed usually produce less methane than cows eating less. Forage and concentrate ratios in the diets are another factor, with high forage diets resulting in more methane being produced in the rumen. Considerable amounts of CH4 are also released from animal manure. Although the CH4 emissions are variable depending on manure handling and storage, dairy manure is the largest contributor to total CH4 emission from animal manure. The US EPA (2015) estimated that dairy manure contributed 52% (1,271 kilotones) of total CH4 emitted from animal manure (2,456 kilotones) in 2013. The contribution of CH4 emissions from dairy operations to total CH4 emissions in the US was 12%, and the contribution of dairy operations (enteric and manure) to total greenhouse gas emissions (CO2, CH4, and N2O) in the US (2013) was only 1.2% (CH4 and N2O emissions from manure). Therefore, dairy operations have relatively less impact on global warming potential compared with other environmental impacts from N and P.
Figure 3. Relationship between dietary dry matter intake and enteric methane production (FAO, 2013).
In summary, major nutrients lost from dairy operations are N and P. Nitrogen and P are lost during manure handling and storage in barns and during field application. Methane is emitted from the rumen as feed energy loss and manure. Because manure production from dairy operations is highest of the livestock operations in Ohio, efforts to minimize nutrient losses from dairy manure needs to be made. Since the contribution of animal operations to total greenhouse gas emissions in the US is relatively quite small, mitigating surface water (nutrient runoff) and air (ammonia emissions) pollution should be a primary focus in dairy operations.
Alvarez-Fuentes, G., J. A. D. R. N. Appuhamy, and E. Kebreab. 2016. Prediction of phosphorus output in manure and milk by lactating dairy cows. J. Dairy Sci. 99:1-12.
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Hristov, A. N. 2011. Technical note: Contribution of ammonia emitted from livestock to atmospheric fine particulate matter (PM2.5) in the United States. J. Dairy Sci. 94:3130-3136.
NRC. 2003. Air emissions from animal feeding operations: Current knowledge, future needs. The National Academy Press, Washington, DC.
Ohio EPA. 2010. Ohio Lake Erie Phosphorus Task Force Final Report. Ohio Environmental Protection Agency, Division of Surface water, Columbus, Ohio.
OSU Extension. 2006. Ohio Livestock Manure Management Guide. Bulletin 604, Ohio State University Extension, Columbus.
Tetra Tech, Inc. 2010. Recommended Actions for Grand Lake St. Marys, Ohio. Prepared for Ohio Environmental Protection Agency and U.S.
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