Housing and Manure Removal
Nitrogen loss begins soon after manure is excreted by the animal. For swine and cattle, urine N is primarily in the form of urea. In poultry, excretions are high in uric acid, which is transformed into urea through aerobic decomposition. When mixed with urease enzymes prevalent in fecal material, urea N can quickly transform into ammonia, which is highly volatile and easily diffused into the surrounding air (Monteny and Erisman, 1998). The rate of this transformation is a function of the total ammoniacal (ammonia plus ammonium) N content, temperature, moisture content, and the pH of the manure (Hartung and Phillips, 1994; Sommer and Hutchings, 1995). As just presented, the ammoniacal N portion excreted is primarily related to the way animals are fed. Urease activity is relatively low below a temperature of 10°C, but it increases exponentially over higher temperatures. If the pH of manure is held below 6, ammonia release is low. At normal pH levels of 7 or more, the ammoniacal N is readily volatilized as ammonia. Urease activity and ammonia loss are also reduced by drying manure to less than 40% moisture content (Groot Koerkamp, 1994; Sommer and Hutchings, 1995). Under typical temperature, pH, and moisture conditions, urea transformation is very rapid, reaching a maximum rate within 2 h of deposition (Monteny and Erisman, 1998). Because of the extra transformation steps required for uric acid, ammonia loss from poultry manure is slower, requiring a couple days to reach maximum transformation and volatilization rates (Groot Koerkamp, 1994).
As ammonia is produced, other factors that control ammonia loss include the exposed manure surface area and the air movement across this surface (Hartung and Phillips, 1994; Sommer and Hutchings, 1995). Depositing manure to a greater depth creates less surface area. With less exposure, ammonia has less opportunity to escape. The volatilization rate from the manure surface is a function of the ammonia mass transfer coefficient and the difference in concentration or partial pressure of gaseous ammonia between that at the surface and that in the ambient air. This mass transfer coefficient is related to the temperature and velocity of the air at the surface. With air movement, more ammonia is carried away from the microenvironment near the surface, increasing the volatilization rate.
Housing design and manure removal practices have a large effect on the rate of N transformation and loss. Strategies are available that reduce this N loss, and new techniques are being developed. Because of the differences in housing design and animal handling among poultry, swine, and cattle, each species will be discussed separately.
A common poultry housing system is the high-rise laying hen house. Normally, excreted manure drops from the cage to the floor or into a pit. This manure is often removed annually, so the housing system includes long-term storage. Composting can occur during this storage period, stimulating greater N emissions (Groot Koerkamp, 1994). Average annual N losses from this type of facility are reported to be about 50% of the total N excreted (Table 2). Use of a deep litter housing system also produces relatively high N loss, but this loss may be half that of the high-rise structure (Groot Koerkamp, 1994; van Horne et al., 1998).
The most effective approach to controlling N loss in poultry housing is to increase the manure DM content (Groot Koerkamp, 1994
). Depositing excreted manure onto a belt or dropping board allows partial drying. In commercial layer farms, the use of catching boards and scrapers reduced volatile N loss from the facility by up to 60% (Yang et al., 2000
). Nitrogen loss across four facilities using different manure handling designs was proportional to the DM content of the stored manure in each. Use of dropping boards scraped twice daily increased manure DM content to 72% compared to a DM content of 50% or less when dropped directly into a pit. Nitrogen loss was 25% of the N fed for the system, producing the drier manure and 41% with the wetter manure. Minimum emission is achieved if a manure DM content of 60% is reached within 50 h after excretion (Groot Koerkamp, 1994
Ammonia loss in the housing facility can be greatly reduced by frequently removing manure from the facility. Manure can be removed by belts or flushing. Belt removal twice a week with or without drying can reduce ammonia emission from the housing facility by 60 to 90%, and more-frequent removal can provide an additional reduction (Groot Koerkamp, 1994; Hartung and Phillips, 1994; van Horne et al., 1998). Flushing provides an estimated 80% reduction (van Horne et al., 1998). Once removed from the structure, though, scraped or flushed manure must normally be stored up to a year, during which continuing loss will occur. Thus, these loss reductions do not describe the full system. Storage losses will be discussed further in the next section. Through simulation of full production systems, the use of manure removal belts with partial manure drying was found to reduce total N emission by 21% (van Horne et al., 1998).
Aviary housing systems have been developed in Europe to promote improved welfare for laying hens. These systems provide greater freedom of movement for the hens, nest boxes, and a dust bathing area with litter. A disadvantage is that these housing systems have up to three times the ammonia emission compared to traditional battery cage systems with daily manure removal (Groot Koerkamp and Bleijenberg, 1998). The increased emission is primarily due to ammonia volatilization from the litter. A litter drying system was tested that reduced ammonia emission to a level slightly lower than that of the traditional cage system (Groot Koerkamp et al., 1998).
Filtration to remove ammonia from air exiting the housing facility is another option for reducing ammonia emissions. Emission reductions of up to 80% can be achieved through biofiltration or scrubbing of the air (van Horne et al., 1998). Simulation of this approach predicted a 26% annual reduction in ammonia emission from a typical layer facility. Practical application of this technology is constrained due to a relatively high cost and technical problems created by the large amount of dust in poultry houses (Groot Koerkamp, 1994).
Swine and cattle can be produced on a deep-litter system, in which bedding such as straw or sawdust is used to absorb and cover urine and feces. A complex decomposition process occurs in the deep litter. The primary microbial processes include aerobic and anaerobic degradation of organic matter, urea hydrolysis, nitrification, denitrification, and N immobilization (Jeppsson, 1999). The bedded pack normally provides 3 to 12 mo of manure storage, with ammonia volatilization occurring throughout this period (Table 2). Microbial processes of nitrification and denitrification can also convert ammonia into inert dinitrogen gas. Normally, conditions are such that these processes do not run to completion, and nitrous and nitric oxides are produced and emitted (Groenestein and Van Faassen, 1996).
Reports on the N emissions from deep-litter or bedded systems are inconsistent. Some report less loss whereas others report more compared to systems that do not use bedding (Hartung and Phillips, 1994). Proper management in tending the bedding can likely reduce ammonia emission. Two deep-litter systems using various amounts and types of sawdust were compared to a traditional slatted floor in fattening pigs (Groenestein and Van Faassen, 1996). In this study, the deep-litter systems were able to reduce ammonia emission by as much as 50% compared to the use of slatted floors; however, total N loss was greater due to the loss of nitrous oxide. Thus, deep litter systems were not environmentally beneficial.
The most common swine housing facility uses a slatted floor to allow fecal and urine excretions to drop into a pit below the floor for removal. The portion of the floor area that is slatted affects ammonia emission. Reducing the slatted floor area from 50% of the pen floor to 25%, reduced ammonia emission by 20% in the rearing of pigs (Aarnink et al., 1996). During the fattening of pigs, emission was reduced by only 10% with this change in floor design. Aarnink et al. (1996) concluded that reducing the slatted floor and slurry pit area in swine housing decreased ammonia emission from the slurry pit but increased the fouling and emission from the floor. Combining a partially slatted floor over a slurry channel with a sloped floor that is flushed several times a day reduced ammonia emissions by 30% (Hartung and Phillips, 1994). By maintaining flushing liquid in the channel, up to a 70% reduction in ammonia emission was maintained relative to a traditional slatted floor (Hartung and Phillips, 1994).
The ventilation rate of the housing facility can affect the rate of ammonia emission. Emission from pig slurry stored under a slatted floor was approximately doubled by increasing the air exchange rate from two air changes per hour to seven (Hartung and Phillips, 1994). Air temperature must also be considered, though, because warmer temperatures in the facility due to low ventilation rates will increase volatile loss. Thus, optimal management of air temperature and ventilation can reduce N loss while maintaining or improving animal productivity.
Ammonia concentrations in swine confinement buildings have been reduced using an acid and oxidation treatment (Jensen, 2002). The treatment included spraying a water and sulfuric acid mixture on the floor, adding sulfuric acid to the manure, and oxidizing the manure. Ammonia concentrations in the facility were reduced from 8 to 10 ppm down to 1 to 2 ppm and pig performance was improved. The reduction in ammonia emission kept more N in the manure, improving its fertilizer value.
Common housing systems for cattle include tie stall barns, free stall barns, and open feedlots. The lowest ammonia emission is normally found in a tie stall facility (Table 2). Because animal movement is limited, most manure is deposited in a relatively small area and normally in a deep gutter. With less exposed surface, volatile loss is reduced. In addition, manure is normally removed on a daily basis, allowing less time for loss to occur. In an evaluation of 34 dairy farms in Sweden, ammonia concentrations in tie stall barns with solid manure handling were about half those in free stall barns with liquid manure (Swensson and Gustafsson, 2002).
Deep litter or a bedded pack is sometimes used, particularly for housing young stock. In a comparison of a bedded area and a flat solid floor with manure removed three times a week, ammonia emission from the solid floor was 40% of that from the bedded area (Jeppsson, 1999). Use of a 60% peat and 40% chopped straw bedding mixture reduced ammonia emission by 57% compared to long straw, which provided an emission similar to that from the solid floor. As stated earlier, a major concern with deep litter is that incomplete nitrification and denitrification processes produce two environmentally undesirable gases, nitrous oxide and nitric oxide.
The most common housing for dairy cows, particularly on larger farms, is the free stall system. Cubicles are provided for resting, and animals have the freedom to move to the feeding area on an open floor. Manure is primarily deposited on the open floor. Common floor designs are a solid floor that is frequently scraped or flushed or a slatted floor from which manure drains between the slats into a pit below. Nitrogen emission from these floor systems is nearly all in the form of ammonia. Under warm conditions with fecal and urine materials well-mixed on the floor, N loss is high, with most of the urea transforming into ammonia and volatilizing to the atmosphere. Under cold winter conditions, N loss is relatively low. On average, about 16% of the excreted N is lost from the free stall area (Table 2). Loss from a slatted floor may be a little greater than that from a scraped floor (Kroodsma et al., 1993; Monteny and Erisman, 1998), but the pit underneath the floor in this type of facility normally provides long-term manure storage. Thus, a complete comparison must include storage loss as well.
On solid floors, floor shape and surface characteristics can influence ammonia loss. A small, 3% slope of the floor allows urine to drain away from the feces, reducing ammonia emission by 21% compared to solid or slatted level floors (Braam et al., 1997a). A double-sloped floor with a urine gutter in the center reduced ammonia emissions by 50% (Braam et al., 1997b). Spraying this floor with water following scraping provided further reduction, with a total emission reduction of 65% compared to the reference floors. A grooved solid floor system was evaluated that included perforations through the floor. The grooves enabled urine to move away from fecal material and then drain through the floor perforations. Compared to a traditional slatted floor system, the grooved and perforated floor reduced ammonia emissions by 46% (Swierstra et al., 2001). Using smoother, coated floors reduced urease activity but had little effect on overall ammonia emission (Braam and Swierstra, 1999).
Manure is removed from solid floors by mechanical scraping or flushing with water. Scraping of solid or slatted floors appears to have little effect on ammonia emission (Kroodsma et al., 1993). The spreading and mixing of feces and urine caused by scraping maintains rates of urease activity and ammonia emission that are similar to those of a manure-covered floor. Thus, increasing the scraping frequency had little effect on ammonia emission (Braam et al., 1997a). Flushing reduced ammonia emission up to 70% immediately after the flush, and short and more-frequent (every 2 h) flushing cycles provided the greatest reduction (Kroodsma et al., 1993). Further analysis indicated that flushing could reduce average annual ammonia emissions from the facility by 14 to 17% (Ogink and Kroodsma, 1996). Compared to traditional scraped or slatted floor systems, the amount of manure was roughly doubled through the inclusion of the flush water.
Greater reductions in ammonia emission can be obtained with less flushing solution by adding a disinfectant or acid to deactivate or slow urease activity. Spraying a dilute formaldehyde solution on slatted or sloped solid floors has reduced long-term ammonia emissions by 50% and 87%, respectively (Ogink and Kroodsma, 1996; Monteny and Erisman, 1998). Reductions of up to 60% were achieved by a combination of the acidification of slurry in a shallow pit and regular flushing of the slats with the acidified slurry (Monteny and Erisman, 1998). Animal and human safety is a major concern when using these treatments, so a safe flushing solution must be developed to allow practical application of this kind of technique.
The greatest housing N loss occurs when cattle are on a feedlot (Table 2). Losses of 40 to 90% of the excreted N are reported by the time feedlot pens are cleaned (Eghball and Power, 1994; Bierman et al., 1999). Most of this loss is emitted into the atmosphere, but portions are also lost through runoff from rain and leaching into the soil below the feedlot. In the drier climate of the Great Plains, 3 to 6% of the excreted manure N is lost in runoff (Eghball and Power, 1994). Although nitrate leaching into groundwater does not appear to be a problem, very high nitrate-N levels have been measured in soils under abandoned feedlots (Eghball and Power, 1994). In a comprehensive feeding experiment, Bierman et al. (1999) found that 9 to 19% of the N excreted by cattle on various finishing diets was removed in the manure scraped from the lot at the completion of the study. Nitrogen lost in runoff was 5 to 19% of the excreted N, with 10 to 16% leached into the soil. The remaining 57 to 67% was assumed to be lost by volatilization. Volatile loss would primarily be in the form of ammonia, but denitrification products would also occur.