Strategies for Reducing Nitrogen Excretion
Conservation of N in animal production must begin by improving the N use efficiency of the animals. On dairy farms today, 20 to 30% of the N consumed by the herd is in the protein of the milk and meat produced, and the remainder is excreted in manure (Dou et al., 1996; Kohn et al., 1997; Oenema et al., 2001a). Pasture-fed dairy animals are at the lower end of this range, and pasture-produced beef have a N use efficiency of less than 10% (Hutchings et al., 1996). When finishing beef in a feedlot, about 10% of the N intake is retained in body tissue (Bierman et al., 1999). In poultry or swine production, where the protein needs of the animals can be more closely met, this efficiency may average 30 to 35% and even approach 40% (Mohan et al., 1996; Jongbloed et al., 1997; Lee et al., 1998; Lindberg and Andersson, 1998; Lenis and Jongbloed, 1999; Han et al., 2001). Through various management techniques, these utilization efficiencies can be increased. The maximum possible efficiency varies with animal species, age, stage of lactation, and so on, but this theoretical limit is about 50%.
Thus, large amounts of N are excreted in the production of all animal species (Table 1). By reducing excreted N, losses in all manure handling processes throughout the N cycle are potentially reduced. Nitrogen excretion is directly related to the animal’s N (protein) intake, so less protein must be fed per unit of production. Two general strategies can be used to reduce N excretion. The first is to reduce the protein fed by improving the match between the protein quality fed and that required by the animal. The other is to improve animal productivity. As more milk, meat, or egg product is produced per animal, the maintenance requirement of protein per unit of production is reduced. Thus, the animal product can be produced with less N consumed and excreted. Although improved productivity can increase N use efficiency, greater improvements are generally obtained through strategies that improve protein-feeding efficiency.
Improved Diet and Feeding
A common goal in feeding all animal species is to provide the right amount and quality of protein to maximize production at a minimum feed cost. As the balance between the protein fed and the animal’s requirement is improved, less N is excreted and production may be improved. The major deterrent in reaching a nearly perfect balance is feed cost and the resulting farm profit. Improved formulation of protein in rations normally requires the use of more-expensive feed ingredients. Less of this feed may be required, which can offset at least some of the increased cost, but often this leads to greater overall feed cost per unit of production. An economic incentive is required to encourage industry adoption of a feeding practice.
Feed protein generally refers to crude protein. Crude protein is defined as N content times 6.25, which assumes 16 g of N/100 g of protein in feeds. Proteins are composed of amino acids, which are required for the maintenance, growth, and productivity of animals (NRC, 1994; NRC, 1998; NRC, 2000; NRC, 2001). Matching the amino acid levels in rations to that required by the animal is very complex. There are 20 primary amino acids in proteins. Different amounts of each amino acid are required, and these amounts vary with animal age and other characteristics. Providing the precise amount of each amino acid required at any point in the animal’s life is essentially impossible, but improved feeding management can approach this goal.
Amino acids are generally classified as either essential or nonessential. Essential amino acids are those that must be obtained directly from the feed. Nonessential amino acids can be synthesized by the animal from other amino acids fed in excess in the diet. Since nonessential amino acid requirements are more easily met, the emphasis in formulating diets must be given to the essential group. Although there are similarities between ruminant and nonruminant species, different systems are used to balance feed protein levels with animal needs.
In nonruminants, such as poultry and swine, the optimal dietary pattern among essential amino acids that corresponds to the needs of the animal is often referred to as the ideal protein (NRC, 1998). This ideal protein or ideal amino acid pattern varies with the characteristics of the animal, such as sex, age, genotype, and production function (Han and Lee, 2000). The digestibility of each amino acid also affects its bioavailability to the animal, which further complicates the matching of available protein to animal needs. This has led to the use of the apparent ideal digestibility of various amino acids in feeds to match the animal’s requirements for those amino acids.
Dietary crude protein can be reduced through the supplementation of synthetic amino acids to reduce N excretion from pigs and poultry (Jongbloed et al., 1997; Sutton et al., 1999; Nahm, 2002). Reductions of 2 to 4 percentage units of dietary protein have been made without decreasing weight gain or feed conversion (Han and Lee, 2000). In a review of swine production data, amino acid supplementation with low-CP diets was found to reduce N excretion by 3 to 62%, depending on the size of the pig, on the reduction in dietary CP, and on the initial CP content in the control diet (Kerr, 1995; Sutton et al., 1999; Han et al., 2001). An average reduction in N excretion per unit of dietary CP reduction is about 8.5%. In a review of poultry data, dietary changes made to reduce CP content using synthetic amino acids reduced N excretion by 10 to 27% in broiler production and 18 to 35% in chick and layer production (Nahm, 2002). The production and use of synthetic amino acids for animal feed has grown rapidly over the past few years, with an average annual growth rate of 6% (Han and Lee, 2000).
With less N excreted, the potential for N loss is reduced. Most excess excreted N is in a form that is easily transformed into ammonia (Han et al., 2001), so the potential reduction in N emission during manure handling and storage is at least equal to and likely greater than the reduction in excretion. With a 41% reduction in N excretion, ammonia emission from a swine facility was reduced 47 to 59% (Kay and Lee, 1997).
Dietary carbohydrates can also be manipulated to reduce ammonia emission from swine manure. Including bacterially fermentable carbohydrates in diets has reduced the ratio between urinary N and fecal N (Lenis and Jongbloed, 1999). Because fecal N is less easily degraded to ammonia, emission is reduced. Slurry pH is also reduced, which further reduces the potential for urease activity and ammonia volatilization. Use of raw potato starch in the diets of growing pigs reduced ammonia emission by 13% (Lenis and Jongbloed, 1999).
A linear relationship was found between the intake of dietary nonstarch polysaccharides and ammonia emission (Cahn et al., 1998). For each 100-g increase in the intake of the nonstarch polysaccharides, slurry pH decreased by about 0.12 unit and ammonia emission decreased 5.4%. Thus, replacing cornstarch in the diet with components high in fermentable carbohydrates appeared to increase volatile fatty acid concentration in manure, which lowered pH. Urinary pH can also be reduced by manipulating the dietary cation-anion difference. By adding acidifying Ca salts to the diet instead of CaCO3, urinary pH was reduced by 1.6 to 1.8 units. This reduced ammonia emission by 26 to 53% (Lenis and Jongbloed, 1999).
As animals grow, their nutrient needs change. Phase feeding provides a strategy for improving the match of the diet to the growth stage of the animal. The largest benefit is attained going from a single phase to a two-phase strategy where a 10% reduction in N excretion was found (Hartung and Phillips, 1994; Han et al., 2001). Adding an additional phase to a two-phase feeding strategy for growing pigs was predicted to provide a 6% reduction in N excretion (Lenis, 1989). Multiphase feeding can be accomplished by mixing a diet high in N with a low-N diet in decreasing proportions each week throughout the growth period. Multiphase feeding reduced urinary N excretion in swine production by 15% (van der Peet-Schwering et al., 1996). With less excreted N, ammonia emission was reduced 17%. A combination of three-phase feeding and the use of synthetic amino acids reduced N excretion by more than 30% (Hartung and Phillips, 1994). Using separate diets for pregnant and lactating sows has also shown as much as a 24% reduction in N excretion over a yearly cycle (Lenis, 1989).
Other feeding strategies that have been studied to reduce N emissions include feed additives to inhibit urease activity or to bind ammonia (Lenis and Jongbloed, 1999). In theory, urease inhibitors will increase N utilization, reduce urea degradation, and thus reduce ammonia emission. Binding agents, such as clay minerals, should reduce the volatility of ammonia. Thus far, experimental work with these additives has shown small and inconsistent effects on ammonia emission (Lenis and Jongbloed, 1999).
The digestive process in ruminant animals includes the added step of rumen fermentation. Ruminally degraded protein (RDP) provides a mixture of peptides, free amino acids, and ammonia for microbial growth and protein synthesis (NRC, 2001). Ruminally synthesized microbial protein typically supplies most of the amino acids passing to the small intestine. Ruminally undegraded protein (RUP) is the second most important source of amino acids that are absorbable within the intestinal tract. Thus, the rate of degradation of proteins is very important in supplying the right amount and type of amino acids. Crude protein can be divided into five fractions (A, B1, B2, B3, and C) according to degradation rate (NRC, 2001). The A fraction, which includes nonprotein N, is essentially immediately available for degradation in the rumen, and the C fraction is considered nondegradable or never available. The remaining B fractions are potentially degradable true protein that fall within three ranges of degradation rate. Rumen degradable protein is defined to include the A component and a portion of the B components, and RUP includes the remainder of the B components and the entire C component.
The goal in feeding ruminants such as cattle is to supply the right amount of protein with the proper balance of degradation rates to provide the appropriate amino acids to the intestinal track. Most forage protein is highly degradable, so there is normally little problem meeting the RDP requirement. Total protein can be overfed to meet the minimum RUP requirement, but this leads to the excretion of considerable excess N. This excess N also requires energy to digest, which may reduce animal performance. The better solution is to feed rumen-protected proteins, which bypass the rumen but degrade within the intestinal tract. Industrially produced amino acids in pure form may also be fed to meet specific amino acid needs.
A direct relationship between N excretion and protein intake is well documented. Protein needs of cattle, particularly lactating cows, are normally best met by reducing the total protein in the diet through the supplementation of feeds high in RUP (NRC, 2001). As the balance between animal needs and feed availability is improved, animal performance is normally maintained and perhaps improved. With all diets balanced to meet the RDP and RUP requirements of lactating dairy cows, urine N excreted from a high-protein diet (18% CP) was 2.3 times greater than that from a low-protein diet (12% CP; Tomlinson et al., 1996). Fecal N excretion was only 25% greater using the high-protein diet, which illustrates that excess protein is primarily excreted in urine. In a similar experiment comparing low- (14% CP) and high- (19% CP) diets, laboratory-measured ammonia emissions were about three times greater from the manure excreted on the high-protein diet (Frank et al., 2002). In an experiment in which ammonia emissions were monitored through various storage practices, emissions were reduced 70% using a low (12.5% CP) diet compared to a high (17.5% CP) diet (Külling et al., 2001). Nitrous oxide emissions from stored manure were also reduced using the diet of lower protein.
A simulation study by Rotz et al. (1999b) demonstrated the potential whole-farm benefit of improved protein feeding. Compared to the use of soybean meal as the sole protein source, the use of a feed mix with lower RDP reduced N excretion by 39 kg/cow per year. This reduced volatile N loss from the farm by 27%, with a small reduction in N leaching loss. Using the more expensive but less degradable protein feed improved the annual net return of the farm by $45 to $70/cow depending on other management strategies used. Using a more precise or ideal protein feeding strategy only reduced volatile N loss an additional 9%.
Group-feeding cattle of similar age or stage of lactation provides a strategy to better match the diet to the animal’s nutrient needs. An optimum of six milking animal groups was found for a simulated farm (St-Pierre and Thraen, 1999). Compared to feeding the milking herd in a single group, the use of the six-group strategy reduced N excretion by 8%. In a full lactation study with high-producing dairy cows, a reduction in dietary protein from 17.5 to 16% was possible around wk 30 of lactation without an adverse affect on production (Wu and Satter, 2000). This reduction in protein fed reduced N excretion by about 14% over the full lactation.
The type and amount of forage fed can also affect protein intake and N excretion by cattle. Alfalfa silage is normally very high in degradable protein. In contrast, corn silage is low in total protein and this protein is generally less degradable. Dairy cow diets balanced to meet protein and energy needs using moderate to high amounts of corn silage have provided milk productions similar to those attained with alfalfa-based diets with lower total protein intake and greater protein use efficiency (Dhiman and Satter, 1997). In this full lactation study, high-producing dairy cows on a diet in which one-third of the forage was corn silage excreted 6% less N compared to cows with all of their forage coming from alfalfa silage. With two-thirds of the forage from corn silage, N excretion was reduced by 15%. In a whole-farm simulation study, substituting corn silage for alfalfa silage provided up to a 30% reduction in N excretion for the dairy herd, improving the overall farm balance of N available to that required for crop uptake (Borton et al., 1997).
Diet can also affect the portion of the total N excreted in feces vs. urine. Feedlot cattle fed a diet with 7.5% roughage had 7% more of the total N excreted in the feces, and feeding wet corn gluten feed gave 12% more of the total N in the feces compared to an all-concentrate diet (Bierman et al., 1999). Fecal N has the advantage of being less volatile in the feedlot and during manure handling. In this experiment, however, total N excretion and volatile N loss were lowest with the all-concentrate diet.
Improved Production Efficiency
Steps taken to improve the productivity of animals (rate of gain, milk, or egg production) will normally improve feed efficiency. Thus, growing animals will reach their weight goal faster or gain more weight over a fixed period. Either way, more product is obtained per unit of feed expended on maintenance of the animal. Van Heugten and van Kempen (2000) determined that improving feed efficiency in swine production by 0.1 point (lowering feed per unit of gain from 3.0 to 2.9) resulted in a 3.3% reduction in nutrient excretion (assuming similar growth and nutrient retention). For dairy cows, increasing milk production yields more milk per unit of feed required for animal maintenance. Although increased production often requires increased feed intake, the net result is normally less N intake and excretion per unit of milk produced. A simulated 25% increase in milk production was found to reduce N excretion per unit of milk by 8% (St-Pierre and Thraen, 1999). Reductions in N excretion that are achieved through improved production efficiency are generally small, so such strategies have a lesser role for potential reductions in the N that is lost to the environment.
Productivity increases in dairy production can be obtained using numerous strategies, including genetic, feeding, and handling improvements. Hormone and other injections and various feed additives can also be used. Bovine somatotropin injection, increased milking frequency, and extended photoperiod have all improved milk production. Simulation of these three strategies predicted reductions in herd N excretion per unit of milk produced by 7.8, 7.0, and 3.6%, respectively (Dunlap et al., 2000). Combining all three strategies decreased excreted N per unit of milk by about 15%. If this N is efficiently cycled through the farm, the reduction in what is lost to the environment should be similar or slightly greater than the reduction in excretion. On a simulated dairy farm, a 20% increase in milk production decreased volatile N loss per unit of milk produced by 12% and leaching loss by 16% (Rotz et al., 1999b).
Improving the overall longevity of animals in the herd or flock can also improve N use efficiency a small amount. Steps taken to reduce the replacement rate or age at first production in dairy, cow-calf, and layer operations will allow a given production with the growth and maintenance of fewer replacement animals. For example, reducing the replacement rate of a dairy herd from 35 to 30% will reduce the required replacement animal numbers by 15%. On a farm where all replacements are raised, the manure N excretion for the herd will be reduced by about 5% with about 6% less N loss from the farm (Rotz, unpublished simulation data).
Feed processing can influence feed intake, digestibility, animal production, and the excretion of N. Common processing techniques include grinding and pelleting. Grinding to obtain the proper particle size is particularly important in maximizing feed efficiency in swine and poultry feeding (Nahm, 2002). Reducing particle size from 1,000 to 400 µm was found to improve nutrient digestibility, which reduced N excretion in the manure of finishing pigs by about 30% (Han et al., 2001; Nahm, 2002). For each 1% improvement in digestibility, N waste per kilogram of meat produced was decreased about 1.4% (Han et al., 2001). Pelleting of feeds may also improve average daily gain and the feed efficiency in swine production by about 6% (Hancock et al., 1996; Van Heugten and van Kempen, 2000; Han et al., 2001). This processing of the feed has reduced DM and N excretion in the feces by about 22%. Considering that 20 to 25% of the total N excreted is in the feces, overall N excretion is reduced about 5%.
Feed additives, such as enzymes, antibiotics, probiotics, organic acids, and growth hormones, may also reduce N excretion. Enzymes can improve feed digestibility and nutrient availability. They have been used in swine diets to improve feed efficiency, but their effect on N excretion appears to be small (Han et al., 2001). Antibiotics, probiotics, and organic acids have all been used to improve feed efficiency in swine production. Their effect on N excretion was often a 5% reduction or less, but reductions of up to 25% have sometimes been reported (Han et al., 2001). Growth hormones have had greater effects, reducing N excretion by 12 to 38% (Han et al., 2001).