Reducing the environmental impact of swine production through nutritional means

Livestock production is becoming more concentrated in many parts of the world and pork production is no exception. This is particularly evident when examining a recent Canadian report (Saskatchewan Agriculture Food and Rural Revitalization, Statistics Canada, 2003) showing the pig densities per square mile in different countries of the world (Table 1).

More and more people residing in rural areas are not accustomed to practices associated with crop and livestock production. Certainly, producers want to remain profitable in order to continue in the pork business in the future. However, most if not all pork producers also maintain the goal of producing pork in a socially acceptable and environmentally sound manner (Coffey, 1999). The actual numbers of pigs produced has not changed drastically since 1900 (Table 2). In fact, the environmental impact of pork production is likely less today than 100 years ago when examined on a per pig basis. However, the same number of pigs is being produced on fewer farms, which increases the environmental risk in a concentrated area (Table 2). In 2000, 51% of the pigs marketed were coming from operations that marketed more than 50,000 pigs annually (National Pork Board, 2002).

The general public is increasingly concerned with the environmental circumstances under which food is produced. Public concerns center around soil (accumulation and runoff of minerals from land where manure is applied), water (surface and ground water), and air quality environmental issues. Federal, state, and local laws have and will continue to impose regulations designed to address the environmental concerns associated with crop and livestock production. Jongbloed and Lenis (1998) reported that many countries have limited the use of livestock manure or the number of animals per hectare of land. Livestock producers must implement these regulations in times when margin or profit per animal sold continues to decline (National Pork Board, 2002). The combination of laws, neighbor relations, and economic concerns along with the concentration of pork and other livestock production brings about challenges for producers. The objective of this paper is to review the nutritional methods to reduce the environmental impact of swine production.


Table 1. Pig density for selected countries1.


¹Adapted From Saskatchewan Agriculture Food and Rural Revitalization, Statistics Canada.


Table 2. Inventory of pigs, number of operations and lean meat produced per pig in the US from 1900 to 2000.1


¹Coffey et al., 1999 and National Pork Board Pork Facts, 2002/2003.


Feeding practices and excretions

Food animals convert feed nutrients into various animal products including meat, milk, eggs, wool, hides, draft power, feces, urine, and gases (fermentation and respiration). The amounts of nutrients required depend on the animal’s genetic capacity for each of these products under a variety of environmental conditions. Today, producers are able to control what the pig eats because modern swine systems utilize confinement facilities in a very large proportion of the industry. Relatively few animals are raised in outdoor systems where pigs can forage for food other than the diet provided by the producer. As a result, manure concentrations of elemental nutrients in swine are direct functions of nutrient inputs in the diet (Van Horn and Powers, 2004). Until recently, livestock producers were generally concerned only with maximizing food animal production. However, when including livestock production into a whole farm plan, often maximization of production output does not make economic sense. A better approach would be to develop a farm plan that optimizes livestock production for the operation as a whole.

There are several ways that diets fed to pigs can be altered to reduce the nutrients excreted and the odors produced from pork production thereby reducing environmental impact. Those nutrients that are generally considered to cause the most environmental concern are phosphorus, potassium, carbon dioxide, methane, ammonia and the nitrogenous compounds nitrite/nitrate, and nitrous oxide. Kornegay and Harper (1997) outlined means to accomplish a reduction in the excretion of the components of manure that pose the greatest environmental concern:

It is clear that not all dietary nutrients ingested are absorbed, utilized, and retained by the pig. Kornegay and Harper (1997) outlined the percentages of various nutrients digested and retained by grow–finish pigs (Table 3). The percentage of nutrient digested ranged from 5% to 88% and the percentage of nutrient retained ranged from 10% to 70%. This clearly points out that it is not possible for 100% utilization of nutrients by pigs or other livestock. However, several factors can influence the quantity of each nutrient retained or excreted (Kornegay and Verstegen, 2001). These factors include age, class, and nutritional status of the pig; source, quality, amount, and proportion of nutrients provided in the diet; processing methods; and environmental influences (Kornegay and Verstegen, 2001).


Table 3. Nutrient digestion and retention by growing – finishing swine.1


¹Adapted from Kornegay and Harper, 1997


The nutrient requirements of swine are well documented (NRC, 1998). However, these requirements are based on research conducted on pigs that do not have the genetic capacity for lean accretion shown in some modern pig lines. Additionally, as shown in Table 3, many nutrients have a range of The nutrient requirements of swine are well documented (NRC, 1998). However, these requirements are based on research conducted on pigs that do not have the genetic capacity for lean accretion shown in some modern pig lines. Additionally, as shown in Table 3, many nutrients have a range of digestibility and retention values that are dependent on feed quality, processing, and other factors. For these reasons, many nutritionists and feed suppliers commonly recommend nutrient levels that include safety margins, resulting in diets that are formulated to exceed NRC requirements. Nutritionists commonly over-fortify diets to ensure that the pig has a supply of nutrients that allows the animal to meet its genetic capacity for growth under a variety of conditions.

There are two requirements needed to minimize the amount of nutrients excreted and maximize the nutrient quantities digested and retained. First, the pig’s nutritional needs must be accurately known for every genetic type, size and gender of pig, under all environmental conditions. Second, the bioavailability of each nutrient from every feed source must be known. Unfortunately, neither is known precisely at the present time.


NITROGEN

Feeding diets that more closely provide the pig with the ideal amino acid pattern has been shown to reduce the amount of nitrogen excreted. Traditionally, corn– soybean meal diets have an excess of some amino acids in order that the requirement of the most limiting amino acid, lysine, is met. However, excess protein can be reduced substantially through balancing diets using a reduced amount of soybean meal and including crystalline amino acids (Carter et al., 1996). By including the crystalline amino acids lysine, threonine, and tryptophan, nitrogen excretion was reduced by nearly 30% (Bridges et al., 1995). Similarly, feeding diets that contain protein sources that are highly digestible reduces the amount of each specific nutrient excreted when the ration is balanced based upon bioavailability of the most limiting nutrient. Kerr and Easter (1995) reviewed several articles and estimated that for each percentage point reduction in crude protein obtained by balancing the diet using crystalline amino acids, nitrogen excretion is reduced by approximately 8 percentage points. These results have been more recently supported by those of Sutton et al. (1998), Otto et al. (2003), and Shriver et al. (2003).


PHOSPHORUS AND OTHER MINERALS

Poor availability of minerals can pose a greater environmental problem. Of particular concern are the heavy metals and other minerals. Table 3 shows the digestibility and retention values of several minerals. It is clear that mineral bioavailability can vary substantially. Awareness of this variation causes many nutritionists to over-fortify minerals in swine diets, the excess of which is excreted. Phosphorus, zinc, and copper, specifically, have received attention (Moore et al., 2001). Phosphorus builds up in soils where manure is applied and can pose runoff problems, which in turn contributes to eutrophication of surface waters. Land application of minerals has become an increasing environmental concern and is now regulated in many livestock-producing regions.

As previously indicated, phosphorus is one of the minerals fed in relatively high amounts due to its poor availability and its importance in various metabolic functions in the pig. The relatively high inclusion rates are due to a high proportion of the phosphorus in feedstuffs commonly fed to pigs being in an unavailable form (phytate). Cromwell and Coffey (1991) reported that as much as two thirds of the phosphorus in corn-soybean meal swine diets is in phytate form.

One way to increase phosphorus bioavailability is by including the enzyme phytase in swine rations. This enzyme releases a portion of the phytate making it available to the pig, which in turn reduces the need for the inclusion of inorganic phosphorus in the diet. Figure 1, adapted from Kornegay et al. (1998) shows the percentage reduction in phosphorus excreted when phytase is supplied in swine diets with reduced inorganic phosphorus inclusion. McMullen and Karsten (2001) reported that the inclusion of phytase combined with reduced inorganic phosphorus resulted in approximately 20% reduction in phosphorus excreted. Additionally, this study also consistently demonstrated improved feed efficiency of pigs fed diets containing phytase. Pierce et al. (2001) demonstrated that phytase did not improve feed efficiency, but did tend to improve rate of gain. Lyons and Cole (2000) summarized several studies, showing that a 0 to 0.20 percentage decrease in dietary phosphorus combined with the use of phytase reduced phosphorus excretion 23 to 53%.

There are numerous other studies that demonstrate improved grow–finish swine performance when phytase is added to diets. This can have positive environmental impact several ways. First, the direct effect of reduced phosphorus in the manure of pigs fed diets containing phytase and reduced amounts of added inorganic phosphorus. Second, if feed efficiency is improved, the same number of pigs would excrete less nutrients as it would take less feed to produce the same amount of gain. Third, if rate of gain is improved and pigs are sold at the same weights, the pigs would be on feed for fewer days thus reducing the amount of feed needed for maintenance and reducing the amount of nutrients excreted. Lastly, it has been documented that phytate increases the dietary requirement for zinc. If phytase is utilized and the phytate thereby hydrolyzed, then the amount of dietary zinc can be reduced, thereby reducing the potential environmental impact of excess dietary zinc.




Figure 1. Reduced phosphorus excretion from grow-finish pigs fed increasing amounts of supplemental phytase (adapted from Kornegay et al., 1998).


Trace minerals are becoming an environmental concern in areas that rely on repeated land application of manure. Metals accumulate in the soil when fed in excess to pigs. These minerals pose problems, as they can adversely affect the growth of aquatic organisms or even become toxic to some fish like blue gill, minnows, and rainbow trout (Davis, 1974). Additionally, certain metals tend to accumulate in the food chain and pose a toxicity problem to sensitive animal species, such as sheep. Use of minerals with high bioavailability and phase feeding to meet physiological needs can substantially reduce mineral amounts excreted and the environmental risk of soilapplied manure.

Pierce et al. (2001) demonstrated that swine fed diets containing organic minerals (Bioplex™) had similar performance to those fed inorganic sources, but had a 46% decrease in fecal copper concentrations (Figure 2). Leeson (2003) found that using trace minerals with greater bioavailability (Bioplex™ trace mineral) did not affect body weight gain and had little effect on feed efficiency of broilers even when fed at 20% of the inorganic trace mineral level. The authors are conducting a similar study with swine although the lowest level of the Bioplex™ trace mineral inclusion is not as low as that used in the Leeson (2003) broiler study.


PHASE FEEDING

Another way to reduce the amount of nutrients excreted is to meet the pig’s requirements for growth more precisely. This can be accomplished many ways. It is well known that different genetic lines have different capacities for lean growth deposition (Schinckel and deLange, 1996; Thompson et al., 1996). Producers should know the genetic capacity of their pigs for growth, lean deposition, milk production, etc. in order to match diet nutrient fortification with level of production. Additionally, it is well known that the requirements of nursery and grow–finish swine change as body weight and feed intake increase. The Iowa State Life Cycle Swine Nutrition Guide (Holden et al., 1996) demonstrates how diets can be modified for pigs of different body weights. Similarly, it has been shown (Schinckel et al., 1996) and accepted by the industry that the nutrient requirements of barrows and gilts are different and indeed recommendations for feeding them are different (Holden et al., 1996). The most appropriate way to meet the nutrient requirements of pigs and minimize the excreted nutrients would be to change the diet on a daily basis. However, this is not practical and in most cases not more than 4-6 diet changes are recommended for barrows and gilts throughout the grow–finish phase (Holden et al., 1996). The majority of US pork producers are implementing some form of phase feeding and a large portion implement split sex feeding. In this manner, pigs of different genders and live weights can be fed diets that more closely meet their needs. This type of feeding method reduces the environmental impact to a larger degree when compared to a feeding method that utilizes a diet that meets the needs of a younger, smaller animal and is fed throughout the full grow – finish period.




Figure 2. Effect of copper source on fecal copper content of growing pigs (Pierce et al., 2001).


Henry and Dourmad (1993) demonstrated that nitrogen output could be reduced when grow-finish pigs were fed two or three diets rather than a single phase feeding regimen (Table 4). Clearly, when feeding based on averages (either live weight or across sexes), the compromise implied will affect nutrient excretion. First, when feeding by average weight whether for one phase or multiple phases, at some point the pig is overfed while at other times the pig is underfed. Obviously, when the pig is overfed the unused nutrients are converted to waste, which represents an environmental burden. However, there is also an environmental impact associated with underfeeding when deficiencies result in slower growth and/or poorer feed efficiency.


Table 4. Effect of phase feeding on nitrogen excreted in pigs from 25 to 105 kg. <sup>1


a</sup>Crude protein changed at 55 kg
^bCrude protein changed at 50 and 75 kg
¹from Henry and Dourmad, 1993


It has been demonstrated that improving growth and feed efficiency can reduce the environmental impact of livestock production. As previously mentioned, genetic lines that have higher lean content typically have better feed efficiency compared to lines that have less lean growth capacity. However, there are numerous other factors that can influence the growth and feed efficiency of grow-finish pigs. Certainly, health challenges can reduce growth and feed efficiency and could contribute to increased environmental impact of pork production. Further, diseases that result in poorer absorption of dietary nutrients would have a negative environmental impact. Maintaining a healthy herd can beneficially affect both the pork producer’s bottom line and the environment. Other environmental and management factors can affect feed efficiency and, hence, alter the environmental burden associated with the farm. Such factors include poor ventilation, overcrowding, mycotoxins in feed, and a host of other factors. Additionally, proper feed processing influences nutrient availability by reducing anti-nutritional factors (Han et al., 2001).


FEED WASTAGE

Another obvious way to reduce the environmental impact of pork production is to control feed wastage. Van Heugten and Van Kempen (1999) indicated that feed wastage ranges from 2 to 20% depending on the country and particular study involved. Producers should pay close attention to feeder adjustment to ensure that wastage is minimized. Other factors, such as feeder design, feeder placement, and maintenance can influence feed wastage.


GENETIC INFLUENCES

Crocker and Robinson (2002) demonstrated that there is a genetic component to nutrient content in swine excreta. Table 5, adapted from Crocker and Robison, shows the genetic differences in various nutrient contents in the excreta of swine. Pigs from the maternal line excreted less phosphorus, calcium, copper, zinc, and iron than did the paternal line or the F1 cross of the two lines. These differences, on a daily basis, may be the result of slower growth and reduced feed intake of the maternal line. However, one should examine the total amount of nutrient excreted over the entire growing phase. Some lines grow faster and excrete more nutrients per day, but excrete less total nutrients throughout the growing phase when compared to lines that grow slower, excrete fewer nutrients on a daily basis, but require a greater number of days to reach the same weight. The Crocker and Robison (2002) study also showed that gilts excrete lower levels of all nutrients compared to barrows.


PROTEIN

Nutritional strategies exist to reduce odor and its components. The obvious method to reduce ammonia emissions would be to reduce the amount of nitrogen excreted (Powers, 2002). This can be accomplished by not over-fortifying crude protein. As previously mentioned, it is possible to meet the amino acid needs of the pig more closely by using crystalline amino acids without over-fortifying the diet with protein sources such as soybean meal. This will allow the diet to more closely meet the pig’s amino acid needs rather than over-fortifying the diet with some amino acids such that other amino acid requirements are met. Excretion of excess amino acids requires energy, which could otherwise be used for growth. For each 1% decrease in dietary crude protein, on average a corresponding ammonia reduction of 10% is observed (Sutton et al., 1997; Kay and Lee, 1997; Blair et al., 1995; Jacob et al., 1994; Aarnink et al., 1993). However, the diets using several crystalline amino acids have tended to be more costly than those diets using soybean meal and thus producers resist implementation.


Table 5. Least square means (± SE) of output of nutrients by swine genetic groups.¹


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^aAdjusted for total weight of pigs (n=8) in pen and feed disappearance. Total output of liquid and solid material for 3 days.
^{x, y, z}Values with different superscripts in the same row differ significantly (P<0.01).
¹Adapted from Crocker and Robison (2002).


While reducing dietary crude protein lowers ammonia emissions, it may not help reduce the offensiveness of swine manure odor. A study by Otto et al. (2003) demonstrated that an ammonia emission reduction was obtained when dietary crude protein was reduced from 15% to 9% or 6% in diets that were supplemented with crystalline amino acids. This study also reported that odors from manure from pigs fed these diets were more offensive than the manure from pigs fed the control 15% crude protein diet.


FIBER AND CARBOHYDRATES

Sutton et al. (1998) reported that adding low levels of an oligosaccharide and cellulose reduced the nitrogen content of fresh swine manure, especially the ammonia nitrogen fraction. This study indicated that inclusion of these products stimulates fermentation in the colon, reducing the amount of ammonia in the fresh manure. An additional benefit from the addition of cellulose to the pig’s diet was that the pH of fresh and stored manure was reduced, which would help control ammonia volatilization. This study did not evaluate the economics of the diets or cost of gain for the pigs on test. Other tests have used non-starch polysaccharides and found that total nitrogen excretion did not differ but route of excretion shifted from urine to fecal-excreted nitrogen (Canh et al., 1997). Fecal-excreted nitrogen is less volatile than urine nitrogen.


SULFUR

Many of the odors found most offensive have the ‘rotten egg’ smell indicative of sulfur containing compounds. Shurson et al. (1998) concluded that reducing the amount of dietary sulfur resulted in a 2 to 40% reduction in odor concentrations. One might also theorize that allowing pigs to consume water with high sulfur content might also contribute to manure odor unless other dietary sources of sulfur (i.e. trace mineral mixes) are adjusted accordingly.

Reduced sulfur compounds are formed under anaerobic conditions (Mackie et al., 1998). This type of condition is typically found when manure is stored in deep pits or lagoons and is released when the storage structure is agitated. However, other storage systems, like shallow pit recharge and flush, are not likely to facilitate the production of hydrogen sulfide containing compounds within the building. Van Kempen and van Heugten (2003) hypothesized that when these compounds are formed in the anaerobic portion of lagoons, the hydrogen sulfide compounds are metabolized by aerobes in the upper layer of a properly function lagoon. Therefore, strategies are site-specific in nature if they are to be effective.


Diet and air quality

Air quality concerns associated with livestock production, particularly pork production, are largely focused in two areas. First, and foremost, are the odors associated with pork production; and secondly, health concerns associated with exposure to odor compounds (Powers, 2002). Odors from swine manure are emitted during storage, treatment, transport and disposal (Van Horn and Powers, 2004). While the odor-causing compounds emitted are not at toxic levels downwind of swine facilities, the odors can be offensive to those living around swine operations. The offensiveness of livestock odors can be dependent on several factors that are generally subjective. Not all odors smell exactly the same to each person nor is the degree of offensiveness the same from person to person. The odors associated with swine production are caused by the products of anaerobic decomposition of manure (Van Horn and Powers, 2004). Certainly, diet composition can affect the amount of substrate available for anaerobic decomposition. At the present time, the gases most associated with odor concerns are ammonia and hydrogen sulfide. They also are the most commonly regulated. Methane gas is also a concern, but not from an odor standpoint as it is odorless. Methane gas concerns arise from its contribution to greenhouse gases and global warming (Van Horn and Powers, 2004).


Conclusions