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Environmental impacts of feeding DDGS to pigs

Part 15 (and final) in the series "What have we learned about feeding distillers co-products to pigs over the past 20 years?"

Swine diet formulation and feeding management is evolving into a new era of decision making. Nutritionists are beginning to implement "precision nutrition" approaches to customize diet formulations and feeding programs designed for specific farm conditions to capture the greatest economic value in pork production.

In addition, critical decisions are also being made on selecting and using alternatives to growth-promoting antibiotics and selecting feed ingredient sources based on their relative biosecurity risks of introducing foreign animal diseases. Another important consideration is that environmental sustainability has become a new megatrend in global agriculture, and large multi-national companies have begun implementing programs to source feed ingredients based not only on cost and nutritional value, but also on their environmental impact to reduce the overall carbon footprint of pork production systems (Shurson, 2017).

Several researchers have begun conducting life cycle assessments of the environmental impacts of using various feed ingredients in animal feeds. However, the assumptions, breadth and scope of many of these assessments vary among studies and impact the results and their interpretation (Zilberman, 2017). In fact, many of the published studies do not include the economic impact of environmental assessments, they use static instead of dynamic models, and do not take into account the actual measured emission rates from feeding diets, which leads to misleading results.


Installments in the DDGS series
Part 1: 20 years of DDGS lessons in pig diets
Part 2: Varied energy and digestible amino acids levels in DDGS manageable
Part 3: Work continues to evaluate performance responses from feeding DDGS
Part 4: Managing carcass yield, pork fat quality when feeding corn DDGS
Part 5: Reaching an understanding of fiber characteristics of corn DDGS
Part 6: Enzymes, pre-treatment improve fiber and nutrient digestibility
Part 7: DDGS show greater antioxidant capacity than in corn grain
Part 8: Need better understanding of energy levels in distillers corn oil
Part 9: Corn DDGS a good source of digestible phosphorus for swine
Part 10: Feeder design and diet management impact performance with DDGS diets
Part: 11: Feeding DDGS diets to gestating and lactating sows
Part 12: DDGS present handling and storage considerations
Part 13: Pelleting DDGS diets has benefits, drawbacks
Part 14: Feeding DDGS diets impact swine health
Part 15: Environmental impacts of feeding DDGS to pigs


Although estimates vary substantially, food animal production contributes about 18% of total greenhouse gas emissions (i.e. carbon dioxide, methane, nitrous oxide; Steinfeld et al., 2006) globally, which is primarily attributed to gastrointestinal fermentation of feed in animals and manure storage.

In addition to minimizing GHG emissions and carbon footprint, precision feeding programs must also focus on minimizing nutrient excretion as well as odor and gas emissions from confinement pork production facilities (Lu et al., 2017). Minimizing these negative environmental impacts are important because high nitrogen, phosphorus and trace mineral concentrations in manure that is applied to crop land, can cause soil concentrations of these nutrients to exceed crop removal rates. Nitrate can leach through soils and contaminate ground water supplies, and is considered to be a major pollution concern on livestock farms. Methane and nitrous oxide produced in manure contribute to greenhouse gas emissions, and volatilization of ammonia causes acid rain that has detrimental effects on vegetation and trees.

Furthermore, phosphorus can enter surface waters through soil erosion and increase growth of algae and other aquatic plants, which reduces dissolved oxygen that can cause fish death. In addition, soil accumulation of excessive trace minerals (e.g. copper and zinc) can increase the risk of toxicity of plants and micro-organisms.

Lu et al. (2017) suggested several nutritional strategies that can be effective for minimizing excess nitrogen, phosphorus and trace mineral excretion in manure. First, formulate diets to accurately meet dietary amino acids, phosphorus and trace mineral requirements of animals. Dietary crude protein levels can be reduced by using supplemental synthetic amino acids. Excess phosphorus excretion can be minimized by formulating swine diets on a digestible phosphorus basis and adding supplemental phytase. Using multiple phase-feeding programs to adjust diet formulations more frequently as nutrient requirements change during the various stages of production can substantially minimize excess nutrient excretion in manure.

Using high bioavailable sources of phosphorus and trace minerals and avoid excesses of these nutrients when formulating diets can minimize excess mineral content in manure. Effective feed additives such as enzymes, probiotics, prebiotics and others that improve nutrient utilization in animal feeds can also have measurable impacts on reducing production costs, but also minimize potential negative environmental impacts.

Effects of feeding DDGS diets on manure volume and nutrient excretion
Manure volume
Swine diets containing dried distillers grain with solubles contain higher fiber, crude protein and sulfur compared to traditional corn-soybean meal diets (Kerr et al., 2008; Zhang, 2010), which affects nutrient digestibility and excretion (Kerr et al., 2003; Degen et al., 2007; Kil et al., 2010; Anderson et al., 2012). Due to the relatively high fiber content of DDGS, dry matter excretion is increased when feeding DDGS compared to corn-soybean meal diets without DDGS (Almeida and Stein, 2012). This results in increased manure volume produced, which may increase the need for greater manure storage capacity or more frequent manure removal from swine production facilities.

Nitrogen and phosphorus excretion
It is well documented that using crystalline amino acids and phytase in swine diets are effective for improving nutrient utilization efficiency, reducing diet cost, reducing nitrogen and phosphorus excretion in manure, as well as emissions of gases such as ammonia. Kebreab et al. (2016) compared the impact of adding or not adding crystalline amino acids and phytase to swine diets in Europe, North America and South America. Their results showed that using these supplements in pig diets reduced GHG emissions by 56% in Europe, 17% in North America and 33% in South America, compared with feeding diets without supplemental synthetic amino acids and phytase.

These are substantial reductions and it is interesting to note that the North American swine diets used in this comparison contained 14.6% DDGS, but DDGS was not included in European and South American diets. As a result, the use of DDGS in swine diets can be part of the solution for minimizing the negative environmental impacts of pork production.

McDonnell et al. (2011) evaluated the effects of adding zero, 10, 20 or 30% corn DDGS to replace wheat in wheat and barley-based diets, formulated on a net energy, ileal digestible amino acid and digestible phosphorus basis, on nitrogen and phosphorus balance of growing-finishing pigs. As expected, nitrogen intake as well as urinary and total N excretion increased linearly with increasing DDGS levels in the diets (Table 1). This was due to feeding excess nitrogen from DDGS relative to pig requirements, resulting in increased deamination of excess amino acids and increased urinary N excretion. Nitrogen retention was not affected by feeding the 10% and 20% DDGS diets, but feeding the 30% DDGS diet decreased nitrogen retention relative to nitrogen intake. The increased nitrogen excretion commonly observed by feeding DDGS diets to swine can be minimized by using synthetic amino acids to reduce the amount of excess protein (nitrogen) in the diet.

University of Minnesota Table 1: Effects of adding increasing levels of corn DDGS to wheat and barley-based diets on nitrogen and phosphorus balance in growing-finishing pigs (McDonnell et al., 2011)

Table 1: Effects of adding increasing levels of corn DDGS to wheat and barley-based diets on nitrogen and phosphorus balance in growing-finishing pigs (McDonnell et al., 2011)

In contrast, phosphorus intake linearly increased with increasing dietary DDGS levels, but there was no effect on phosphorus excretion or retention. These results indicate that feeding diets containing up to 30% DDGS increases nitrogen excretion but has no effect on phosphorus excretion in growing-finishing pigs when diets are formulated on a digestible amino acid and phosphorus basis.

Baker et al. (2013) compared the phosphorus balance and digestibility between dicalcium phosphate and DDGS in growing pigs and showed that although the standardized total tract digestibility of phosphorus of DDGS was less than dicalcium phosphate, it was quite high (93% and 63%, respectively), and did result in greater fecal P excretion than dicalcium phosphate (Table 2). However, because of the relatively high P digestibility in DDGS, diet inclusion rates may need to be reduced to minimize excess phosphorus excretion in manure. Furthermore, dicalcium phosphate is a much more expensive source of phosphorus in animal feeds, and global supplies of inorganic phosphate reserves are rapidly declining, which makes DDGS an excellent and more sustainable alternative phosphorus source in swine diets.

University of Minnesota Table 2: Comparison of phosphorus intake, excretion and digestibility between dicalcium phosphate and DDGS (adapted from Baker et al., 2013)

Table 2: Comparison of phosphorus intake, excretion and digestibility between dicalcium phosphate and DDGS (adapted from Baker et al., 2013)

The addition of microbial phytase to swine diets has become a common practice to improve phosphorus digestibility, reduce phosphorus excretion in manure, and reduce diet cost by reducing the amount of inorganic phosphate required in the diet. Almeida and Stein (2012) added increasing levels of microbial phytase (zero, 500, 1,000 or 1,599 phytase units) to corn or 50% DDGS diets and showed a linear improvement in standardized total tract digestibility of P in corn (40.9, 67.5, 64.5 and 74.9%, respectively), and P digestibility tended to increase in DDGS diets (76.9, 82.9, 82.5 and 83.0%, respectively). However, the magnitude of improvement in P digestibility by adding phytase to DDGS diets was much less than observed for corn, and may not justify the additional cost of adding high amounts of phytase to swine DDGS diets.

Rojas et al. (2013) compared the phosphorus balance and digestibility of corn, DDGS and corn gluten meal, with and without 600 FTU/kg diet of supplemental phytase, when fed to growing pigs. Total P excretion was greatest for pigs fed the corn diet without phytase supplementation but was reduced by 50% when phytase was added (Table 3).

University of Minnesota Table 3: Effect of microbial phytase supplementation (600 phytase units per kilogram) on fecal phosphorus concentration, excretion, and digestibility of corn, DDGS and corn gluten meal (adapted from Rojas et al., 2013)

Table 3: Effect of microbial phytase supplementation (600 phytase units per kilogram) on fecal phosphorus concentration, excretion, and digestibility of corn, DDGS and corn gluten meal (adapted from Rojas et al., 2013)

However, feeding DDGS without phytase resulted in 40% less P excretion than feeding corn without phytase, and feeding corn gluten meal without phytase resulted in a 60% reduction in P excretion compared to feeding corn. Adding phytase to the corn diet had the greatest magnitude of improvement on reducing phosphorus excretion, with no benefit in the DDGS diets, and some improvement when phytase was added to the corn gluten meal diet. As a result, adding phytase to corn and corn gluten meal diets improves standardized total tract digestibility of P in corn and corn gluten meal, but not DDGS. This lack of response to adding phytase to DDGS diets is a result of the already high P digestibility that occurs from the degradation of phytate during the fermentation process in dry-grind ethanol plants.

Therefore, formulating DDGS diets on a digestible phosphorus basis for swine can dramatically reduce phosphorus excretion in manure compared with feeding corn-based diets.

Effect of feeding DDGS diets on gas and odor emissions
Feeding high fiber diets to pigs has been shown to increase the production of methane (Jarret et al., 2011), which is a major GHG of concern. Some sources of DDGS contain significant concentrations of sulfur, which may increase the sulfur content of manure and lead to an increase in hydrogen sulfide, other reduced-sulfur compounds and odor of swine manure (Blanes-Vidal et al., 2009; Feilberg et al., 2010; Trabue et al., 2011). Furthermore, the relatively high protein relative to lysine content in DDGS results in increased protein and nitrogen content in swine diets which can lead to increased nitrogen excretion and potentially greater ammonia emissions in swine manure. Ammonia and hydrogen sulfide are two of the major gases produced from swine manure during storage.

Several studies have been conducted to determine the effects of feeding DDGS diets to swine on gas and odor emissions from manure. Powers et al. (2009) measured air emissions of ammonia, hydrogen sulfide, methane and non-methane hydrocarbons when feeding diets containing zero or 20% DDGS to growing-finishing pigs, with either supplemental inorganic or organic trace minerals. Although feeding the organic trace mineral sources helped minimize the increased hydrogen sulfide emissions resulting from feeding the 20% DDGS diet, ammonia, methane and non-methane hydrocarbon emissions increased when feeding the DDGS diet. This is the only study that has shown an increase in ammonia and hydrogen sulfide emissions from feeding DDGS diets to pigs. Spiehs et al. (2012) observed no differences when feeding a 20% DDGS diet compared with a corn-soybean meal diet to growing pigs over a 10-week feeding period on total reduced sulfur, ammonia or odor concentrations.

Trabue et al. (2016) fed growing pigs diets containing 35% DDGS over a 42-day period and observed a reduction in manure pH and an increase in manure surface crust coverage, dry matter content, as well as increased concentrations of carbon, nitrogen and sulfur in manure compared with pigs fed a corn-soybean meal diet (Table 4). Warmer temperatures are often observed for manure with greater surface crusting or foam (van Weelden et al., 2015), and is associated with animals fed high fiber diets (Misselbrook et al., 2005; Lynch et al., 2007; Wood et al., 2012) and lower pH (Kerr et al., 2006).

University of Minnesota Table 4: Manure characteristics and air concentrations of odorous compounds from pigs fed corn-soybean meal and 35% DDGS diets (adapted from Trabue et al., 2016)

Table 4: Manure characteristics and air concentrations of odorous compounds from pigs fed corn-soybean meal and 35% DDGS diets (adapted from Trabue et al., 2016)

As a result, the increased crusting of manure (Wood et al. 2012), temperature (Blunden and Aneja, 2008; Blunden et al., 2008; Rumsey and Aneja, 2014), and reduced pH associated with feeding DDGS diets can reduce gas emissions. In fact, ammonia and hydrogen sulfide emissions from manure produced by pigs fed DDGS was less than from those fed a corn-soybean meal diet, but volatile fatty acid and phenolic compound concentrations were greater in manure from pigs fed the DDGS diet (Table 4). It is likely that the increased crusting of manure from pigs fed the DDGS diet reduced hydrogen sulfide emissions by acting as a barrier for emission to the air.

Trabue et al. (2016) also measured emissions of various odor compounds from manure in the same study (Table 5). These data were normalized for pig weight (animal unit) and nutrients consumed. Pigs fed the corn-soybean meal diet had greater ammonia (53% of N consumed) and hydrogen sulfide emissions (9% of S consumed) than pigs fed the 35% DDGS diet (30% of N consumed and 2% of S consumed). These results are consistent with those from another study where ammonia emissions were reduced from feeding DDGS diets to swine (Li et al., 2011), which is likely due to reduced pH of manure (Roberts et al., 2007) and increased microbial activity from more carbon present in manure (Kerr et al, 2006; Ziemer et al., 2009).

University of Minnesota Table 5: Emissions of odorous compounds from stored swine manure for pigs fed corn-soybean meal and 35% DDGS diets (adapted from Trabue et al., 2016)

Table 5: Emissions of odorous compounds from stored swine manure for pigs fed corn-soybean meal and 35% DDGS diets (adapted from Trabue et al., 2016)

However, manure from pigs fed the 35% DDGS diet had greater emissions of volatile fatty acid and phenolic compounds, but there were no differences in indole emissions compared with manure from pigs fed the corn-soybean meal diet (Table 5). These differences were relatively small compared with ammonia and hydrogen sulfide emissions because total volatile organic compound emissions represented less than 1% of the total carbon consumed from feeding both diets. Furthermore, human panelists detected no differences in odor of compounds emitted from manure from pigs fed the two diets, but chemical analysis of individual odorous compounds showed greater hydrogen sulfide and ammonia, and less total volatile fatty acids and phenols in manure emissions from pigs fed the corn-soybean meal diet than those fed the DDGS diet.

The majority (60%) of odorous compounds in swine manure were derived from ammonia and hydrogen sulfide. These data indicate that controlling nitrogen and sulfur excretion when feeding DDGS diets does not change ammonia and hydrogen sulfide emissions because the sulfur content in the DDGS diet was almost twice as high as in the corn-soybean meal diet (Trabue and Kerr, 2014), but manure hydrogen sulfide emissions from manure of pigs fed the DDGS diet was about 30% less than those fed the corn-soybean meal diet.

Carbon dioxide, methane and nitrous oxide are major greenhouse gases of concern in animal production systems. From the same study (Trabue et al., 2016), emissions of the major carbon, nitrogen and sulfur gases were also determined (Trabue and Kerr, 2014). Results showed that carbon dioxide, methane and nitrous oxide emissions, expressed on an animal unit and amount of element consumed basis, were not different between the two diets (Table 6). However, as previously described, ammonia and hydrogen sulfide emission were reduced by feeding the DDGS diet. These results suggest that pigs fed DDGS diets have no greater GHG emissions from stored manure than those fed corn-soybean meal diets.

University of Minnesota Table 6: Emissions of major carbon, nitrogen and sulfur gases in stored swine manure for pigs fed corn-soybean meal and 35% DDGS diets (adapted from Trabue and Kerr, 2014)

Table 6: Emissions of major carbon, nitrogen and sulfur gases in stored swine manure for pigs fed corn-soybean meal and 35% DDGS diets (adapted from Trabue and Kerr, 2014)

Effect of feeding DDGS diets on potential for biogas production
Van Weelden et al. (2016) showed that manure from pigs fed coarse ground diets containing corn and soybean meal had the lowest methane production rate, while pigs fed corn-soybean meal-soybean hulls diets had the greatest methane production, with manure from pigs fed a 35% DDGS diet having an intermediate production rate. However, the biochemical methane production potential was greatest when feeding the 35% DDGS diet compared with corn-soybean meal or corn-soybean meal-soybean hulls diets. These results suggests that for swine farms installing biogas production systems to capture energy from manure, feeding DDGS diets would provide manure with significant amounts of carbon to generate greater amounts of methane than feeding corn-soybean meal diets.

Life cycle assessment of feeding DDGS diets
There is increasing interest in conducting life cycle assessments of the environmental impacts of using various feed ingredients in the swine industry. Lammers et al. (2010) conducted a partial life cycle assessment, which only included production and processing of feed ingredients used in Iowa swine diet formulations (including DDGS), and focused on non-solar energy use and global warming potential. Unfortunately, economic analyses of diets were not considered in this study which provided misleading results.

In another study by Thoma et al. (2011), there was about a 6% increase in the overall carbon footprint of pork production (production to consumption) when DDGS was included in swine diets. This increase was attributed to the additional energy consumed during processing of corn during the ethanol and co-product production process compared with corn grain and soybean meal.

Mackenzie et al. (2016) determined the environmental impacts of using co-products from human food and biofuels supply chains in pig diets in Canadian pork production systems using a more comprehensive life cycle assessment approach. As shown in Table 7, feeding corn DDGS at maximum diet inclusion rates increases non-renewable resource use by 71%, non-renewable energy use by 68% and global warming potential by 30% compared with feeding the control corn-soybean meal diets, on a per kilogram of feed basis. However, including corn DDGS in the diets reduced acidification potential by 20% and eutrophication potential by 22% compared with the corn-soybean meal and all other co-product diets. When environmental impacts were expressed on a kilogram of carcass weight basis, the impacts were less dramatic but in the same direction as when expressed on a per kilogram of feed basis. Because of the increasing interest and importance of reducing the carbon footprint and resource use in pork production, more of these types of comparative studies will be conducted in the future.

University of Minnesota Table 7: Average environmental impact per kilogram of feed of Canadian grower-finisher diets when co-product ingredients are included at maximum inclusion rates compared with a corn-soybean meal control diet. (adapted from Mackenzie et al. 2016)

Table 7: Average environmental impact per kilogram of feed of Canadian grower-finisher diets when co-product ingredients are included at maximum inclusion rates compared with a corn-soybean meal control diet. (adapted from Mackenzie et al. 2016)

Conclusions
The use of DDGS in swine diets can contribute to improved environmental sustainability when using net energy and digestible nutrients when formulating precision diets using accurate net energy and nutrient digestibility estimates, which is essential to minimize excretion of nitrogen and phosphorus in manure. Although DDGS is relatively high in protein and low in lysine and other amino acids relative to the pig's requirements, the widespread availability and cost effectiveness of synthetic amino acids allow nutritionists to reduce dietary crude protein levels, while meeting all of the essential amino acid requirements, and reduce nitrogen excretion in manure.

One of the unique advantages of corn DDGS compared with other grains and grain-based ingredients is its relatively high total and digestible phosphorus content. Formulating swine and poultry diets on a digestible phosphorus basis and using phytase can significantly reduce manure phosphorus excretion. Furthermore, feeding DDGS diets to pigs does not appear to affect methane emissions, but can substantially reduce ammonia and hydrogen sulfide emissions from swine manure. Initial studies comparing DDGS with other co-product or byproduct ingredients indicate that feeding DDGS to pigs may reduce acidification and eutrophication potential by up to 22% compared with the corn-soybean meal and all other co-product diets.

References

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