By Dr. Jerry Shurson and Amanda Palowski, University of Minnesota Department of Animal Science
© 2019 Feedstuffs. Reprinted with permission from Vol. 91, No. 06, June 3, 2019
Although the majority of swine diets containing dried distillers grains plus solubles (DDGS) in the midwestern U.S. are fed in meal form, when these diets must be pelleted, the dietary inclusion rate of DDGS is often restricted due to concerns of reduced pellet quality and pellet mill throughput. As a result, the ability of feed manufacturers and pork producers to capture greater economic value from using higher dietary inclusion rates may be diminished because of constraints imposed on DDGS to meet desired pellet quality and production efficiencies in commercial feed mills.
Pelleting is the most common thermal processing method used in manufacturing swine feeds (Miller, 2012) and provides the advantages of improved feed conversion (due to reduced feed wastage) and improved digestibility of energy and nutrients, which is partly attributed to the partial gelatinization of starch (Richert and DeRouchey, 2010; National Research Council [NRC], 2012). Additional advantages of pelleting diets include reductions in dustiness, ingredient segregation during transport, pathogen presence and sorting large particles in mash, along with improved palatability, bulk density and handling characteristics (Abdollahi et al., 2012; NRC, 2012).
The pellet durability index (PDI) is a common way to measure pellet quality (ASAE, 1997). However, almost any adjustment that’s made to increase pellet durability decreases pellet mill throughput and increases energy cost (Behnke, 2006).
The pellet mill’s production rate affects PDI and energy consumption. Stark (2009) showed that increasing pellet mill throughput from 545 kg to 1,646 kg per hour increased pellet mill efficiency from kg to 112.4 kg/hp hour and linearly reduced PDI from 55.4% to 30.2%.
Production of steam for the conditioning stage and the electricity (kilowatt hours per metric ton) required to operate the feeders, conditioners, pellet mill and pellet cooling system are the primary contributors to energy use and cost during the pelleting process. As much as 72% of the energy used for pelleting is for steam conditioning (Skoch et al., 1983), and Payne (2004) suggested that 15 kWh/mt should be a reasonable goal for pelleting swine diets.
Similar to pellet quality, pellet mill energy consumption also depends on variables such as pellet die diameter, die speed, L-to-D ratio and feed ingredient moisture and chemical composition (Tumuluru et al., 2016). Electrical usage in pellet mills is quantified as units of energy per unit of throughput or time and is commonly described in kilowatt hours per metric ton. Minimizing energy consumption per metric ton of pelleted feed can be achieved by maximizing the production rate, which is affected by diet characteristics and die volume (Fahrenholz, 2012).
Changing the pitch of the conditioner paddles (Briggs et al., 1999) can be used to increase retention time (heat) and in- crease PDI (Gilpin et al., 2002)
However, the ability of steam pressure to improve PDI is inconsistent. Cutlip et al. (2008) reported that increasing steam pressure resulted in only small improvements in PDI, whereas Thomas et al. (1997) reported that there is no clear relationship between steam pressure and PDI.
This poor relationship was also observed in an earlier study that found no effect of steam pressure on PDI or production rate (Stevens, 1987). As a result, Briggs et al. (1999) concluded that using 207-345 kilopascals appears to be sufficient steam pressure for achieving a high PDI in pellets.
The most important factor related to a pellet die is the thickness (L) of die relative to hole diameter (D), commonly described as the L-to-D ratio, or L:D. As L:D increases (thicker die), pellet durability increases due to increased friction and die retention time, but pellet mill throughput is reduced and energy consumption is increased (Traylor, 1997).
The starch and protein content of swine diets plays a significant role in PDI. Maximum PDI can be achieved in diets containing 65% starch, while low-starch diets with high protein content decrease pellet durability (Cavalcanti and Behnke, 2005a); Cavalcanti and Behnke (2005b) showed that increasing protein content in corn, soybean meal and soybean oil diets increased PDI. In fact, the starch and protein content of the diet has been shown to have a greater effect on PDI than conditioning temperature (Wood, 1987).
Increasing dietary lipid content decreases PDI (Cavalcanti and Behnke, 2005a). Adding 1.5% or 3.0% fat has been shown to decrease PDI by 2% and 5%, respectively (Stark et al., 1994). Adding fat to diets before pelleting may also reduce energy consumption, but there are many interactions among chemical components of diets that affect energy use (Briggs et al., 1999).
Moisture content of the mash is another major factor that contributes to pellet durability and energy consumption during pelleting. Gilpin (2002) showed that increasing mash moisture content increased PDI and reduced energy consumption. Furthermore, adding five percentage points of moisture to mash before pelleting has been shown to increase PDI when pelleting high-fat diets (Moritz et al., 2002).
Third, although its relatively high protein content contributes to plasticizing the protein during pelleting, which improves pellet quality, the relatively high oil content in DDGS may contribute to reduced pellet quality, depending on the inclusion rate and the amount of other fats or oils in the diet. However, a relatively high DDGS oil content may contribute to improved pellet mill production rates. Fourth, although some feed ingredients contain fiber that serves as a natural binder contributing to the production of good-quality pellets, the high amount of insoluble fiber in DDGS reduces production rates of pellet mills. Last, DDGS has moderate bulk density, which can contribute to reduced production rates, depending on the density and amounts of other ingredients in the feed formulation.
The particle size of DDGS varies from 294 to 1,078 µm among sources (Kerr et al., 2013).
Knauer (2014) evaluated the effects on pellet quality of regrinding soybean meal (1,070 µm versus 470 µm) and DDGS (689 µm versus 480 µm) and adding 0% or 30% DDGS to swine finishing diets. His results showed that adding 30% DDGS to diets improved modified PDI by 9.5%, and regrinding soybean meal improved PDI by 4.7%, but regrinding DDGS had no effect on PDI. He also evaluated the effects of pelleting diets containing two DDGS particles sizes (640 µm versus 450 µm) and two levels of pellet fines on the growth performance of finishing pigs and observed no effects. These results suggest that reducing DDGS particle size by regrinding does not improve pellet quality.
Pelleting swine diets has been shown to improve the digestibility of starch (Freire et al., 1991; Rojas et al., 2016), lipids (Noblet and van Milgen, 2004; Xing et al., 2004) and dry matter, nitrogen and gross energy (Wondra et al., 1995a). Pelleting nursery pig diets containing 30% DDGS improved apparent total tract digestibility of dry matter, organic matter, gross energy and crude protein compared to feeding a meal diet (Zhu et al., 2010).
More recently, Rojas et al. (2016) evaluated the effects on energy and nutrient digestibility of extruding and pelleting corn/soybean meal and corn/soybean meal/25% DDGS diets. They found that pelleting and extrusion improved apparent ileal digestibility of gross energy, starch, crude protein, dry matter, ash, acid hydrolyzed ether extract and amino acids (Table 2). Furthermore, pelleting increased the metabolizable energy (ME) content by 97 kcal/kg of dry matter, and extruding increased ME by 108 kcal/ kg of dry matter, but the combination of extruding and pelleting did not improve the ME content in the DDGS diets compared to the meal form (Table 3). Similarly, pelleting the corn/soybean meal diet improved ME by 81 kcal/kg of dry matter, and extruding and pelleting increased ME by 89 kcal/kg, but extrusion alone did not improve the ME content.
Therefore, the greatest improvement in digestibility for most nutrients in the DDGS diets was achieved with extrusion, but the combination of extrusion and pelleting did not generally improve nutrient digestibility beyond that obtained with extrusion. Several other studies have shown that apparent ileal digestibility of amino acids in swine diets improves with pelleting and extrusion (Muley et al., 2007; Stein and Bohlke, 2007; Lundblad et al., 2012), but this is not always the case (Herkleman et al., 1990).
Feeding pelleted diets containing 15% DDGS had no effect on average daily gain, reduced average daily feed intake and improved gain:feed compared with feeding 15% DDGS diets in meal form to growing/finishing pigs (De Jong et al., 2016). However, when feeding growing/ finishing pigs pelleted diets containing 30% DDGS, there was a trend for improved overall growth rate with no effect on feed intake, and feed conversion was improved compared with meal diets (Fry et al., 2012; Overholt et al., 2016).
In a recent study, De Jong et al. (2016) fed pelleted or meal diets containing 15% DDGS and showed no differences in hot carcass weight, carcass yield, backfat depth, loin depth and percentage carcass lean. In contrast, Overholt et al. (2016) fed pelleted diets containing 0% or 30% DDGS to growing/finishing pigs and reported an increase in hot carcass weight and 10th-rib backfat thickness and a reduced percentage of carcass lean compared with meal diets.
However, there was no effect of DDGS inclusion rate on carcass characteristics, including loin muscle quality. Although feeding pelleted diets reduced the weight of the gastrointestinal tract and improved carcass yield, feeding diets containing DDGS increased the weight of the gastrointestinal tract and contents, resulting in reduced carcass yield.
Storage bin design can be a significant cause or a potential solution to the flowability problems with feed containing DDGS. Hilbrands et al. (2016) evaluated feed flow of 40% DDGS diets using three different designs of commercially available feed storage bins and showed that feed bin design affects flow rate during discharge of meal diets, but installing passive agitators increased feed flow in all bin designs.
Cochrane et al. (2017) showed that conditioning and pelleting temperatures greater than 54.4°C appear to be effective in reducing the quantity and infectivity of PEDV in swine feeds. In fact, their results showed that pelleting diets inactivated PEDV at a faster rate (30 seconds) and at a much lower temperature than those (145°C and 10 minutes) reported by Trudeau et al. (2016).
It is unknown if pelleting swine diets reduces the quantity and infectivity of other pathogens, but it appears to be an effective strategy to partially reduce the risk of transmission of PEDV from feed mills to swine farms.
Pellets produced with low PDI generally result in an increased amount of fines that may reduce swine growth performance. Stark et al. (1993) evaluated the effects of pellet quality on the growth performance of pigs in both the nursery and finishing phases. In the nursery phase, pigs fed a pelleted diet with 25% added fines had a 7% reduction in feed conversion compared to pigs fed a pelleted diet screened for fines. In the finishing phase, increasing the amount of fines in the diet resulted in a linear trend for decreased feed conversion, resulting in less of an advantage for feeding pelleted diets.
However, Knauer (2014) also evaluated the effects of feeding pelleted diets containing two particles sizes of DDGS (640 µm versus 450 µm) and two levels of pellet fines and observed no effects on the growth performance of finishing pigs.
However, pelleting DDGS diets increases cost, may reduce vitamin and enzyme activity and may increase the percent- age of fines. There are multiple interactions among factors involved in pelleting DDGS diets, but identifying the key factors and understanding their relative significance can be useful for improving pellet quality and production efficiency.
© 2019 Feedstuffs. Reprinted with permission from Vol. 91, No. 06, June 3, 2019
Pelleting is the most common thermal processing method used in manufacturing swine feeds (Miller, 2012) and provides the advantages of improved feed conversion (due to reduced feed wastage) and improved digestibility of energy and nutrients, which is partly attributed to the partial gelatinization of starch (Richert and DeRouchey, 2010; National Research Council [NRC], 2012). Additional advantages of pelleting diets include reductions in dustiness, ingredient segregation during transport, pathogen presence and sorting large particles in mash, along with improved palatability, bulk density and handling characteristics (Abdollahi et al., 2012; NRC, 2012).
Pelleting factors
The three main goals of manufacturing high-quality pelleted swine diets are to achieve high pellet durability and pellet mill throughput while minimizing the energy cost of the pelleting process. Pellet durability refers to the ability of pellets to remain intact during bagging, storage and transport until reaching the feeders in the animal production facility while minimizing the proportion of fines (Cramer et al., 2003; Amerah et al., 2007).The pellet durability index (PDI) is a common way to measure pellet quality (ASAE, 1997). However, almost any adjustment that’s made to increase pellet durability decreases pellet mill throughput and increases energy cost (Behnke, 2006).
The pellet mill’s production rate affects PDI and energy consumption. Stark (2009) showed that increasing pellet mill throughput from 545 kg to 1,646 kg per hour increased pellet mill efficiency from kg to 112.4 kg/hp hour and linearly reduced PDI from 55.4% to 30.2%.
Production of steam for the conditioning stage and the electricity (kilowatt hours per metric ton) required to operate the feeders, conditioners, pellet mill and pellet cooling system are the primary contributors to energy use and cost during the pelleting process. As much as 72% of the energy used for pelleting is for steam conditioning (Skoch et al., 1983), and Payne (2004) suggested that 15 kWh/mt should be a reasonable goal for pelleting swine diets.
Similar to pellet quality, pellet mill energy consumption also depends on variables such as pellet die diameter, die speed, L-to-D ratio and feed ingredient moisture and chemical composition (Tumuluru et al., 2016). Electrical usage in pellet mills is quantified as units of energy per unit of throughput or time and is commonly described in kilowatt hours per metric ton. Minimizing energy consumption per metric ton of pelleted feed can be achieved by maximizing the production rate, which is affected by diet characteristics and die volume (Fahrenholz, 2012).
Optimal pellet quality
Steam conditioning.
Steam conditioning of the mash is considered the most important factor in achieving high pellet durability. A high conditioning temperature increases PDI and decreases energy consumption (Pfost, 1964) due to de- creased mechanical friction (Skoch et al., 1981). Starch gelatinization decreases as conditioning temperature increases (Abdollahi et al., 2011).Changing the pitch of the conditioner paddles (Briggs et al., 1999) can be used to increase retention time (heat) and in- crease PDI (Gilpin et al., 2002)
However, the ability of steam pressure to improve PDI is inconsistent. Cutlip et al. (2008) reported that increasing steam pressure resulted in only small improvements in PDI, whereas Thomas et al. (1997) reported that there is no clear relationship between steam pressure and PDI.
This poor relationship was also observed in an earlier study that found no effect of steam pressure on PDI or production rate (Stevens, 1987). As a result, Briggs et al. (1999) concluded that using 207-345 kilopascals appears to be sufficient steam pressure for achieving a high PDI in pellets.
Pellet die.
Characteristics of the pellet die affect pellet durability, mill throughput and energy consumption and include: metal properties, hole design, hole pattern and number of holes (Stark, 2009). The types of metals in the die affect the amount of friction generated and subsequent temperature increases as the mash passes through the die (Behnke, 2014).The most important factor related to a pellet die is the thickness (L) of die relative to hole diameter (D), commonly described as the L-to-D ratio, or L:D. As L:D increases (thicker die), pellet durability increases due to increased friction and die retention time, but pellet mill throughput is reduced and energy consumption is increased (Traylor, 1997).
Diet particle size.
Many feed manufacturers perceive that diet particle size has a significant influence on PDI of pellets, but there is no convincing research evidence to support this. Theoretically, fine- and medium-ground particle sizes may provide more surface area for moisture absorption from steam and result in chemical changes that may improve pellet quality while preventing large particles from serving as natural breaking points for producing fines. Furthermore, low- and medium-particle size ingredients and diets may improve pellet die lubrication and increase production rates. However, large particles can cause fractures in pellets, making them more prone to breakage (California Pellet Mill Co., 2016).
Stevens (1987) showed that the particle size of ground corn had no effect on production rate or PDI. Similarly, Stark et al. (1994) reported that reducing the diet particle size from 543 to 233 microns only slightly increased PDI, while Reece et al. (1985) showed that increasing the diet particle size from 670 to 1,289 microns only slightly decreased PDI.
Stevens (1987) showed that the particle size of ground corn had no effect on production rate or PDI. Similarly, Stark et al. (1994) reported that reducing the diet particle size from 543 to 233 microns only slightly increased PDI, while Reece et al. (1985) showed that increasing the diet particle size from 670 to 1,289 microns only slightly decreased PDI.
Diet composition.
Diet composition is an important factor that affects pellet quality and manufacturing efficiency because of its effects on die lubrication and abrasion as well as the bulk density of the feed (Behnke, 2006). As a result, various feed ingredients have been characterized based on “pelletability factors” (Payne et al., 2001). While it is theoretically possible to use these relative feed ingredient pelletability factors as constraints in diet formulation, it is infeasible in practice because the primary goal in diet formulation is to meet nutritional needs at a low cost rather than manipulating formulations to optimize PDI.The starch and protein content of swine diets plays a significant role in PDI. Maximum PDI can be achieved in diets containing 65% starch, while low-starch diets with high protein content decrease pellet durability (Cavalcanti and Behnke, 2005a); Cavalcanti and Behnke (2005b) showed that increasing protein content in corn, soybean meal and soybean oil diets increased PDI. In fact, the starch and protein content of the diet has been shown to have a greater effect on PDI than conditioning temperature (Wood, 1987).
Increasing dietary lipid content decreases PDI (Cavalcanti and Behnke, 2005a). Adding 1.5% or 3.0% fat has been shown to decrease PDI by 2% and 5%, respectively (Stark et al., 1994). Adding fat to diets before pelleting may also reduce energy consumption, but there are many interactions among chemical components of diets that affect energy use (Briggs et al., 1999).
Moisture content of the mash is another major factor that contributes to pellet durability and energy consumption during pelleting. Gilpin (2002) showed that increasing mash moisture content increased PDI and reduced energy consumption. Furthermore, adding five percentage points of moisture to mash before pelleting has been shown to increase PDI when pelleting high-fat diets (Moritz et al., 2002).
Third, although its relatively high protein content contributes to plasticizing the protein during pelleting, which improves pellet quality, the relatively high oil content in DDGS may contribute to reduced pellet quality, depending on the inclusion rate and the amount of other fats or oils in the diet. However, a relatively high DDGS oil content may contribute to improved pellet mill production rates. Fourth, although some feed ingredients contain fiber that serves as a natural binder contributing to the production of good-quality pellets, the high amount of insoluble fiber in DDGS reduces production rates of pellet mills. Last, DDGS has moderate bulk density, which can contribute to reduced production rates, depending on the density and amounts of other ingredients in the feed formulation.
The particle size of DDGS varies from 294 to 1,078 µm among sources (Kerr et al., 2013).
Knauer (2014) evaluated the effects on pellet quality of regrinding soybean meal (1,070 µm versus 470 µm) and DDGS (689 µm versus 480 µm) and adding 0% or 30% DDGS to swine finishing diets. His results showed that adding 30% DDGS to diets improved modified PDI by 9.5%, and regrinding soybean meal improved PDI by 4.7%, but regrinding DDGS had no effect on PDI. He also evaluated the effects of pelleting diets containing two DDGS particles sizes (640 µm versus 450 µm) and two levels of pellet fines on the growth performance of finishing pigs and observed no effects. These results suggest that reducing DDGS particle size by regrinding does not improve pellet quality.
Pelleting DDGS diets
Pellet durability
Limited studies have been conducted to evaluate the pellet durability of swine diets when DDGS is added, and the results are inconsistent. Fahrenholz et al. (2008) used a pellet die that measured 3.97 mm x 31.75 mm with a conditioning temperature of 85°C and found that as DDGS levels increased, PDI values and bulk density decreased. However, although PDI was slightly reduced as DDGS inclusion rates increased (90.3 PDI for 0% DDGS, 88.3 PDI for 10% DDGS and 86.8 PDI for 20% DDGS), he suggested that the practical significance of this reduction was of minimal importance.
Stender and Honeyman (2008) observed a more dramatic decrease in PDI (from 78.9 to 66.8) when comparing pelleted diets containing 0% and 20% DDGS, respectively. However, Feoli (2008) showed that adding 30% DDGS to corn/ soybean meal swine diets increased PDI from 88.5 to 93.0.
De Jong et al. (2013) found no differences in PDI values (93.3 versus 96.9), percentage of fines (1.2% versus 8.0%) and production rate (1,098 versus 1,287 kg per hour) among pelleted corn/soybean diets and 30% DDGS diets for nursery pigs using a pellet die of 3.18 mm x 3.81 mm. The inconsistent results suggest that several interactions among processing variables may have contributed to differences in PDI of the DDGS diets in these studies.
The lipid content of diets and feed ingredients affects pellet quality and production rate. Yoder (2016) evaluated the effects on PDI of adding either 15% or 30% of reduced-oil or high-oil DDGS to corn/soybean meal swine finisher diets. Diets were pelleted using conditioning temperatures of 65.6°C or 82.2°C and a 4.0 mm x 32 mm die. Throughput was maintained at a constant rate of 680 kg per hour. Pellet quality was evaluated using four pellet durability tests: standard PDI (ASABE S269.4, 2007), modified PDI using three 19 mm hex nuts, Holmen NHP 100 for 60 seconds and Holmen NHP 200 for 240 seconds.
The dietary inclusion rate (15% or 30%) of DDGS and conditioning temperature had no effect on PDI, but PDI was greater for diets containing reduced-oil DDGS (88.0%) than high-oil DDGS (82.8%). Furthermore, the method used to determine pellet quality dramatically affected PDI, with the highest value obtained for standard PDI (95%), followed by modified PDI (91%), Holmen NHP 100 (89%) and Holmen NHP 200 (67%).
The results of this study indicate that relatively high PDI can be achieved in corn/soybean meal-based swine finishing diets containing up to 30% DDGS and that adding reduced-oil DDGS improved PDI by about five percentage points compared with adding high-oil DDGS to the diet. However, caution should be used when comparing PDI values among studies because the use of various PDI test methods can lead to differences in interpretation of acceptable PDI.
Stender and Honeyman (2008) observed a more dramatic decrease in PDI (from 78.9 to 66.8) when comparing pelleted diets containing 0% and 20% DDGS, respectively. However, Feoli (2008) showed that adding 30% DDGS to corn/ soybean meal swine diets increased PDI from 88.5 to 93.0.
De Jong et al. (2013) found no differences in PDI values (93.3 versus 96.9), percentage of fines (1.2% versus 8.0%) and production rate (1,098 versus 1,287 kg per hour) among pelleted corn/soybean diets and 30% DDGS diets for nursery pigs using a pellet die of 3.18 mm x 3.81 mm. The inconsistent results suggest that several interactions among processing variables may have contributed to differences in PDI of the DDGS diets in these studies.
The lipid content of diets and feed ingredients affects pellet quality and production rate. Yoder (2016) evaluated the effects on PDI of adding either 15% or 30% of reduced-oil or high-oil DDGS to corn/soybean meal swine finisher diets. Diets were pelleted using conditioning temperatures of 65.6°C or 82.2°C and a 4.0 mm x 32 mm die. Throughput was maintained at a constant rate of 680 kg per hour. Pellet quality was evaluated using four pellet durability tests: standard PDI (ASABE S269.4, 2007), modified PDI using three 19 mm hex nuts, Holmen NHP 100 for 60 seconds and Holmen NHP 200 for 240 seconds.
The dietary inclusion rate (15% or 30%) of DDGS and conditioning temperature had no effect on PDI, but PDI was greater for diets containing reduced-oil DDGS (88.0%) than high-oil DDGS (82.8%). Furthermore, the method used to determine pellet quality dramatically affected PDI, with the highest value obtained for standard PDI (95%), followed by modified PDI (91%), Holmen NHP 100 (89%) and Holmen NHP 200 (67%).
The results of this study indicate that relatively high PDI can be achieved in corn/soybean meal-based swine finishing diets containing up to 30% DDGS and that adding reduced-oil DDGS improved PDI by about five percentage points compared with adding high-oil DDGS to the diet. However, caution should be used when comparing PDI values among studies because the use of various PDI test methods can lead to differences in interpretation of acceptable PDI.
Energy and nutrient digestibility.
More recently, Rojas et al. (2016) evaluated the effects on energy and nutrient digestibility of extruding and pelleting corn/soybean meal and corn/soybean meal/25% DDGS diets. They found that pelleting and extrusion improved apparent ileal digestibility of gross energy, starch, crude protein, dry matter, ash, acid hydrolyzed ether extract and amino acids (Table 2). Furthermore, pelleting increased the metabolizable energy (ME) content by 97 kcal/kg of dry matter, and extruding increased ME by 108 kcal/ kg of dry matter, but the combination of extruding and pelleting did not improve the ME content in the DDGS diets compared to the meal form (Table 3). Similarly, pelleting the corn/soybean meal diet improved ME by 81 kcal/kg of dry matter, and extruding and pelleting increased ME by 89 kcal/kg, but extrusion alone did not improve the ME content.
Growth performance.
Several studies have shown an improvement in feed conversion (Wondra et al., 1995a; Nemechek et al., 2015) and growth rate (Wondra et al., 1995a; Myers et al., 2013; Nemechek et al., 2015) when feeding swine pelleted versus meal diets. A reduction in feed intake is often observed when feeding pelleted versus meal diets, which has been attributed to a reduction in feed wastage (Skoch et al., 1983; Hancock and Behnke, 2001) and improved energy digestibility (NRC, 2012).Feeding pelleted diets containing 15% DDGS had no effect on average daily gain, reduced average daily feed intake and improved gain:feed compared with feeding 15% DDGS diets in meal form to growing/finishing pigs (De Jong et al., 2016). However, when feeding growing/ finishing pigs pelleted diets containing 30% DDGS, there was a trend for improved overall growth rate with no effect on feed intake, and feed conversion was improved compared with meal diets (Fry et al., 2012; Overholt et al., 2016).
Carcass composition and yield
Several studies have shown no effect of feeding pelleted or meal diets on carcass characteristics (Wondra et al., 1995a; Myers et al., 2013; Nemechek et al., 2015), but some studies have shown an increase in carcass yield (Fry et al., 2012) as well as increased back and belly fat (Matthews et al., 2014) when feeding pigs pelleted diets.In a recent study, De Jong et al. (2016) fed pelleted or meal diets containing 15% DDGS and showed no differences in hot carcass weight, carcass yield, backfat depth, loin depth and percentage carcass lean. In contrast, Overholt et al. (2016) fed pelleted diets containing 0% or 30% DDGS to growing/finishing pigs and reported an increase in hot carcass weight and 10th-rib backfat thickness and a reduced percentage of carcass lean compared with meal diets.
However, there was no effect of DDGS inclusion rate on carcass characteristics, including loin muscle quality. Although feeding pelleted diets reduced the weight of the gastrointestinal tract and improved carcass yield, feeding diets containing DDGS increased the weight of the gastrointestinal tract and contents, resulting in reduced carcass yield.
Feed handling and storage
Pelleting DDGS diets is useful for reducing ingredient segregation, improving flowability in bins and feeders and reducing pigs’ sorting of different-sized particles of diets in feeders (Clementson et al., 2009; Ileleji et al., 2007). Flowability can be reduced when DDGS diets are fed in meal form, which can reduce the rate of feed delivery to feeders and can cause bridging in feeders, leading to out-of-feed events that may increase stress and the likelihood of gut health problems and reduce growth performance in pigs (Hilbrands et al., 2016).Storage bin design can be a significant cause or a potential solution to the flowability problems with feed containing DDGS. Hilbrands et al. (2016) evaluated feed flow of 40% DDGS diets using three different designs of commercially available feed storage bins and showed that feed bin design affects flow rate during discharge of meal diets, but installing passive agitators increased feed flow in all bin designs.
Mycotoxin-contaminated DDGS.
Deoxynivalenol (DON), or vomitoxin, is one of the most common mycotoxins found in corn and DDGS that reduces pigs’ feed intake and growth performance. Although most detoxification treatments have been ineffective (Friend et al., 1984; Danicke et al., 2004; Doll et al., 2005), adding sodium bisulfate and thermal treatment has been shown to be effective in converting DON to a nontoxic form (Young et al., 1987; Danicke et al., 2004).
Therefore, Frobose et al. (2015) conducted four experiments to determine the effects of pelleting conditions (conditioning temperatures of 66°C and 82°C and retention times of 30 or 60 seconds within temperature) and the addition of sodium metabisulfate to DDGS contaminated with 20.6 mg/kg of DON. Pelleting conditions had no effect on DON concentrations, but when sodium metabisulfate was added to DDGS at increasing concentrations, DON concentrations were reduced. Furthermore, when DON-contaminated DDGS diets containing sodium metabisulfate were pelleted and fed to nursery pigs, daily gains and feed intakes increased. These results suggest that adding sodium metabisulfate to DON-contaminated DDGS prior to pelleting nursery pig diets helps reduce the negative effects of DON on growth performance.
Therefore, Frobose et al. (2015) conducted four experiments to determine the effects of pelleting conditions (conditioning temperatures of 66°C and 82°C and retention times of 30 or 60 seconds within temperature) and the addition of sodium metabisulfate to DDGS contaminated with 20.6 mg/kg of DON. Pelleting conditions had no effect on DON concentrations, but when sodium metabisulfate was added to DDGS at increasing concentrations, DON concentrations were reduced. Furthermore, when DON-contaminated DDGS diets containing sodium metabisulfate were pelleted and fed to nursery pigs, daily gains and feed intakes increased. These results suggest that adding sodium metabisulfate to DON-contaminated DDGS prior to pelleting nursery pig diets helps reduce the negative effects of DON on growth performance.
Inactivation of porcine epidemic diarrhea virus (PEDV).
PEDV can be transmitted by feed and feed ingredients (Dee et al., 2014; Schumacher et al., 2015). However, PEDV is a heat-sensitive virus, so the temperature and time of exposure of swine feeds during the pelleting process may reduce the infectivity of PEDV in complete feeds (Pospischil et al., 2002; Nitikanchana, 2014; Thomas et al., 2015).Cochrane et al. (2017) showed that conditioning and pelleting temperatures greater than 54.4°C appear to be effective in reducing the quantity and infectivity of PEDV in swine feeds. In fact, their results showed that pelleting diets inactivated PEDV at a faster rate (30 seconds) and at a much lower temperature than those (145°C and 10 minutes) reported by Trudeau et al. (2016).
It is unknown if pelleting swine diets reduces the quantity and infectivity of other pathogens, but it appears to be an effective strategy to partially reduce the risk of transmission of PEDV from feed mills to swine farms.
Increased diet cost.
Pelleting diets increases diet cost (Wondra et al., 1995b), but this is acceptable if the economic benefits resulting from improved growth performance, reduced mortality and improved handling and bulk density exceed the added cost.Low PDI and increased fines may affect growth performance.
However, Knauer (2014) also evaluated the effects of feeding pelleted diets containing two particles sizes of DDGS (640 µm versus 450 µm) and two levels of pellet fines and observed no effects on the growth performance of finishing pigs.
Smaller particle size for pelleting diets may increase gastric ulcer incidence.
Gastric lesions and ulcers are a common problem in swine production (Grosse Liesner et al., 2009; Cappai et al., 2013) and contribute to significant financial losses (Friendship, 2006). Hyperkeratosis, mucosal erosions and bleeding ulcers have been observed more commonly in pigs fed pelleted diets than mash diets (Mikkelsen et al., 2004; Canibe et al., 2005; Cappai et al., 2013; Mosseler et al., 2014; Liermann et al., 2015).
Although the reasons for this occurrence are not well defined, several researchers have suggested that diet particle size is a contributing factor. Vukmirovic et al. (2017) indicated that a further reduction in particle size occurs during the pelleting process but concluded, after summarizing results from all published studies, that swine diets containing less than 29% of particles smaller than 400 µm are low risk for ulcer occurrence.
De Jong et al. (2016) reported that pigs fed pelleted diets (with or without 15% DDGS) for at least 58 days of the 118 grower/finisher feeding period had a greater prevalence of stomach ulcerations and keratinization than pigs fed meal diets. However, these researchers also observed that alternating between feeding pelleted diets and meal diets during the finishing phase may help maintain improvements in feed conversion while reducing the incidence of stomach ulcerations.
Similarly, Overholt et al. (2016) fed growing/finishing pigs meal or pelleted diets containing 0% or 30% DDGS and found that pigs fed pelleted diets had greater gastric lesion scores in the esophageal region compared to pigs fed a meal diet, but the addition of 30% DDGS to the diets had no effects on the incidence of gastric lesions.
Although the reasons for this occurrence are not well defined, several researchers have suggested that diet particle size is a contributing factor. Vukmirovic et al. (2017) indicated that a further reduction in particle size occurs during the pelleting process but concluded, after summarizing results from all published studies, that swine diets containing less than 29% of particles smaller than 400 µm are low risk for ulcer occurrence.
De Jong et al. (2016) reported that pigs fed pelleted diets (with or without 15% DDGS) for at least 58 days of the 118 grower/finisher feeding period had a greater prevalence of stomach ulcerations and keratinization than pigs fed meal diets. However, these researchers also observed that alternating between feeding pelleted diets and meal diets during the finishing phase may help maintain improvements in feed conversion while reducing the incidence of stomach ulcerations.
Similarly, Overholt et al. (2016) fed growing/finishing pigs meal or pelleted diets containing 0% or 30% DDGS and found that pigs fed pelleted diets had greater gastric lesion scores in the esophageal region compared to pigs fed a meal diet, but the addition of 30% DDGS to the diets had no effects on the incidence of gastric lesions.
Pelleting may increase lipid peroxidation and reduce vitamin and feed enzyme activity.
Because the pelleting process involves heat and moisture, these conditions can contribute to increased lipid peroxidation (Shurson et al., 2015) and reduced vitamin activity (Pickford, 1992).
Jongbloed and Kemme (1990) determined that when swine diets containing phytase are pelleted at conditioning temperatures of >80°C, phytase activity is reduced. Consequently, this reduces the effectiveness of phytase for improving phosphorus digestibility. Although many factors in the pelleting process may affect phytase activity, as conditioning temperatures increase, phytase inactivation increases (Simons et al., 1990).
No studies have been conducted to determine the effects of pelleting on other types of feed enzymes (e.g., carbohydrases and proteases), but thermal treatment may partially reduce the activity of some forms of these enzymes.
Jongbloed and Kemme (1990) determined that when swine diets containing phytase are pelleted at conditioning temperatures of >80°C, phytase activity is reduced. Consequently, this reduces the effectiveness of phytase for improving phosphorus digestibility. Although many factors in the pelleting process may affect phytase activity, as conditioning temperatures increase, phytase inactivation increases (Simons et al., 1990).
No studies have been conducted to determine the effects of pelleting on other types of feed enzymes (e.g., carbohydrases and proteases), but thermal treatment may partially reduce the activity of some forms of these enzymes.
Prediction equations to improve pellet quality of DDGS diets for swine.
The inconsistent results reported in pellet durability, production rates and energy usage among published studies for swine indicate that there are many interactions among the various factors that affect these important measures. To address the complexity of these interactions and predict the effects of adding DDGS to swine diets, Fahrenholz (2012) developed prediction equations for PDI and energy consumption when pelleting these diets.
The PDI equation (R-squared = 0.92) was shown to be highly accurate in predicting PDI compared with actual PDI within a 1.1% variation. The pellet die L:D ratio has the greatest effect on PDI, where decreasing the die thickness from 8:1 (common in the industry) to 5.6:1 decreased PDI by 10.9 units. Increasing conditioning temperature from 65°C to 85°C increased PDI by 7.0 units, and decreasing the supplemental soybean oil content in the diet from 3% to 1% increased PDI by 5.4 units. Decreasing the particle size of ground corn from 462 µm to 298 µm contributed to a small, 0.5-unit increase in PDI. Similarly, reducing the feed production rate from 1,814 kg to 1,360 kg per hour increased PDI by only 0.6 units and had a minimal effect on PDI. THE energy consumption equation (R- squared = 0.95) was also shown to be highly accurate in predicting kilowatt hours per ton with actual energy use within a 0.3% variation. Increasing the conditioning temperature from 65°C to 85°C had the greatest effect on reducing energy consumption (2.7 kWh per ton), while a thinner die L:D (5.6:1) reduced energy use by 1.3 kWh per ton. No other factor — including changing corn particle size from 462 to 298 microns, percent soybean oil (fat) from 1% to 3%, percent DDGS from 0% to 10%, production rate from 1,360 kg to 1,814 kg per hour or retention time from 30 to 60 seconds — affected energy consumption by more than 1.0 kWh per ton.
The PDI equation (R-squared = 0.92) was shown to be highly accurate in predicting PDI compared with actual PDI within a 1.1% variation. The pellet die L:D ratio has the greatest effect on PDI, where decreasing the die thickness from 8:1 (common in the industry) to 5.6:1 decreased PDI by 10.9 units. Increasing conditioning temperature from 65°C to 85°C increased PDI by 7.0 units, and decreasing the supplemental soybean oil content in the diet from 3% to 1% increased PDI by 5.4 units. Decreasing the particle size of ground corn from 462 µm to 298 µm contributed to a small, 0.5-unit increase in PDI. Similarly, reducing the feed production rate from 1,814 kg to 1,360 kg per hour increased PDI by only 0.6 units and had a minimal effect on PDI. THE energy consumption equation (R- squared = 0.95) was also shown to be highly accurate in predicting kilowatt hours per ton with actual energy use within a 0.3% variation. Increasing the conditioning temperature from 65°C to 85°C had the greatest effect on reducing energy consumption (2.7 kWh per ton), while a thinner die L:D (5.6:1) reduced energy use by 1.3 kWh per ton. No other factor — including changing corn particle size from 462 to 298 microns, percent soybean oil (fat) from 1% to 3%, percent DDGS from 0% to 10%, production rate from 1,360 kg to 1,814 kg per hour or retention time from 30 to 60 seconds — affected energy consumption by more than 1.0 kWh per ton.
Conclusion
In addition to improving growth performance and nutrient digestibility, there are several advantages to pelleting DDGS diets, such as partially inactivating PEDV and perhaps reducing the toxicity of vomitoxin when sodium bisulfate is added prior to pelleting.However, pelleting DDGS diets increases cost, may reduce vitamin and enzyme activity and may increase the percent- age of fines. There are multiple interactions among factors involved in pelleting DDGS diets, but identifying the key factors and understanding their relative significance can be useful for improving pellet quality and production efficiency.
References
The extensive list of references may be obtained by emailing tim.lundeen@farm-progress.com.
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