August 28, 2019
By Jerry Shurson and Amanda Palowski, University of Minnesota Department of Animal Science
Although the majority of swine dried distillers grain with solubles diets in the Midwestern United States are fed in meal form, when these diets must be pelleted, diet inclusion rate of DDGS is often restricted because of 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 the diet inclusion rate 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 has been partially attributed to the partial gelatinization of starch (Richert and DeRouchey, 2010; NRC, 2012). Additional advantages of pelleting diets include reduced 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).
Factors affecting pellet durability, production rate and energy consumption
The three main goals of manufacturing high-quality pelleted swine diets are to achieve high pellet durability and pellet mill throughput, while minimizing 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). Pellet quality is commonly measured by the pellet durability index (ASAE, 1997).
However, almost any adjustment made to increase pellet durability decreases pellet mill throughput and increases energy cost (Behnke, 2006). Production rate of the pellet mill affects PDI and energy consumption. Stark (2009) showed that increasing pellet mill throughput from 545 kilograms per hour to 1,646 kilograms per hour increased pellet mill efficiency from 73.3 to 112.4 kilogram per horse power hour, and linearly reduced PDI from 55.4% to 30.2%. Production of steam for the conditioning stage, and electricity (kilowatt hours per 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 kilowatt hours per metric ton should be a reasonable goal for pelleting swine diets.
Similar to pellet quality, energy consumption of pellet mills 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 as kilowatt-hours per ton (Fahrenholz, 2012). Minimizing energy consumption per ton of pelleted feed can be achieved by maximizing the production rate, which is affected by diet characteristics and die volume (Fahrenholz, 2012).
Installments in the DDGS series
Part 13: Pelleting DDGS diets has benefits, drawbacks
Achieving optimal pellet quality and manufacturing efficiency
Steam conditioning of the mash is considered to be the most important factor in achieving high pellet durability. High conditioning temperature increases PDI and decreases energy consumption (Pfost, 1964) due to decreased 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 increase PDI (Gilpin et al., 2002). However, the effects of steam pressure on improving PDI are 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 where there was 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
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 that is 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 the L to D ratio 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 lubrication of the pellet die and increased 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 particle size of ground corn had no effect on production rate or PDI. Similarly, Stark et al. (1994) reported that reducing diet particle size from 543 to 233 microns only slightly increased PDI. Likewise, Reece et al. (1985) showed that increasing particle size of the diet from 670 to 1,289 microns only slightly decreased PDI.
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 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, this is infeasible in practice because the primary goal in diet formulation is to meet the nutritional needs at a low cost, rather than manipulating formulations to optimize PDI.
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), whereas Cavalcanti and Behnke (2005b) showed that increasing protein content in corn, soybean meal and soybean oil diets increased PDI. In fact, 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), where adding 1.5% to 3% 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, the addition of 5 percentage points of moisture to mash before pelleting has been shown to increase PDI when pelleting high fat diets (Moritz et al., 2002).
Physical and chemical characteristics of DDGS
The chemical composition of DDGS is not conducive for achieving high pellet durability, production rate or decreasing energy use because it contains very low starch content as well as relatively high crude fat and NDF content compared with other common feed ingredients. In fact, DDGS is classified as having low pelletability and a medium degree of abrasiveness on pellet die. There are several reasons for DDGS to be classified as low pelletability (Table 1).
First, DDGS has relatively low moisture content which may require adding moisture to the diet in addition to steam provided in the pellet mill, to achieve a good quality pellet. However, this dependent on the diet inclusion rate of DDGS and the overall moisture content of the diet. Secondly, the starch content of DDGS is low and may be partially gelatinized during the production process, which is not conducive to improving pellet quality. Third, although the relatively high protein content of DDGS contributes to plasticizing the protein during pelleting, which improves pellet quality, the relatively high oil content in DDGS may contribute toward reduced pellet quality depending on diet inclusion rate and amount of other fats or oils in the diet. However, relatively high oil content in DDGS may contribute to improved pellet mill production rates. Fourth, although some feed ingredients contain fiber that serves as a natural binder that contributes to producing good quality pellets, the high amount of insoluble fiber in DDGS reduces production rates of pellet mills. Lastly, 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.
Particle size of DDGS varies from 294 to 1,078 µm among sources (Kerr et al., 2013). Knauer (2014) evaluated the effects of regrinding soybean meal (1,070 vs. 470 µm) and DDGS (689 vs. 480 µm), and the addition of zero or 30% DDGS to swine finishing diets on pellet quality. His results showed that adding 30% DDGS to diets improved modified PDI by 9.5%, regrinding soybean meal improved PDI by 4.7%, but regrinding DDGS had no effect on PDI. Knauer (2014) also evaluated the effects of pelleting diets containing two particles sizes of DDGS (640 vs. 450 µm) and two levels of pellet fines on 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 for swine
Limited studies have been conducted to evaluate pellet durability of swine diets when DDGS is added, and results are inconsistent. Fahrenholz et al. (2008) used a pellet die that measured 3.97 mm × 31.75 millimeters, and a conditioning temperature of 85 degrees 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 (0% DDGS = 90.3 PDI; 10% DDGS = 88.3% PDI; 20% DDGS = 86.8% PDI), he suggested that the practical significance of this reduction was of minimal practical 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 to 96.9), percentage of fines (1.2 to 8.0%), and production rate (1,098 to 1,287 kilogram per hour) among pelleted corn-soybean diets and 30% DDGS diets for nursery pigs using a pellet die of 3.18 millimeters × 3.81 millimeters. The inconsistent results from these studies suggest that there are several interactions among processing variables that may have contributed to differences in PDI of DDGS diets among these studies.
Lipid content of diets and feed ingredients affects pellet quality and production rate. Yoder (2016) evaluated the effects of adding 15 or 30% reduced-oil, and 15 or 30% high-oil DDGS to corn-soybean meal swine finisher diets on PDI. Diets were pelleted using conditioning temperatures of 65.6 degrees C or 82.2 degrees C and a 4.0 millimeters × 32 millimeters die. Throughput was maintained at a constant rate of 680 kilograms per hour. Pellet quality was evaluated using four pellet durability tests (standard PDI, ASABE S269.4, 2007; modified PDI using three 19- millimeters hex nuts; Holmen NHP 100 for 60 seconds; Holmen NHP 200 for 240 seconds). Diet 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%) compared with high-oil DDGS (82.8%). Furthermore, the method used to determine pellet quality dramatically affected PDI, where the highest value was 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 adding reduced-oil DDGS improves PDI by about 5 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
Pelleting swine diets has been shown to improve digestibility of starch (Freire et al., 1991; Rojas et al., 2016), lipid (Noblet and van Milgen, 2004; Xing et al., 2004), as well as 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 of extruding and pelleting corn-soybean meal and corn-soybean meal-25% DDGS diets on energy and nutrient digestibility. 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 ME content by 97 kilocalories per kilogram DM, extruding increased ME by 108 kilocalories per kilogram DM, but the combination of extruding and pelleting did not improve ME content in the DDGS diets compared the meal form (Rojas et al., 2016; Table 3).
Similarly, pelleting the corn-soybean meal diet improved ME content by 81 kilocalories per kilogram DM, and extruding and pelleting increased ME content by 89 kilocalories per kilogram, but extrusion alone did not improve 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 digestibility of nutrients 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).
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 pelleted diets compared to meal diets to swine. A reduction in feed intake is often observed when feeding pelleted diets compared to 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 pelleted diets containing 30% DDGS were fed to growing-finishing pigs, there was a trend for improved overall growth rate with no effect on feed intake, and feed conversion was improved compared with feeding 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 backfat and belly fat (Matthews et al., 2014) when feeding pelleted diets to pigs.
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, 10th rib backfat thickness, and reduced percentage of carcass lean from feeding pelleted diets compared with meal diets. However, there was no effect of DDGS diet inclusion rate on carcass characteristics including loin muscle quality. Although feeding pelleted diets reduced the weight of the gastrointestinal tract and improve carcass yield, feeding diets containing DDGS increased the weight of the gastrointestinal weight 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 sorting of different size particles of diets by pigs in feeders (Clementson et al., 2009; Ileleji et al., 2007). Flowability of DDGS diets can be reduced when DDGS diets are fed in meal form, which can reduce the rate of feed delivery to feeders, as well as bridge in feeders leading to out-of-feed events that can increase stress and the likelihood of gut health problems and reduced 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 the flow rate during discharge of meal diets, but installing passive agitators increase feed flow in all bin designs.
Mycotoxin contaminated DDGS and DDGS diets
Deoxynivalenol (vomitoxin) is one of the most common mycotoxins found in corn and DDGS, that reduces feed intake and growth performance of pigs. Although most detoxification treatments have been ineffective (Friend et al., 1984; Dänicke et al., 2004; Döll et al., 2005), adding sodium bisulfate and thermal treatment has been shown to be effective in converting DON to a non-toxic form (Young et al., 1987; Dänicke et al., 2004). Therefore, Frobose et al. (2015) conducted four experiments to determine the effects of pelleting conditions (conditioning temperatures of 66 degrees C and 82 degrees C and retention times of 30 and 60 seconds within temperature) and the addition of sodium metabisulfate to DDGS contaminated with 20.6 mg/kg deoxynivalenol. Pelleting conditions had no effect on DON concentrations, but when sodium metabisulfate was added at increasing concentrations to DDGS, DON concentrations were reduced. Furthermore, when DON-contaminated DDGS diets containing sodium metabisulfate were pelleted and fed to nursery pigs, ADG and ADFI were increased. These results suggest that adding sodium metabisulfate to DON-contaminated DDGS prior to pelleting nursery pig diets is effective in reducing the negative effects of this mycotoxin on growth performance.
Inactivation of porcine epidemic diarrhea virus
Porcine epidemic diarrhea virus can be transmitted by feed and feed ingredients (Dee et al., 2014; Schumacher et al., 2015). However, PEDV is a heat-sensitive virus and 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 degrees C appear to be effective in reducing the quantity and infectivity of PEDV in swine feed. 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 degrees 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 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). However, this increased cost is acceptable if the economic benefits resulting from improved growth performance, reduced mortality, and improved handling and bulk density exceed this added cost.
Low PDI and increased percentage of fines may affect growth performance
Pellets that are produced with low PDI generally results in an increased amount of fines which 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 that was 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 advantage of feeding pelleted diets (Stark et al., 1993). However, Knauer (2014) also evaluated the effects of feeding pelleted diets containing two particles sizes of DDGS (640 vs. 450 µm) and two levels of pellet fines and observed no effects on growth performance of finishing pigs.
Low particle size used for pelleting diets may increase the incidence of gastric ulcers
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 more commonly observed in pigs fed pelleted diets compared to feeding mash diets (Mikkelsen et al., 2004, Canibe et al., 2005; Cappai et al., 2013; Mößeler 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). Vukmirovic et al. (2017) also indicated that further reduction in particle size occurs during the pelleting process, but concluded from summarizing results from all published studies that swine diets containing less than 29% of particles less than 400 um are low risk for ulcer occurrence. De Jong et al. (2016) reported that when pigs were fed pelleted diets (with or without 15% DDGS) for at least 58 days of the 118 grower-finisher feeding period, there was a greater prevalence of stomach ulcerations and keratinization compared with pigs that were fed meal diets.
However, these researchers also observed that alternating between feeding pelleted diets with 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 meal or pelleted diets containing zero or 30% DDGS to growing-finishing pigs and found that pigs fed pelleted diets had greater gastric lesion scores in the esophageal region compared to pigs that were 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 ³ 80 degrees C, phytase activity is reduced. Consequently, this reduces the effectiveness of phytase for improving phosphorus digestibility. Although there are many factors in the pelleting process that 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 to predict PDI and energy consumption when pelleting these diets.
The PDI equation (R2 = 0.92) was shown to be highly accurate in predicting PDI compared with actual PDI within 1.1% variation. The pellet die L:D ratio has the greatest effect on PDI where decreasing die thickness from 8:1 (common in the industry) to 5.6:1 decreased PDI 10.9 units. Increasing conditioning temperature from 65 degree C to 85 degrees C increased PDI by 7.0 units, and decreasing supplemental soybean oil content in the diet from 3% to 1% increased PDI by 5.4 units. Decreasing particle size of ground corn from 462 µm to 298 µm contributed to a small, 0.5 unit increase in PDI. Similarly, reducing feed production rate from 1,814 to 1,360 kilograms per hour increased PDI by only 0.6 units, and had minimal effect on PDI.
This energy consumption equation (R2 = 0.95) was also shown to be highly accurate in predicting kWh/ton with actual energy use within 0.3% variation. Increasing conditioning temperature from 65 degrees C to 85 degrees C had the greatest effect on reducing energy consumption (2.7 kWh/ton), while a thinner die L:D (5.6:1) reduced energy use by 1.3 kWh/ton. No other factor (corn particle size — 462 to 298 microns; percent soybean oil = fat — 1 to 3%; percent DDGS — 0 to 10%; production rate — 1,360 to 1,814 kilograms per hour; or retention time — 30 to 60 seconds) affected energy consumption by more than 1.0 kWh/ton.
There are several advantages of pelleting DDGS diets in addition to improving growth performance and nutrient digestibility, such as partially inactivating PED virus and may help reduce the toxicity of vomitoxin when combined with addition of sodium bisulfate prior to pelleting. However, pelleting DDGS diets increases diet cost, may reduce vitamin and enzyme activity, and may increase the percentage 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.
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