Evaluation of dietary selenium sources and levels on growth performance, carcass characteristics, selenium concentrations, and blood biochemistry of growing-finishing pigs

The experimental protocol for this study was reviewed and approved by the Institutional Animal Care and Use Committee of Chungnam National University, Daejeon, Korea (approval #: 202006A-CNU-090).

A total of 160 pigs ([Landrace × Yorkshire] × Duroc; initial average body weight [BW] = 28.17 ± 3.03 kg) were assigned to one of five dietary treatments (4 pigs/pen; 8 replicates/treatment) in a randomized complete block design (block = initial BW and sex). Dietary treatments were 1) a non-Se-fortified diet based on corn and soybean meal (CON), 2) CON + 0.3 ppm ISe (ISe3), 3) CON + 0.5 ppm ISe (ISe5), 4) CON + 0.3 ppm OSe (OSe3), and 5) CON + 0.5 ppm OSe (OSe5). The experimental diet was formulated according to the nutritional requirements of growing and finishing pigs, except for Se [17] (Table 1). This study was conducted on two phase feeding programs, with the grower phase from experimental day 1 to 49 and the finisher phase from experimental day 50 to 98. The ISe and OSe products (sodium selenite, 1,000 ppm; Se-yeast, 1,000 ppm, Sel-plex, respectively) were obtained from commercial suppliers (Daone Chemical, Siheung, Korea; Alltech Korea, Seoul, Korea, respectively). Diets were provided in mash form, and pigs had ad libitum access to the feed and water throughout the study. All pigs were housed in same sized pen where ambient temperature, humidity, and lighting program were automatically controlled.

BW of individual pigs and feed residuals in the feeder after supply were weighed and recorded on a pen basis at the end of each phase to calculate average daily gain (ADG), average daily feed intake (ADFI), and gain to feed ratio ([G:F], feed efficiency). Blood samples were randomly collected from six pigs per dietary treatment via the jugular vein of pigs using 10 mL serum tubes (BD Vacutainer Systems, Franklin Lakes, NJ, USA) at the end of each phase. The collected blood samples were centrifuged at 3,000 × g for 15 minutes at 4°C to obtain serum samples and stored at -80°C for further blood analyses. On the last day of the study, one pig per pen with a BW similar to market weight was individually weighed, recorded, and transferred to a commercial slaughterhouse (FarmStory Hannaeng LPC, Seoul, Korea). The day before slaughter, pigs had completely restricted access to feed for 12 hours, but had been allowed access to water. The slaughter process and carcass characteristics evaluation were conducted according to the conventional procedures of the Korea Institute for Animal Products Quality Evaluation. After dividing the carcass into two parts, the liver and kidney were collected before the evisceration process. Pork loins (longissimus muscles) were collected from near the 10th ribs on the right side of the carcass for further analysis. The collected tissue and loin samples were stored at −20°C until Se concentration analysis. Carcass characteristics were evaluated using hot carcass weight, and backfat thickness.

The serum samples were analyzed for total protein, calcium, inorganic phosphorus, magnesium, total cholesterol, triglyceride, glucose, albumin, creatinine, blood urea nitrogen (BUN), aspartate aminotransferase, and alanine aminotransferase using a clinical auto analyzer (Toshiba Acute Biochemical Analyzer-TBA-40FR, Toshiba Medical Instruments, Tokyo, Japan) with specific kits (Wako Pure Chemical Industries, Osaka, Japan) [18]. The other serum samples were analyzed for porcine insulin-like growth factor-1 (IGF-1) using enzyme-linked immunosorbent assay kit (MyBioSource, San Diego, CA, USA) according to the provided manufacturer’s instructions. The concentration of serum IGF-1 was determined using a microplate reader (Epoch microplate spectrophotometer, BioTek Instruments, Winooski, VT, USA).

The proximate composition of the pork loin was evaluated based on moisture, crude protein, crude fat, and ash contents according to the Association of Official Analytical Chemists (AOAC) method [19]. To determine Se concentration in the diets, liver, kidney, pork loin, and serum, the samples were digested in a digestion block (N-biotek, South Korea), acted with 2,3-diaminonaphthalene solution, and analyzed with a fluorescence spectrometer (RF-6000, Shimadzu, Kyoto, Japan) using the fluorometric method [20], as reported in the AOAC (method 996.16) [19].

Data were analyzed using the MIXED procedure of SAS (SAS Institute, Cary, NC, USA) in a randomized complete block design (block = initial BW and sex) with the pen as the experimental unit. Statistical models for growth performance, carcass characteristics, proximate composition of pork loin, Se concentrations, and blood biochemistry included dietary treatments as main effect and blocks as random effects. Contrast statements were applied to determine the dietary Se effects (source, level, and source × level interaction). Statistical significance and tendency among dietary treatments were considered at p < 0.05 and 0.05 ≤ p < 0.10, respectively.

There were no clinical lesions and/or signs of disease associated with Se deficiency or toxicity in all pigs fed the dietary treatments throughout the study. According to the National Research Council, the requirement of dietary Se for pig is 0.15 to 0.30 ppm [17], and the Food and Drug Administration suggests that the dietary Se content in swine feed should not exceed 0.30 ppm [21]. In this study, analyzed dietary Se content in the dietary treatments were as follows: 0.081 (CON), 0.464 (ISe3), 0.623 (ISe5), 0.458 (OSe3), and 0.628 ppm (OSe5). The effects of dietary Se sources and levels on the growth performance of growing-finishing pigs are shown in Table 2. Pigs fed OSe tended to have increase (p < 0.10) G:F in the grower phase compared with those fed ISe. However, there were no differences in the growth performance during the finisher and overall phase among dietary treatments. Most previous studies have shown that different sources and levels of dietary Se did not affect the growth performance of pigs [6,13,22,23]. However, some previous studies have reported that dietary OSe improved the growth performance of pigs compared with dietary ISe or non-Se-fortified diet [8,14,24]. Although the results of improved G:F in the OSe group compared with the ISe group cannot be easily explained, the antioxidant capacity of dietary Se [2,3] or its interaction with reproductive hormones [2,24,25] is assumed to be direct or indirect effects due to the higher bioavailability of dietary OSe than dietary ISe.

Pigs fed ISe had higher (p < 0.05) hot carcass weight in finisher phase than those fed OSe (Table 3). However, there were no differences in dressing percentage, and backfat thickness among dietary treatments. Hot carcass weight is the weight at which the head and internal organs are removed after slaughter and before chilling and is used to evaluate meat quantity rather than meat quality. There may be an error in that high live weight is proportional to high carcass weight, but this is not simple. Therefore, the dressing percentage, expressed as a ratio of live weight, and backfat thickness are considered in the production of high-quality livestock products. In a previous study, hot carcass weight was positively correlated with fat and muscle thickness, as well as negatively correlated with lean yield [26]. Although the increased hot carcass weight did not lead to an increase in backfat thickness in our study, the increased hot carcass weight in the ISe group may be related to bone and/or skeletal muscle development. This is because the bone and muscle are the main tissues in the body that retain Se [3,27]. In addition, adequate dietary Se plays an important role in the proliferation and differentiation of bone cells via the regulation of ROS [28]. Se deficiency can be associated with muscular dystrophy because it induces oxidative stress through decreased expression of SeP genes such as GPX and TRXR [29]. However, dietary Se has been reported to prevent white muscle disease caused by Se deficiency in pigs [30]. Moreover, in a previous in vitro study, it was reported that among different Se sources, sodium selenite reduced intracellular ROS levels in myocytes [31]. Taken together, different dietary Se sources may have differences in bioavailability and cellular metabolism depending on the body tissues.

As shown in Table 4, the crude protein content of pork loin was different among dietary treatments (p < 0.05). Additionally, dietary ISe increased (p < 0.05) the crude protein content of pork loin compared with dietary OSe. The crude ash of pork loin was decreased (p < 0.05) as Se level increased from 0.3 ppm to 0.5 ppm. The interaction between dietary Se source and level was observed on moisture (p < 0.05), crude protein (p < 0.10), and crude fat contents (p < 0.05) of pork loin. In a previous study, dietary ISe had higher moisture content and lower crude protein and fat contents in pork loin than dietary OSe [8]. However, a previous study reported that dietary Se did not affect the crude protein and crude fat contents of pork loin, regardless of Se source [7]. Meat quality should be considered through indicators such as water holding capacity (WHC), color, and pH because proximate composition has limitations in evaluating meat quality. Previous studies have consistently shown that dietary Se was more effective than non-Se, especially dietary OSe than Ise, in reducing drip, pressing, or cooking loss of meat [5–8,22]. Interestingly, dietary Se did not affect meat color and/or pH, regardless of Se source or even level. The effect of dietary Se on the WHC of meat could be related to the upregulation of muscular SeP W, which has antioxidant properties [6,32,33]. However, dietary ISe resulted in a higher drip loss as well as paler-colored muscle than dietary OSe [5]. Although our study only analyzed the proximate composition of pork loin, based on previous studies, dietary ISe may reduce meat quality than dietary OSe. In addition, because the crude protein content of pork loin had a negative correlation with cooking loss [34], meat quality evaluation should be supported by additional research.

As expected, dietary Se had higher (p < 0.05) Se concentration in the liver, kidney, pork loin, and serum than non-Se-fortified diet (Table 5). In addition, pigs fed dietary OSe or high level of Se had higher Se concentration in the liver (p < 0.10 and p < 0.05, respectively) and kidney (p < 0.05 and p < 0.05, respectively) than those fed dietary ISe or low level of Se, and both results showed an interaction (p < 0.05) between source and level. Furthermore, dietary OSe or 0.5 ppm Se had higher (p < 0.05 and p < 0.05, respectively) pork loin Se concentration than dietary ISe or 0.3 ppm Se. However, the differences in the source and level on pork loin did not show any interactions. Pigs fed high level of Se tended to have higher (p < 0.10) serum Se concentration than those fed low level of Se. Dietary Se is absorbed in the small intestine, transported to the liver through the bloodstream, and then distributed to other tissues through the bloodstream after SeP production and metabolism in the liver [21,35]. Therefore, the liver is the main organ responsible for regulating Se metabolism in the body. In addition, the liver mirrors the degree of intestinal absorption [36]. The kidney plays a major role in the utilization of Se to protect the cellular membranes involved in performing their function as well as the excretion of Se [14,37]. Consistent with our study, the previous studies also showed that the Se level in the kidney of pigs was higher than that in the liver or pork loin [5,12,14,23]. Moreover, the Se concentrations in the liver, kidney, and pork loin were higher in dietary OSe than the ISe [12,14,23]. These results indicate that the OSe is more effective than the ISe for absorbing Se from the small intestine and retaining the levels for Se metabolic utilization in the body. Se level in tissues reflects long-term status of animals, while Se level in blood along with urine reflects the short-term status of Se intake [13,37]. Unlike Se concentrations in tissues, this study showed that Se concentration in serum differed only at the Se level, regardless of the Se source. Interestingly, some previous studies have reported that the effects of Se source on serum Se concentration were reduced in the finishing period, but not in the growing period [14,23]. Moreover, serum Se concentration was high when dietary ISe was added at a low level (i.e. 0.5 mg/kg), whereas when dietary OSe was added, serum Se concentration also increased with increasing Se level [5]. These results suggest that there are differences in the concentrations of Se retained in the blood of pigs at different growth stages depending on the source as well as the level of Se. Furthermore, based on the reference values of Se levels in blood for Se deficiency or toxicity [24], our result supported that the pigs were neither deficient nor in toxicity condition and that it was not associated with health problems during the study. However, the blood Se level in the CON group was at the marginal level, indicating that the addition of dietary Se to feed should be considered.

In the grower phase, pigs fed dietary OSe tended to have higher (p < 0.10) serum BUN concentration than those fed dietary ISe (Table 6). However, there were no differences in other biochemical indices among dietary treatments. On the other hand, high level of dietary Se had lower concentrations of serum total protein (p < 0.05), calcium (p < 0.05), inorganic phosphorus (p < 0.05), magnesium (p < 0.05), total cholesterol (p < 0.10), albumin (p < 0.10), and creatinine (p < 0.05) in the finisher phase than low level of dietary Se. BUN is a useful predictor of protein status in animals because it is related to nitrogen utilization efficiency [38]. In addition, blood BUN has been reported to be negatively correlated with feed efficiency and lean growth in pigs [39]. However, in the current study, dietary OSe supplementation resulted in higher blood BUN during the grower phase than dietary ISe, which is expected to result in low protein and amino acids utilization, but in fact resulted in high feed efficiency in growing pigs. When considering the protein metabolism, previous results of dietary Se on crude protein digestibility or nitrogen retention have been inconsistent [14,23], but our results indicated that dietary OSe may have a negative effect on protein synthesis in the grower phase. Blood creatinine level, along with BUN, is related to the health of the liver and kidney, which are the main organs involved in amino acids deamination and urea synthesis. Additionally, phospholipid hydroperoxide GPX, one of the SeP, is known to inhibit lipid peroxidation due to its ability to reduce lipid and cholesterol peroxides [4,40], and its regulations by dietary Se were confirmed [24,29]. Our results indicated that increasing dietary Se level affected not only Se concentration in the liver and kidney, but also the nutritional metabolism of the organs that play an important role in Se metabolism and excretion. In addition, because blood values reflect the nutritional, physiological, and health status of animals [15,16], blood metabolic changes caused by dietary Se supplementation appear to have affected blood total protein and albumin, which are components of blood proteins. Furthermore, blood biochemistry results may be related to previous study showing that different source and level of dietary Se influence the retention and excretion of macro-minerals such as calcium, phosphorous, and magnesium [13].