Introduction

Selenium (Se) is an essential trace element in human and animal life1. Nano-Se, as a natural antioxidant with high biological activity, has low toxicity compared to both organic and inorganic Se2. Not only can it be absorbed and utilized by human body, but also it can exert the unique antioxidant and immune regulatory functions of organic and inorganic Se2. Se can reduce oxidized low-density lipoprotein, reduce lipid deposition on arterial walls, increase the activity of glutathione peroxidase in platelets, and inhibit platelet aggregation3. Se also participates in cellular oxidative phosphorylation and energy metabolism, eliminating free radicals while undergoing lipid peroxidation, thereby enhancing the antioxidant capacity of animals4.

Curcumin (CUR) is a natural substance extracted from plants in the ginger family and is the main active ingredient in turmeric, which has various biological functions such as antibacterial, anti-inflammatory, antioxidant, liver protection, and anti-tumor5. It can promote pig growth and improve pork quality. CUR has antibacterial and anti-inflammatory functions, and its main mechanism of action is to inhibit inflammatory factors by inhibiting biological activity related signaling pathways. CUR combines with intracellular free radicals to produce stable phenolic compounds, clearing intracellular free radicals, reducing malondialdehyde content, inhibiting lipid metabolism and oxidation, and improving pork quality5. Research has found that CUR can regulate the innate immune system and digestive function of pigs6. Adding 200 mg·kg−1CUR to the diet of intrauterine growth retardation (IUGR) piglets may enhance the antioxidant function of live IUGR pigs and alleviate oxidative stress through the Nrf2 signaling pathway7. Glycyrrhizic acid (GA) is isolated from the roots and rhizomes of licorice and has antiviral, anti-inflammatory, and hepatoprotective effects8. GA can maintain the balance of gut microbiota and improve metabolic disorders, promoting healthy growth of piglets.

With the aggravation of metabolic process of sows in the later stage of pregnancy, the reactive oxygen species (ROS) produced in metabolic process is excessive and antioxidant capacity is decreased, resulting in oxidative stress9. IUGR piglets usually refer to newborn mammals whose fetal or postpartum growth and development are impaired due to intrauterine dysfunction10. Due to the congenital lack of antioxidant system in newborn animals, the physiological oxidative stress occurring during the delivery of mother is easy to be transmitted to offspring, resulting in offspring piglets suffering from more serious oxidative stress11, thus bearing serious pressure12. Previous study found that it could improve the survival ability of IUGR piglets and promote the growth of piglets by improving maternal dietary nutrition13,14. Another study also found that the antiviral effect of GA enhanced body’s immunity15, the addition of nano-Se to the mother diet improved the newborn litter weight of offspring piglets16, and the combination of nano-Se and macleaya cordata extract (MCE) in the sow’s diet also enhanced the resistance of newborn IUGR piglets17. However, there is a lack of research on whether the combination of maternal nano-Se and CUR or glycyrrhiza extract (GE) can promote the reproductive performance of sows and the growth of offspring piglets, or it have a continuation effect on next birth.

Therefore, the aim of this study is to investigate the effects of the combination of nano Se and CUR or GE in the diet of primiparous sows on the reproductive performance of first and second litter sows, in order to determine whether the addition of nano Se and CUR or GE to the diet of primiparous sows has a sustained effect on the reproductive performance of next parity.

Materials and methods

All animal procedures were approved by the Institutional Animal Care and Use Committee of Yibin University (2023036) and the experiment was complied with Laboratory animal—Guideline for ethical review of animal welfare (GB/T 35892 − 2018). Also, all the experiments in the manuscript follows the recommendations in the Animal Research Reporting in Vivo Experiments (ARRIVE) guidelines.

Sources of nano-Se, CUR and GE

Nano-Se (> 99%, 30–70 nm particle size, and 45 nm average particle size) was provided by Sichuan Jilongda Group, China. CUR (> 98%) was purchased from a biotechnology company in Guangdong Province, China. GE (> 95%) was purchased from Natural Products Research Lab, Southwest University of Science and Technology, Mianyang, China.

Experimental design

The farm lies in Weining City of Guizhou Province, China. A total of 54 primiparous sows (Landrace × Yorkshire) were randomly divided into three groups (18 sows per group): (1) CON group, basal diet (0.30 mg·kg−1 Se, sodium selenite); (2) CUR group, basal diet + 0.20 mg·kg−1 Se (nano-Se) + 300 mg·kg−1 CUR; (3) GE group, basal diet + 0.20 mg·kg−1 Se (nano-Se) + 500 mg·kg−1 GE. The trial lasted for approximately 180 days from day 90 of gestation of primiparous sows to next birth.

Feeding management

The animal experiment was conducted in the breeding pig farm from April to October 2023, amounting to approximately 180 days. Sows were raised separately in the pregnancy house (2.0 × 0.6 m) from d 90 to d 107 of pregnancy. At d 7 before delivery, sows were transferred to the delivery room. The delivery bed was made of cast iron leakage plate with an area of 2.1 × 3.6 m, and the piglet incubator area was 55 cm × 105 cm. The temperature in the sow delivery room was in the range of 25–30 ℃, and the humidity was 40–60%. After piglets were weaned at 25 days of age, sows were moved to the limit bar of the pregnancy house (2.0 × 0.6 m). Sows drank freely during the whole experiment. The immunization and disinfection measures were carried out according to the farm regulation. The basic diet for lactating sows referred to Liu et al.18, while the basal diet for pregnant sows was same as commercial feed purchased from the breeding farm. CUR and GE were added in the form of nutrient packs (diluted to 2 kg with glucose and the daily intake for each sow was calculated, then were added along with the feed). The basal diet of sows was shown in Table 1, and the feeding procedure of sows was shown in Table 2.

Table 1 The basal diet composition and nutrient levels of Sow diets (DM basis).
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Table 2 The feeding procedures of sows during pregnancy and lactation.
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Sow and piglet performance

At birth, the number of total piglets, live piglets, healthy piglets, live stillborn, and IUGR piglets (the birth weight of live piglets was less than 0.80 kg) were recorded. The birth interval between the first piglet and the last piglet was recorded, and the farrowing duration (FD) were calculated. Each piglet was weighed at birth. The feed intake of each sow was recorded daily, and the average daily feed intake (ADFI) during lactation was also calculated. The ADFI is equal to the daily feeding amount minus the remaining amount and the amount of wasted feed.

Sample preparation and analysis

At farrowing, blood samples were randomly collected from the limbal vein of eight sows in each group with vacuum collection vessels containing no anticoagulant, centrifuged at 3,500 r/min for 10 min. Then the serum was separated and stored at − 80 ℃ until further analysis. The levels of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), malondialdehyde (MDA), interleukin 6 (IL-6), interleukin 10 (IL-10), interleukin 1β (IL-1β), and tumor necrosis factor α (TNF-α) were analyzed using commercial kits (Nanjing Jiancheng Bioengineering Institute, Nanjing, China).

Statistical analyses

Statistical analyses were conducted using GLM of SPSS (SPSS, version 23.0, Inc., Chicago, Illinois, USA). Differences among treatment means were determined using the ANOVA test. Variability in the data was expressed as SEM. A probable level of p < 0.05 indicated statistical significance.

Results

Reproductive performance of primiparous sows

There were no significant differences on the reproductive performance of primiparous sows among the three groups (p > 0.05, Table 3), but the live and healthy piglets in the GE group were 0.83 piglet and 1.17 piglet higher than those in the CON group, respectively. The FDs in the CUR and GE groups were reduced by 33 min and 44 min compared with the CON group, respectively (p > 0.05, Table 3).

Table 3 The reproductive performance of primiparous sows.
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Reproductive performance of next birth (parity 2)

There were no significant differences on the reproductive performance of next parity among the three groups (p > 0.05, Table 4), but the live and healthy piglets in the GE group were 1.14 piglet and 0.68 piglet higher than that in the CON group, respectively. The FDs in the CUR and GE groups were reduced by 25 min and 32 min compared with the CON group, respectively (p > 0.05, Table 4).

Table 4 The reproductive performance of the sows (parity 2).
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The growth performance of piglets during lactation

The litter weight and litter weight gain of piglets of primiparous sows during lactation in the GE group were 9.28 kg higher than those in the CON group (p < 0.05, Table 5).

Table 5 The growth performance of offspring piglets (parity 1, day 25).
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The antioxidant capacity in the sow serum

Compared with the CON group, the SOD and GSH-Px levels in the sow serum from the GE group were significantly increased, and the serum MDA content was significantly decreased (p < 0.05, Table 6). The combination of nano-Se and CUR increased significantly the serum GSH-Px level in the sow at delivery and decreased significantly the serum MDA content compared with the CON group (p < 0.05, Table 6).

Table 6 The antioxidant function in the serum of primiparous sows at parturition.
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The immune functions in the sow serum

GE in combination with nano-Se decreased significantly the IL-6 and IL-1β levels of the serum in the primiparous sow at delivery, and increased the IL-10 and TNF-α levels in the sow serum (p < 0.05, Table 7). CUR in combination with nano-Se decreased significantly the IL-1β level and increased significantly the serum TNF-α level (p < 0.05, Table 7).

Table 7 The immune function in the serum of primiparous sows at parturition.
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Discussion

Reproduction performance of primiparous sows and next birth

Adding an appropriate dose of minerals in the diet is conducive to promoting animal growth and reproduction11,19. Se can improve the fertility of female animals and promote the uterus of female animals to be in the best state20. In the later stage of pregnancy, females are supplemented with Se, then Se can enter the piglets through the placental barrier or milk, improve the antioxidant capacity of piglets, and thus promote the growth of piglets13. Lin et al.16 found that nano-Se (0.7 mg·kg−1 Se) added in the sow diet during late pregnancy and lactation increased significantly the birth weight of piglets. Li et al.17found that the combination of nano-Se and MCE in the sow diet during late pregnancy shortened significantly the sow labor process. Liu M21. observed that the addition of nano- Se (0.3 mg·kg−1 Se) to sow diets increased the weaning litter weight and lactation litter weight gain of piglets. In this experiment, the combination of nano-Se and GE shortened the FDs of sows and increased significantly the litter weight gain of piglets during lactation, indicating that the combination of nano-Se and GE might reduce the oxidative stress of sows during farrowing by improving the resistance of sows, so as to promote the growth of offspring piglets. Adding nano-Se and GE to sows’ diets during late pregnancy also significantly improved the litter weight gain of offspring piglets during lactation, which further indicated that nano-Se and GE also had a synergistic effect on piglets. However, the combination of nano-Se and GE in the sow diet during late pregnancy had no significant effect on the reproductive performance of sows, but can enhance the number of live and healthy piglets of primiparous sows and parity-2 sows.

The antioxidant capacity of the sow serum

The antioxidant system of animals is usually in dynamic equilibrium. When the animal body is stimulated and damaged by the external factors, the balance between the body’s oxidation system and antioxidant system is out of balance, leading to oxidative stress in animals22. Oxidative stress is mainly caused by oxidative reduction dysfunction induced by ROS and reactive nitrogen species (RNS), leading to lipid and protein oxidation, as well as DNA and cell damage23. There is evidence to suggest that sow delivery can lead to fetal oxidative stress, which may be related to an imbalance between free radical accumulation and cellular adaptive antioxidant capacity. Antioxidant enzymes are proteins present in the cellular environment that can eliminate free radicals in the body by regulating the activity and quantity of antioxidant enzymes. GSH itself contains thiol groups that are prone to oxidation, and it can quickly and effectively eliminate excessive free radicals in the body. As an important indicator for evaluating the body’s oxidative stress status, it is an important factor in measuring the body’s antioxidant capacity. MDA is an important indicator reflecting endogenous lipid peroxidation in the body. SOD and GSH-Px activities, as well as MDA content, can reflex the level of antioxidant capacity and the degree of cell damage24,25. In vivo, SOD and GSH-Px can remove superoxide anion26, H2O227, and free radicals28,29, so as to reduce cell damages.

As an essential trace element for animals, Se is a key element to maintain the normal life activities of organisms, such as reproductive process30,31,32. In addition, Se can also reduce lipid peroxides and hydrogen peroxides, thereby avoiding oxidative damages33,34,35. Nano-Se can specifically inhibit the production of free radicals, maintain the normal morphology of cell membrane, and ensure the normal operation of cells36. Research has found that Se can improve the reproductive capacity of female animals and promote their uterus to be in optimal condition37. Therefore, Se is commonly used as a nutritional additive for breeding livestock. Natural plant extracts have been widely used in sow production due to their biological functions such as antibacterial activity, immunomodulatory function, antioxidant propety, and growth-promoting effect. CUR is a commonly used traditional Chinese medicine, and as a plant pigment extracted from turmeric, CUR has effects such as promoting growth, antagonizing inflammatory reactions, and antioxidant properties38. Adding 750 mg·kg−1extracts of lonicera japonica and scutellaria baicalensis extracts to the feed of sows (parity 3–4) increased significantly the number of live and healthy piglets and reduced significantly the number of stillbirths39.

Nano-Se and CUR or GE have strong antioxidant and free radical scavenging abilities, and can protect cells from damage by inhibiting the production of free radicals17. For breeding animals, compared with inorganic Se, organic Se is easier to deposit in the fetus through the animal placenta or milk, which may enhance the immune function of piglets40. The previous study showed that the addition of 50–250 mg·kg−1GE to the diet of weaned piglets improved the antioxidant capacity of piglets and promoted significantly the animal growth41. Supplemental nano-Se increased expression profiles of antioxidant enzymes in the liver and spleen of weanling pigs42. Li et al.17 found that the newborn litter weight of offspring piglets was significantly higher than that from the CON group when nano-Se was used in the feed of sows in the late pregnancy period or in combination with MCE. This study indicated that the addition of nano-Se and GE to the sow’s diet during late pregnancy increased significantly the serum SOD and GSH-Px levels, and decreased significantly the MDA content of sows at delivery. Nano-Se and GE might indirectly improve the antioxidant capacity of sows by reducing oxidative stress during delivery, so as to enhance the resistance of offspring piglets. The result was consistent with the increase of litter weight of offspring piglets during lactation, which further indicated that nano-Se and GE improved the growth performance of offspring piglets by improving the ability of sows to resist the oxidative stress.

The immune function of the Sow serum

Infected by foreign pathogens or viruses, inflammation occurs in the body, and the level of pro-inflammatory factor IL-6 in the serum decreases, while anti-inflammatory factors IL-10 and TNF-α levels will slightly increase43. Due to the congenital lack of immune antibodies, newborn piglets have a very weak ability to resist external adverse pressure and disease invasion44. Inflammation in piglets can cause diarrhea or death, which in turn affects their growth performance and can cause significant economic losses to pig farmers45. Due to the immature development of their own immune system, suckling piglets are easily infected by exogenous pathogenic microorganisms, which can cause slow growth or death. The immune level of lactating piglets during lactation is mainly provided by their mother. Therefore, it is particularly important to enhance the resistance of offspring by increasing the immune level of sows. Previous study showed that nano-Se and MCE reduced the level of proinflammatory factors in weaned piglets and improve the immune function of animals17. Organic Se can increase the level of endogenous antioxidant enzymes in the mice and increase the levels of immune factors46. Adding CUR to the feed of lactating sows can significantly improve the growth performance of piglets and improve their immune functions47. Maternal MCE and nano-Se can improve the immune function of IUGR piglets39. GE can treat rotavirus-induced enteritis by coordinating antiviral and anti-inflammatory effects48. CUR or GE may partially block the release of TNF-α, reduce ROS production, and inhibit the necrotic apoptosis pathway, thereby further reducing inflammation and oxidative damage49,50. In this study, the combination of nano-Se and GE or CUR decreased significantly the IL-10 and IL-1β levels in sow's serum, and increased significantly the IL-6 and TNF-α levels in sow's serum. The changes of immune factors in the sow serum indicated that the maternal immune function was improved and transmitted to offspring through milk or blood, thus indirectly improving the immune function, inflammatory response, and the resistance of offspring piglets.

Conclusion

In this study, it was concluded that dietary supplemented with nano-Se and GE in primiparous sow exerted a synergistic effect. This effect was achieved by enhancing the antioxidant function and immune defense of sows, thereby promoting the growth of offspring piglets and improving the reproductive performance of sows in their subsequent litters.