Effects Of Dietary Supplementation With Folic Acid On Growth Performance And Immune Function In Weanling Pig

Three experiments were conducted to determine the effects of dietary supplementation with folic acid (FA) on performance and immune function in weanling piglets. On the basis of immune suppression model and immune activation model established in piglets respectively, the mechanism on immunoregulation function and disease-resistant effectiveness of FA was explored initially, and the optimal doses of dietary FA for weanling piglets under different immune states were determined respectively. As the basic evidences for disease-resistant theory of dietary FA., the results may advance our understanding of how dietary folate affects the immune function during weaning, which could be applied to pig production also. Experiment 1. Effects of dietary supplementation with folic acid on performance and immune function of weanling piglets infected porcine circovirus-2 (PCV-2)A 2x3 factorial (two states:PCV-2 challenge or not; three added FA levels as 0,0.3, or 15 mg/kg repectively) randomized block design was conducted in this study. Fourty-five crossbred (Duroc x Landrace x Yorkshire) castrated piglets were selected from a PCV-2 negtive pig herd, which were newly weaned at 28±1 days of age with an average initial body weight of 8.44±0.12 kg. Alloted equally into 3 three trial diet groups, piglets were fed ad libitum for 35 days individually. Seven piglets from each trial diet group were challenged with PCV-2 in the morning of d8. Blood and tisssue samples were conllected on dl, d7 (without PCV-2 challenge), d21 & d35. The growth performance, diarrhoea incidence, intestinal morphology, immune organs index, histopathological change, proliferation and apoptosis of marrow cell, apoptosis of peripheral lymphocyte, proportion of circulating T lymphocytes subsets, and serum levels of cytokines or immunoglobulins (Ig) were measured respectively.Results showed that:(1) The model was established efficiently, according to the depressed performance and intensified immunosuppression and increased tissue damage under PCV-2 challenge; (2) Without PCV-2 infection, different levels of immune tissue damages were also found among the piglets with intense stress during weaning; Dietary supplementation with FA 15mg/kg improved folate status in weaning piglets with elevating the serum FA (P<0.01), and obviously restored the tissue damage of immune organs with enhancing cell proliferation of pig marrow (P<0.05), decreasing the apoptosis ratio of peripheral lymphocyte (P=0.08), elevating proportions of CD3+ (P<0.05), CD3+CD4+ (P<0.01) and CD3+CD8+ (P=0.10) cell in circulating lymphocytes; (3) Under PCV-2 challenge, the piglets fed basis diet had the severest immunosuppression and tissue damages; Dietary supplementation with FA 0.3 mg/kg relieved partly the immunosuppression and tissue damages resulting from PCV-2 challenge, with enhancing cell proliferation of pig marrow on d35 (P<0.05), decreasing the apoptosis ratio of marrow cell (P<0.05), and elevating proportions of CD3+CD4+ (P<0.05) cell and CD3+CD8+ cell (P<0.05) in circulating lymphocytes; dietary supplementation with 15 mg/kg FA decreased the apoptosis ratio of peripheral lymphocyte in intermediate stage (P<0.05) and reduced cell proliferation of pig marrow in later stage (P<0.05), which profited to the reproduce and diffusion of PCV-2 in body but disadvantaged restoration of immune system in piglets; (4) With or without PCV-2 challenge, the same supplemental level of dietary FA had different effects on performance and immune function in piglets, which implied that the suitable dose of FA might be different in term of different infection status. Experiment 2. Study on optimizing dietary folic acid level for 6-10kg weanling pigletsIn order to test further the hypothesis that sufficient folate intakes during weaning would improve immune function and performance, and to optimize the dietary supplementation with folic acid (FA) in 6-10kg weanling piglets, a single factorial randomized block design was conducted with 6 dietary FA supplemented levels at 0,0.3,3.0,6.0,9.0 or 15.0 mg/kg. A total of 108 crossbred (Landrace×Yorkshire) castrated weanling piglets (21±1 days of age) were used with an average initial body weight of 6.93±0.07 kg. The piglets were allocated by initial body weight into 36 feeding cages with 3 piglets per cage. Six cages were allotted randomly into 6 dietary groups. Piglets were fed ad libitum for 24 days. Blood samples were collected on d24. The growth performance, diarrhoea incidence, circulating T lymphocytes subsets, and serum concentrations of cytokines (IFN-y, TNF-a and IL-4), nitrogen monoxide and immunoglobulin G were measured respectively. Results showed that dietary FA levels tended to influence the BW on d24 (P<0.1), the ADG of 8-24d and 0-24d (P<0.1), and the G/F of 8-24d and 0-24d (P<0.01); Dietery supplementation with FA at 0.3 or 3mg/kg elevated the proportion of circulating CD3+CD8+ subset (P<0.05), decreased the CD3+CD4+/CD3+CD8+ ratio (P<0.05), and increased the serum IFN-y level (P<0.05). It indicates that dietary supplementation with FA during weaning can improve the immune function and the feed efficiency in piglets; The optimal FA intake on performance are 1.4-1.5mg/d during 0-7d and 2.9-3.0mg/d during 8-24d respectively for weanling piglets. Experiment 3. Dietary supplementation with folic acid relieves lipopolysaccharide (LPS)-induced immune responses in pigletsAfter feeding piglets with the diets supplemented FA at different levels (0,0.3,3.0,6.0,9.0 & 15.0 mg/kg) for 24 days, a 4×2+2 factorial design was conducted in this study. Seven pigs per dietary group were selected to be injected intramuscularly E. coli lipopolysaccharide (LPS) 200μg/kg BW on d1 d4 and d7 respectively; and sterial normal sodium at similar volumes were injected intramuscularly into other piglets selected from the groups supplemented FA at 0,0.3,3.0 & 9.0 mg/kg. A total of 70 crossbred (Landrace×Yorkshire) castrated piglets at 45±1 days of age were used in study, which had an average initial body weight of 10.89±0.14 kg. The piglets were alloted by initial bodyweight into 10 groups with 7 replicates per group. Piglets were fed ad libitum for 21 days. Blood samples were collected at 3h post-injection on d7 and at the end of trial on d21 respectively. The growth performance, proportion of circulating T lymphocytes subsets, and serum concentrations of glucose, cortisol, cytokines (IL-1β, IL-10, TNF-a & IL-4), nitrogen monoxidum and immunoglobulin G were measured. Results showed that, (1) LPS challenge depressed the performance of 0-14d including ADG, ADFI & G/F (P<0.01), significantly increased the serum concentration of cortisol, glucos, IL-1β, IL-10 & TNF-a, decreased CD3+CD8+ subsets (P<0.01) in circulating lymphocytes, elevated CD3+CD4+/CD3+CD8+ (P<0.01), activated and damaged the immune system, and the immune activation model was established successfully; (2) Without LPS challenge, the piglets fed basis diet had a decreased performance, a decreased CD3+ subsets (P<0.01) in circulating lymphocytes, and a lower serum NO level (P<0.05); Dietary supplementation with FA was necessary for maintaining immune system development and gaining a better performance in piglets; (3) Under LPS challenge, dietary supplementation with large dose FA could improve immune function, supplementation with FA 3-15mg/kg could relieve the physical stress and inflammatory reaction, and supplementation with FA 15mg/kg could elevate the CD3+ subsets in circulating lymphocytes, regulate the Thl/Th2 balance and improve the humoral immunity; The suitable dose of FA intake could promote convalescence, growth and feed efficiency in piglets after LPS challenge. (4) Among the groups with or without LPS challenge, there were found the different effects and suitable levels of dietary supplementation with FA; Without LPS challenge, the optimizal FA intake doses were 3.6-4.3mg/d & 0.44-0.55mg/d during 0-7d & 8-21d respectively for piglets; Under LPS challenge and the convalescence, the optimizal FA intake doses for piglets were 7.2mg/d,6.7-7.2mg/d & 4.0-4.5mg/d during 0-7d,8-14d & 15-21d respectively. ImplicationsThe results showed that insufficient intake of dietary FA might depress the performance, decrease the proliferation of immunocytes and antivirotic capability, and the immune system is vulnerable to activation in weanling piglets. Dietary supplementation with FA could improve folate status and performance (ADG & G/F), regulate immune function (including promoting the proliferation and differentiation of immunocytes, influencing the secretion of immune molecules, and regulating the CD3+, CD3+CD4+ & CD3+CD8+ subsets in circulating lymphocytes,) and relieve the damage resulting from immune activation and immune suppression. Under different immune statuses, the suitable levels of dietary supplementation with FA were different in weaning piglets. With the nomal immune status, the suitable FA intake doses for piglets were 1.4-1.5mg/d,2.9-3.Omg/d,3.6-4.3mg/d & 0.44-0.55mg/d during 0-7d,8-24d,25-31d & 32-45d after weaning respectively. Under LPS challenge (0-7d) and the convalescence (8-14d & 15-21d), the suitable FA intake doses for piglets were 7.2mg/d, 6.7-7.2mg/d & 4.0-4.5mg/d respectively. Following the growth of weaning piglet, (with the age, bodyweight and feed intake increasing), or following the convalescence lasting after immune activation, the level of dietary FA should be down-regulated gradually.