Recent years have witnessed the great advantages of reducing dietary crude protein (CP) with free amino acids (AA) supplementation for sustainable swine industry, including saving protein ingredients, reducing nitrogen excretion, feed costs and the risk of gut disorders without impairing growth performance compared to traditional diets. However, a tendency toward increased fatness is a matter of concern when pigs are fed low-protein (LP) diets. In response, the use of the net energy system and balanced AA for formulation of LP diets has been proposed as a solution. Moreover, the extent to which dietary CP can be reduced is complicated. Meanwhile, the requirements for the first five limiting AA (lysine, threonine, sulfur-containing AA, tryptophan, and valine) that growing-finishing pigs fed LP diets were higher than pigs fed traditional diets, because the need for nitrogen for endogenous synthesis of non-essential AA to support protein synthesis may be increased when dietary CP is lowered. Overall, to address these concerns and give a better understanding of this nutritional strategy, this paper reviews recent advances in the study of LP diets for swine and provides some insights into future research directions.

Background: Protein releases amino acids faster than starch releases glucose in digestive tract of pigs fed low-protein (LP) diets. Poor synchronization of dietary glucose and amino acids supply leads to compromised nitrogen efficiency. Dietary starch patterns modulation may improve this situation. Methods: Growing barrows (29.7 ± 2.0 kg) were randomly allotted into 5 dietary treatments with LP diets consisting of different purified starches. Treatments included: waxy corn starch (W LP), corn starch + waxy corn starch (C + W LP), corn starch (C LP), pea starch + waxy corn starch (P + W LP) and pea starch (P LP). In the experiment, growth performance, protein deposition, nutrient metabolism, and fecal microbial community of pigs were investigated. In vitro starch digestion was used for predicting the in vivo glucose response. Results: Dietary starch in vitro glucose release profile was determined by starch source and the ratio of amylopectin and amylose. C + W LP treatment showed decreased total nitrogen excretion and plasma citrulline concentration and improved plasma leptin concentration among treatments (P < 0.05). Besides, the highest nitrogen apparent biological value, whole-body protein deposition and growth performance and lowest urinary nitrogen excretion were also observed in C + W LP treatment. Compared with the other groups, C + W LP and C LP showed increased plasma pyruvate, IGF-1, and lipase concentrations (P < 0.05). The W LP group presented dramatically increased plasma alanine and urea nitrogen concentration and decreased aldolase and leptin concentrations (P < 0.05). Dietary starch patterns did not make an impact on bacterial richness and diversity, but changed the taxonomic and functional structures of the microbial communities. Microbial protein fermentation product (isobutyrate and isovalerate) presented increased in P LP treatments compared with the other treatments (P < 0.05). Conclusions: Dietary starch patterns modulation can regulate dietary glucose release profile, nutrient metabolism, protein turnover, and fecal microbial fermentation in pigs. The optimal dietary glucose release profile effectively strengthened whole-body protein deposition and improve nitrogen efficiency and growth performance in growing pigs fed LP diets.

This study investigated the effects of modulation of the amino acid profile on growth performance and gut health in weaned pigs fed an antibiotic-free, low-protein diet. In experiment 1, 5 treatments were included: a control diet with antibiotics; a low-protein diet with antibiotics; a low-protein diet without antibiotics (LP); a LP diet with 10% more dietary essential amino acids (LP110); and an LP110 diet with 12% more dietary Met + Cys, Thr and Trp. The intestinal digestive enzyme activity and morphology were improved with the increase in dietary essential amino acid levels, while the growth performance was decreased, indicating that the dietary amino acid level was too high. In experiment 2, all 5 treatments of experiment 1 were included, plus a LP diet with 5% more dietary essential amino acids (LP105) and an LP105 diet with 6% more dietary Met + Cys, Thr and Trp. The LP105 treatment showed optimal feed efficiency, a reduced plasma endotoxin concentration, and an increased fecal lactate concentration and increased abundances of Prevotellaceae and Roseburia bacteria. Our results demonstrate that the optimal amino acid profile in an antibiotic-free, low-protein diet can efficiently improve growth performance and gut health and modulate the fecal microbial structure in weaned pigs.

The aim of this study was to investigate the intergeneric transfer of vancomycin resistance gene vanA between probiotic enterococci in the fermentation progress of soybean meal and in the digestive tract of growing pigs. One vanA genotype vancomycin resistant E. faecium strain, Efm4, and one chloramphenicol-resistant E. faecalis strain, Efs2, were isolated from twenty-nine probiotic basis feed material / additive samples. For in vitro conjugation, Efm4 and Efs2 were used as starter to ferment soybean meal. For in vivo conjugation, thirty growing pigs were randomly assigned to five groups (n = 6), treated with a basic diet, or supplemented with 10% fermented soybean meal, 1% Efm4, 5% Efs2 or a combination of 1% Efm4 + 5% Efs2 for 7 d, respectively. Fecal samples of pigs in each group were collected daily for the isolation and dynamic analysis of Efm4, Efs2 and transconjugants. The sequence types (STs) of Efm4, Efs2 and transconjugants were analyzed by multilocus sequence typing (MLST). The vanA harboring plasmid in Efm4 and transconjugants was analyzed by S1-pulsed field gel electrophoresis (PFGE) and further verified by multiple alignments. The results showed that, in FSBM, transconjugants were detected 1 h after the fermentation, with a conjugation frequency of ~ 10− 3 transconjugants / recipient. Transconjugants proliferated with Efm4 and Efs2 in the first 8 h and maintained steadily for 10 d till the end of the experiment. Additionally, in vivo experiment showed that transcojugants were recovered in one of six pigs in both FSBM and Efm4 + Efs2 groups, with conjugation frequency of ~ 10− 5 and ~ 10− 4, respectively. MLST revealed the ST of Efm4, Efs2 and transconjugants was ST1014, ST69 and ST69, respectively. S1-PFGE confirmed the existence of the vanA-harboring, 142,988-bp plasmid, which was also a multi-drug resistant plasmid containing Tn1546-like transposon. The findings revealed the potential safety hazard existing in the commercial probiotic enterococci in China, because the horizontal transfer from farm to fork could potentially pose a safety risk to the public.

Objective: Bacillus subtilis, a kind of probiotic with broad-spectrum antibacterial function, was commonly used in livestock and poultry production. Recent research suggested that Bacillus subtilis may have antioxidant properties and improve immune response. This study aimed to verify the probiotic function of Bacillus subtilis in the production of broiler chickens.Methods: A total of 324 (1-day-old) Arbor Acres broilers were selected and randomly divided into three groups: basal diet group (Ctr Group), basal diet + antibiotic growth promoter group (Ctr + AGP) and basal diet + 0.5% Bacillus subtilis preparation group (Ctr + Bac). The experiment lasted for 42 days. Muscle, serum and liver samples were collected at 42 days for determination.Results: The results showed that Bacillus subtilis could decrease malondialdehyde content in the serum and liver (p<0.05) and increase superoxide dismutase 1 mRNA expression (p<0.01) and total superoxide dismutase (p<0.05) in the liver. In addition, compared with AGP supplementation, Bacillus subtilis supplementation increased interleukin-10 (IL-10) and decreased tumor necrosis factor-α and IL-1β level in the serum (p<0.05). At 45 minutes after slaughter Ctr + Bac presented a higher a* value of breast muscle than Ctr Group (p<0.05), while significant change in leg muscle was not identified. Moreover, there was no difference in weight, shear force, cooking loss and drip loss of breast and leg muscle between treatments.Conclusion: Our results demonstrate that Bacillus subtilis in diet can enhance antioxidant capacity and optimize immune response of broilers.

Background: Reduced nutrient digestibility due to low-protein (LP) diets occurring in the foregut or hindgut of pigs remains unclear. Methods: Growing barrows (21.7 ± 1.7 kg) were allotted into LP and high-protein (HP) diet treatments. Ileal digesta and feces were collected for in vitro cross-fermentation and microbial sequencing, and cross-feeding assessed nutrient digestibility. Results: No difference in foregut digesta flora and nutrient digestibility between treatments was observed. LP diet caused decreased total tract digestibility of dry matter (DM), organic matter (OM), gross energy (GE), neutral detergent fiber (NDF), and acid detergent fiber (ADF) compared with the HP diet (p < 0.05). The fermentation broth from LP diet-fed pigs induced less full fermentation digestion of DM, OM, crude protein, and GE than HP broth (p < 0.05). Additionally, LP broth fermentation presented lower fermentation gas and short-chain fatty acids (SCFAs) generation than HP group (p < 0.05). This situation above may be related to decreased abundances of Lachnospiraceae, Eubacterium_eligens_group, Roseburia, and Ruminococcaceae_UCG-009, which can efficiently ferment nutrients to produce SCFA. Conclusions: Change in the flora caused compromise in hindgut microbial fermentation digestion leads to decreased total tract nutrient digestibility in pigs fed an LP diet.