1. Introduction
With the rapid development of large-scale and intensive animal husbandry, animal diseases have become more complex and diverse, and veterinary drugs and antibiotic feed additives are widely and increasingly being used. However, frequent and excessive antibiotic use is an increasing concern because of possible antibiotic residues, the development of drug-resistant bacteria, and the inhibition of the animal’s immune system [
1,
2,
3]. Therefore, novel dietary supplements, such as probiotics, prebiotics, and other feed additives, have been used to replace antibiotics to improve animal health and maintain production efficiency [
4,
5,
6]. Yeast cultures (YCs) are widely used as excellent feed additive candidates in animals [
7]. YCs are microecological products formed by yeast after sufficient anaerobic fermentation on specific media and are mainly composed of yeast metabolites, fermented medium, and a few yeast cells [
3,
8]. YCs are rich in vitamins, saccharides, minerals, enzymes, growth-promoting factors, and amino acids, which may benefit animal growth, metabolism, and health [
8,
9].
Many studies have reported the application of YCs in animals because of the potential benefits to animal production yields. For example, supplementing dairy cow diets with YCs improved lactation performance [
10,
11,
12] and increased feed efficiency [
13]. Supplemental YCs increased the carcass weights of calf-fed Holstein steers throughout the growing-finishing period [
14]. Supplementing buffaloes with YC increased milk production by increasing the feed, energy, and nitrogen conversion efficiency [
15]. Adding YCs significantly affected the ruminal pH in rams [
16] and helped prevent and treat acute ruminal lactic acidosis in sheep [
17]. In horses, dietary YC supplementation increased variability in Streptococcus bovis [
18] and aided in fiber digestion [
19]. In pigs, the dietary supplementation of YC improved growth performance and may be a good alternative to antibiotics [
3,
8]. YC feeding benefitted the gut microbiota, growth, and biochemical parameters in grass carp [
7] and promoted feed intake, weight gain, and disease prevention in Nile tilapia [
20]. YC supplementation improved growth performance and affected immune functions, Ca and P digestibility, and intestinal mucosal morphology in broilers [
4] and increased egg weight and decreased egg yolk cholesterol without affecting performance or egg traits [
9].
In summary, YCs have been extensively studied in livestock, especially ruminants, and several studies have been conducted on aquatic animals and poultry. However, no study has reported dietary YC supplementation in geese. Geese are economically important herbivorous waterfowl in central Europe and Asia, especially in China (90% of the world’s production), and are characterized by rapid growth, high fecundity, and strong disease resistance [
21]. Geese provide nutritious meat and eggs worldwide, are a valuable protein source for humans, and provide high-quality liver fat and feathers [
22]. Because of its many benefits, we hypothesized that YC supplementation would influence performance in geese by improving digestion and immune functions. Therefore, this study was conducted to evaluate the effects of YC dietary supplementation on growth performance, nutrient digestibility, blood biochemical parameters, and immune functions in geese.
4. Discussion
In this study, YC supplementation did not affect goose mortality (
p > 0.05), which is consistent with previous studies on laying hens [
9], broilers [
4], quails [
24], and calf-fed Holstein steers [
14]. Conversely, Tangendjaja [
25] reported that YC supplementation in laying hens significantly reduced mortality (
p < 0.05). Similarly, Fathi et al. [
26] found that broilers fed a diet supplemented with 1.25 g/kg of YC had lower mortality rates throughout the experiment (
p < 0.05). Barens et al. [
27] indicated that mortality was significantly reduced in rainbow trout during the first 4 weeks of rearing with YC-supplemented diets (
p < 0.05). The specific mechanisms of YC on health and/or viability remained unclear because the results are affected by multiple factors such as species, environment, and feed.
In the current study, supplemental YC improved body weight gain (BWG) and FCR in geese. In poultry, YC research mainly focused on chickens, and the results suggest that YC benefits BWG and FCR [
2,
4,
9,
26]. At appropriate YC levels, BWG was unattributed to increased feed consumption, and even Liu et al. [
28] indicated that 0.2% dietary YC supplementation decreased feed intake in laying hens. Studies with livestock on cattle [
14,
29,
30], sheep [
31], and pigs [
3,
8,
32] also demonstrated that dietary YC supplementation increased ADG. These benefits may be because YC supplementation influences gastrointestinal microbiota and intestinal morphology to improve nutrient digestion and retention [
3,
10,
33], and it increases the relative abundances of cellulolytic, amylolytic and lactate-utilizing bacteria [
34]. Additionally, YCs contain peptides, organic acids, oligosaccharides, amino acids, and other factors that promote animal growth [
4]. In the present study, BWG and FCR tended to be best at the 2.0% level, possibly because YC interacted with the gastrointestinal system by triggering proper levels. Similarly, low YC levels would more effectively improve immune response performance because demand for the immune response is minimal under low disease challenge or stress conditions [
4]. However, other studies reported that YC supplementation did not increase BWG [
15,
35,
36]. Different responses may be related to different dietary nutritional densities, durations of supplementation, and yeast product formulations, making comparisons among studies difficult [
4,
15].
In the present study, YC supplementation improved Ca, P, CP, and GE digestibilities, with better responses at the 2.0% level for P, CP, and GE. Several studies have demonstrated that probiotics, including YCs, are one approach to improving nutrient digestibility. Shen et al. [
8] indicated that dietary YC supplementation improved GE and CP digestibility in nursing pigs. Gao et al. [
4] and Bradley et al. [
37] showed that supplemental YC improved the utilization of digested P and Ca in poultry. This improvement in nutrient utilization may have been due to YC constituents, including phytase [
4], oligosaccharides [
38], and growth factors [
39]. Previous studies reported that increased villus/crypt ratios [
40] and feed retention times [
41] exert positive effects on nutrient digestibility. The effect of YC on the villus/crypt ratio and feed retention times for geese was not measured in our experiment and, thus, warrants further research. However, other studies showed that YC did not affect energy or protein digestibility [
4,
36]. Dias et al. [
13] found that YC supplementation did not affect the total-tract apparent digestibility of nutrients in dairy cows.
Blood metabolite values are frequently used to assess growth and general metabolic health status in animals [
15,
42]. For example, serum ALT and AST are used to indicate liver damage; TC, TG, HDL, and LDL are used to indicate fat mobilization, and ALP is used to indicate growth [
15]. The ALT and AST results in the present study indicated that YC supplementation negatively affected liver health, possibly because the liver had not adapted to the rapid growth, which was confirmed by ALP. Studies on laying hens [
9], buffaloes [
15], lactating ewes [
43], dairy cows [
44], goats [
45], and rams [
16] reported that YC supplementation did not affect serum ALT or AST. Nursoy and Baytok [
46] found increased TG in YC-fed dairy cows, but Chen et al. [
2] indicated that TG was unaffected in YC-fed broiler chickens. Research on rabbits [
47] and broiler chickens [
48,
49] showed that probiotic supplementation reduced TC and LDL, which was consistent with the results of the present study. However, other studies reported no difference in TC or LDL levels between probiotic-supplemented and control groups [
2,
50]. Evidence has shown that probiotics and their components regulate host lipid metabolism via active substances that deconjugate bile salts in the intestines, thereby preventing bile salts from acting as precursors in cholesterol synthesis [
51,
52]. However, other studies reported inconsistent results, likely because of differing doses, ration compositions, animal species, animal ages, and supplementation strains.
Studies have reported that yeast supplementation modulated immune responses in humans [
53], chicks [
4], pigs [
8], dairy cows [
54], and fish [
20] to maintain health and improve growth performance by reducing pathogenic bacteria and improving gut health. Similarly, cellular studies also support the role of YC in innate immune functions [
55]. In this study, dietary YC increased the serum lysozyme, IL2, IgM, and complement C3 contents, particularly at the 0.5% and 2.0% levels, which is consistent with the findings of Gao et al. [
4] and Fathi et al. [
26] who reported increased lysozyme and IgM in YC-fed birds. Studies have shown that probiotics (e.g., YCs) stimulate animal immunity via the following methods: (i) flora from the probiotics migrate throughout the gut wall and multiply to a limited extent; (ii) dead organisms release antigens that are absorbed and stimulate the immune system [
51]; (iii) the luminal and mucosal gut microbiotas are modulated and protected against inflammation [
53]; (iv) yeast cell wall polysaccharides can positively affect immune functions [
54]; (v) harmful and pathogenic microorganisms are competitively excluded [
56]; (vi) probiotics benefit the intestinal environment and improve intestinal morphology [
3,
57]; (vii) probiotics help maintain a physiological balance of immunopotent cells, thus providing a healthy environment for the immune system; (viii) oligosaccharides in yeast cell walls bind to viruses and function as vaccine adjuvants to increase antibody titers [
26]. Surprisingly, YC did not affect IgA and even reduced IgG in the present study. The reason for this is unknown, and further studies are necessary to interpret these results.
Dong and Wang [
58] indicated that the dietary prebiotic significantly increased immune gene expressions in red swamp crayfish, suggesting that YC potentially affects immune genes. In the present study, YC supplementation upregulated three immune genes,
IFN-γ,
IL-2, and
TNF-α, in the spleen, especially at the 2% level, with
IFN-γ and
IL-2 exhibiting the highest expressions. This result is supported by the protein study of Shen et al. [
8], who reported that YC increased cytokine IFN-γ production by triggering a Th-1 response in the gut. IFN-γ and IL-2 activate macrophages, while TNF-α is produced by macrophages. Therefore, the results may be explained in that YC improves animal immunity by activating macrophages [
4,
8] that can rapidly and efficiently phagocytize the bacteria.
Based on the response of the measured indicators to different levels of YC, the results showed that the optimal YC level was 2% on the whole, and 4.0% YC was not more effective in the present study, which may be due to the high levels of YC causing geese to develop immune tolerance, leading to the wastage of energy and nutrients and suppressing growth performance [
59].