INTRODUCTION
The increasing demand for animal products necessitates the acquisition of economically viable livestock feed alternatives. Thus, it is imperative to identify cost-effective alternative feeds to facilitate the development of animal husbandry. Moreover, the shortage of animal feed has resulted in the escalation of their market prices. Several studies have already demonstrated the potential of utilizing unconventional feed to replace cereal-based feed, while ensuring minimal impact on animal production performance [
1,
2]. Moreover, unconventional feed can have additional benefits such as improving the gut microbiota and other immunological parameters. Therefore, it is particularly important to find inexpensive yet beneficial feed alternatives that can replace conventional animal feed [
3].
The mulberry plant (
Morus alba) is a fast-growing shrub, and its leaves are utilized as the primary food source for silkworms [
4]. In China, the cultivation of mulberry is estimated to cover 700,000 ha, yielding nearly 18,200,000 tons [
5]. For herbivores, mulberry leaves are highly palatable and rich in digestible macronutrients, with a high protein content and essential amino acid profile [
6]. Moreover, the protein content of mulberry leaves is higher than those in traditional forages, making them a potential high-quality protein source for livestock [
7]. Fresh mulberry leaves can be directly incorporated into the diet of animals, whereas ensiled mulberry leaves can be fed to bovine animals, with no adverse effects to their performance and carcass quality [
8,
9]. In addition, mulberry leaves are an excellent protein-rich forage for both monogastric and ruminant animals owing to the leaf’s unique nutrient profile, digestibility, and palatability [
10]. Moreover, the active components of mulberry leaves can also help regulate the antioxidant capacity of laying hens and the antioxidative profile and lipid metabolism of pigs [
11,
12].
Several studies have investigated the effects of a mulberry leaf diet on various poultry and livestock, however, their results have been inconsistent [
11,
13–
29]. Moreover, no systematic review has yet summarized the effects of mulberry leaves on the growth, production performance, gut microbiota, and immunological parameters of poultry and livestock. Therefore, this study conducted a systematic review and meta-analysis to provide both the qualitative and the quantitative results regarding the effects of a mulberry leaf diet on poultry and livestock production.
DISCUSSION
Although mulberry leaves are widely used in poultry and livestock, their effects on growth, production performance, gut microbiota, and immunological parameters remain unclear. This systematic review and meta-analysis investigated the effects of mulberry leaf extract on poultry and livestock. A total of 2,335 poultry and livestock samples from 18 studies with a wide range of interventions were utilized in this review. Regarding growth performance, we noted that a moderate dose of mulberry leaves increased the average daily gain while reducing the feed/meat ratio in pigs, whereas mulberry leaves significantly increased the average daily gain and average daily feed intake in chickens. We noted that mulberry leaves could affect half the carcass weight, slaughter rate, and loin eye area in pigs and reduced the slaughter rate in chickens. Regarding meat quality, we additionally noted that cooked meat percentage, shear force, 24 h pH, crude protein, water, and crude ash in pigs could be all affected by mulberry leaves, whereas mulberry leaves could also impact drip loss, shear force, 45 min pH, 24 h pH, crude protein, and crude ash in chickens. Regarding serum antioxidant capacity and immune organ indices, we noted that the thymus index, spleen index, IgM, and IgG were all affected by mulberry leaves in pigs. Whereas, the serum antioxidant capacity of catalase activity, total antioxidant capacity, superoxide dismutase, and glutathione peroxidase were all elevated in chicken receiving mulberry leaves, except for malondialdehyde, which reduced after receiving mulberry leaves. The inflammatory indexes of IL-1β, IL-2, IL-6, IL-8, and INF-γ in pigs were all significantly increased after using mulberry leaves. Mulberry leaves also affected eggshell strength, yolk color, eggshell thickness, eggshell weight, egg yolk ratio, and egg yolk moisture content.
Our study ultimately found that the use of mulberry leaves increased the average daily gain and feed/meat ratio in pigs, and the average daily gain and average daily feed intake of chickens. Mulberry leaves have been reported to improve muscle formation and development in pigs by modulating the expression of various genes, including
ACOT4,
ECHS1,
HACD1,
NPR1,
ADCY2,
MGLL, and
IRS1, which could affect fatty acid metabolism. Muscle formation and development could be affected by
TNNC1,
TNNT1, and
MYL3 [
14]. Moreover, growth performance could be improved by nutrient digestion and absorption, while the use of mulberry leaves could improve the digestibility of dry matter [
36]. In addition, we noted that mulberry leaves could reduce half-carcass weight, slaughter rate, and loin eye area in pigs, while the use of mulberry leaves in chickens could reduce the slaughter rate. Several reasons could explain these results: i) mulberry leaves contained less crude fiber and neutral detergent fiber; thus, the nutrients were easier to digest and absorb, resulting in superior carcass traits [
20]; and ii) the slaughter rate in chickens was reduced after mulberry leaves were used, which was significantly related to growth performance.
Collectively, the results highlight that mulberry leaves could affect the cooked meat percentage, shear force, 24 h pH, crude protein, water, and crude ash in pigs. Moreover, the use of mulberry leaf was significantly associated with drip loss, shear force, 45 min pH, 24 h pH, crude protein, and crude ash in chickens. This can be attributed to the heightened activity of intestinal amylase resulting from the use of mulberry leaves. As the activity of intestinal digestive enzymes can improve after using mulberry leaves, it can subsequently improve the growth, nutrient digestion, or absorption in animals [
37].
We noted that the inflammatory indices, thymus index, spleen index, IgM, and IgG in pigs could all be affected by mulberry leaves. Mulberry leaves have been reported to improve the digestion and absorption of nutrients, enhance immunity and disease resistance, and inhibit metabolic processes, thus increasing the growth performance of animals [
38,
39]. Furthermore, mulberry leaves presented immunomodulating activity, which could affect varies kind cells and macrophage-dependent immune system responses [
40]. In addition, Mullberry leaves have also been observed to alleviate inflammation via interactions among insulin signaling pathway and TNF-α [
41]. In chickens, the eggshell strength, yolk color, eggshell thickness, eggshell weight, egg yolk ratio, and egg yolk moisture content were all affected by mulberry leaves. The improved eggshell strength can be attributed to mulberry leaf flavonoids that increase the antioxidant capacity of the uterine shell gland and calcium deposition [
27]. Similarly, the dark yolk color, which is significantly related to better egg quality, could be attributed to the carotenoid content of mulberry leaves [
42].
However, this study noted several limitations. First, the basal diet differed across the included studies, which may have affected the observed effects of mulberry leaves. Second, the analysis included various poultry and livestock, with the characteristics of the animals differing, which could have influenced the effects of mulberry leaves. Third, the final dataset included a relatively small number of studies, which could affect the reliability of the pooled conclusion. Lastly, the analysis was based on published articles with the inherent limitations of inevitable publication bias and restricted detailed analysis.
Notably, this study is the first to summarize the effects of mulberry leaves on the growth, production performance, gut microbiota, and immunological parameters of poultry and livestock. In pigs, mulberry leaves affect positively influenced parameters like average daily gain, feed/meat ratio, half-carcass weight, and crude protein, water, and crude ash levels, while lowering the carcass weight and slaughter rates. They also enhance certain aspects of meat quality, such as loin eye area and cooked meat percentage. However, mulberry leaves were also shown to alter the shear force, pH levels, thymus index, spleen index, and immunoglobulins (IgM and IgG). Conversely, in chicken, the use of mulberry leaves positively affected the average daily gain, egg quality attributes like eggshell strength, yolk color, eggshell thickness, and eggshell weight, while also affecting variables such as feed intake, slaughter rate, drip loss, and pH levels. However, mulberry leaves also reduced the egg yolk ratio and moisture content. Nevertheless, the meta-analysis revealed mulberry leaves as potential alternative feed, with both advantages and considerations, for poultry and livestock management.