1. Introduction
Various agricultural residues such as corncob, straw, and cottonseed hull can be used as a medium for mushrooms, and the medium remaining after mushroom collection is called spent mushroom substrate (SMS) [
1]. Although the use of agricultural residues has reduced environmental pollution to a certain extent, with the increasing demand for mushrooms [
2], the remaining SMS after mushroom collection has become a serious source of pollution [
3]. The lack of effective treatment methods for the large amount of SMS generated by the mushroom industry has caused serious pollution to the environment. Nowadays, many studies have been performed on how to make full use of SMS as a waste resource. It was reported that co-composting of SMS with pig manure accelerates the conversion of lignocellulose [
4], and in addition, SMS and pig manure composting improves the growth of tomato and pepper seedlings compared with the use of commercial peat [
5]. Another study showed that SMS could increase soil organic matter and its nutrient content, which was safer for soil than chemical fertilizers [
6].
Pleurotus ostreatus is one of the common edible fungi in daily life. The spent mushroom substrate of
Pleurotus ostreatus (SMS-MP) can be used in nurseries [
7], bioremediation [
8], and biofuel [
9]. Another study found that SMS-MP was found to be a versatile, low-cost organic substrate that could reduce polychlorinated biphenyl (PCB) contamination in soil by activating the oxidation process of highly chlorinated PCBs [
10]. Although SMS has been fully utilized at present, the current accumulation of SMS in the main mushroom-producing areas of China is alarming and has caused serious environmental pollution. Therefore, there is an urgent need to expand the potential use of SMS.
In addition, many studies have been conducted in recent years on the processing methods and advantages of SMS alternative feeds [
11,
12]. SMS can also be used as an animal feed supplement and can be easily digested by ruminants, providing additional animal feed resources [
13]. It has been reported that SMS can be used as a functional feed additive to effectively improve milk production and hematological parameters in dairy cows [
14]. However, there is no relevant study of SMS on the hematology of sika doe and suckling fawn ruminants. We speculate that sika deer can also digest SMS well and that it has no adverse effects on the hematology of sika doe and suckling fawn. Sika deer velvet antler is a valuable nutraceutic in traditional Chinese medicine and is widely recognized in many countries of the world [
15]. Therefore, reducing the cost of raising sika deer is an important research direction to expand the deer industry. Our previous study has shown that 10% of SMS-MP can be safely replaced with a concentrated supplement in male sika deer with no adverse effects on nutrient digestion and hematology [
16]. The purpose of this study was to evaluate the effectiveness of SMS-MP replacing 10% concentrate supplementation on the hematology and digestion in sika doe and the hematology of suckling fawn, reducing feeding costs while avoiding the environmental pollution of SMS-MP.
4. Discussion
The SMS addition to ruminant feed has become increasingly accepted in recent years. However, the effects of feed SMS on sika doe and suckling fawn have not yet been reported. The present study was designed to explore the effects of adding SMS-MP to diets on the hematology of sika does and suckling fawns. Studies have shown that the replacement of 10% of the concentrate supplement with SMS-MP had no effect on hematology of sika does and suckling fawns. It can be concluded that SMS-MP can be used as a concentrate supplement for sika doe.
It was reported that SMS-MP can be used as ruminant feed without any deleterious effects on cattle eating behavior. Similarly, the current study found no effect of SMS-MP on feed intake and apparent digestibility of OM, EE, and CP in sika doe [
20]. However, the SMS-MP reduced the digestibility of DM, which needs further study. Neutral detergent fiber (NDF) promotes digestion in ruminants, and dietary NDF has been reported to alter rumen fermentation and plasma metabolites [
21]. In contrast, studies have shown that NDF has no effect on ruminant fermentability and digestibility in ruminants [
22]. Therefore, the effect of SMS-MP on NDF digestibility for sika doe needs to be further confirmed.
The blood biochemical index is an important parameter for evaluating animal nutrition status [
14]. TP contains ALB and α-, β-, and γ-GLO [
23]; therefore, high concentrations of TP are associated with an increase in serum GLO concentration [
24]. An elevated GLO concentration is probably the sign of chronic inflammation [
25]. The GLO concentration of sika does decreased in the experimental group, indicating that SMS-MP could decrease the chronic inflammation caused by GLO. CHOL is an important component in the synthesis of bile acids and vitamin D in the liver and plays an important role in the formation of cell membranes in the body [
26]. Studies have shown that low serum concentrations of CHOL could increase the risk of a retained placenta in perinatal dairy cows [
27] and could cause fatty liver disease [
28]. The results showed that SMS-MP could decrease the CHOL of sika doe, indicating that the reduction in CHOL may be beneficial to the calving of sika does. Carbohydrates in diets could be broken down into GLU and released in the systemic circulation to provide energy, and they played an important role in the synthesis of fatty acids [
29]. The increased GLU concentration in the serum of SMS-MP-supplemented sika does suggests a lower rate of GLU used when SMS-MP was added in diets. However, the reason for the decreased GLU concentration of suckling fawns with supplemented SMS-MP in their diet is still unknown and needs further study. ALP is a very important enzyme that reflects the mineral metabolism in vivo [
30]. The content of ALP increases physiologically during the growth period of suckling fawn bones, indicating that SMS-MP can promote the growth of suckling fawn bones. Serum BUN concentration is determined by the nitrogen level in the diet and can reflect the nitrogen metabolism of ruminants [
31]. A high concentration of BUN in the serum indicates that the animal cannot fully utilize carbohydrates for energy. In addition, high levels of protein in the body may affect the immune system of cows, eventually leading to hyperammonemia [
32]. There were no significant differences in BUN among the groups of the study. It can be suggested that SMS-MP as a concentrate supplement has no adverse effect on sika doe and suckling fawn.
Colostrum has a large number of immune-related components, mainly immunoglobulins. IgG, IgA, and IgM are three common immunoglobulins, which have the functions of preventing microbial invasion, regulating the homeostasis of the body’s immune environment, and controlling the proliferation of cancer cells [
33]. It is well known that high-quality colostrum plays an important role in improving passive immunity and reducing the occurrence of diseases in newborn calves [
34]. The concentrations of β-carotene and vitamin A in the colostrum of Japanese black cows were positively correlated with the concentrations of IgG and IgM [
35]. During pregnancy, IgG is actively transported from the mother to the fetus through the placenta, thereby enhancing fetal immunity [
36]. The predominant category of immunoglobulins in bovine colostrum and milk is IgG, while the largest proportion of antibodies in human milk is IgA [
37]. Mucosal secretions contain a large amount of IgA, which prevents the invasion of pathogenic microorganisms by resisting the microorganisms on the mucosal surface [
38]. By detecting the concentration of immunoglobulin in serum, the immune status of sika doe and suckling fawn can be observed indirectly. In our study, there were no significant differences in serum IgG, IgA, and IgM levels between the two groups, indicating that SMS-MP did not damage the immune system of the sika does and suckling fawns.
The blood of animals is the common medium that connects all organ systems in the body. Numerous studies have reported that various diseases can cause abnormal amino acid concentrations, and when specific organ systems are metabolically disordered, plasma amino acid concentrations can also be altered [
39,
40]. Ala is a non-essential amino acid formed by the transamination of pyruvate, which reflects the increased concentration of pyruvate under these conditions [
41]. The current study showed that SMS-MP could increase the level of Ala in the plasma of sika doe, indicating that SMS-MP may improve the metabolism of nutrients in the body by increasing the content of pyruvate. As a special amino acid in collagen, Pro plays an important role in the structure of blood vessels. Deficiency of Pro may lead to impaired collagen synthesis and further damage to vascular structures, eventually leading to acute aortic dissection [
42]. Phe is one of the essential amino acids in red blood cells and plasma [
43]. The results indicated that SMS-MP could increase the levels of Phe and Pro, which may have some positive effects on the circulatory system of sika doe, while SMS-MP had no significant effect on the plasma amino acid content of suckling fawn.
SMS-MP can be consumed long-term as a concentrate supplement without harmful effects on the hematology of sika doe and suckling fawn. Although this study emphasized that SMS-MP had no effect on the digestion of sika doe, the digestion of suckling fawn requires further study.