In modern dairy production, the milk yield and metabolic health of dairy goats are crucial for improving farming efficiency. Zinc, as an essential trace element, is a necessary component of various proteins in animal bodies (Swain et al., 2016). It plays a significant role in enhancing production performance, regulating metabolism, promoting the digestion and absorption of nutrients, and maintaining intestinal health (Kujur et al., 2016). In animal diets, the primary issue with using inorganic zinc as a feed supplement is its relatively low bioavailability. To achieve optimal production performance, the dietary zinc content used in animal husbandry often exceeds the recommended safe supplementation levels (Bratz et al., 2013). However, excessive use can lead to zinc overload and environmental pollution (Komatsu et al., 2020).

In recent years, the application of nanotechnology in agriculture has garnered significant attention as it effectively enhances the bioavailability of trace elements in animal diets while simultaneously reducing the risk of environmental pollution (Gopi et al., 2017). In agricultural production, the application of low doses of zinc oxide nanoparticles (ZnONPs) has been shown to have no negative impact on the health and stability of ecosystems, making it a safe and efficient zinc fertilizer (Sun et al., 2022; Xu et al., 2023). Compared to traditional zinc sources, ZnONPs possess advantages such as smaller particle size, larger specific surface area, and higher chemical reactivity, making it a feed additive with potential application value (Mandal et al., 2022). ZnONPs can act as molecular channels and carriers within cells, enhancing the transport and transmission of nutrients and signaling molecules, thereby providing the essential material basis for normal cell growth (Mokone et al., 2022). Compared to conventional zinc oxide, ZnONPs can enhance zinc bioavailability and increase serum carbonic anhydrase activity and growth hormone levels (Tsai et al., 2016). Dietary supplementation with ZnONPs (20 mg/kg) effectively improved the growth performance and feed conversion ratio of broilers, compared to the conventional dose of 60 mg/kg of zinc oxide (Zhao et al., 2014). Similarly, ZnONPs (30 mg/kg) as a replacement for conventional zinc sources have been shown to effectively improve the growth performance and meat quality of New Zealand White rabbits, under heat stress conditions (Kamel et al., 2020; Abdel-Wareth et al., 2022). This is because zinc in nanoparticle form can quickly penetrate the gastrointestinal membrane into the bloodstream, promoting the digestion and absorption of nutrients in the feed and thereby enhancing production performance (Liao et al., 2010). In addition, the bioavailability of zinc in the body plays a crucial role in maintaining and enhancing immune responses (Yusof et al., 2019). Dietary supplementation with ZnONPs (80 mg/kg) increased the antibody titer in laying hens compared to conventional zinc oxide, indicating a better immune response (Abedini et al., 2018). In summary, ZnONPs are more effective than traditional zinc sources and can be used as substitutes for conventional zinc sources, thereby reducing the effective dosage of zinc.

The richness and diversity of intestinal microbiota have a significant impact on maintaining intestinal homeostasis and nutrient absorption (Kim and Isaacson, 2015; Mukherjee et al., 2020). ZnONPs can regulate the balance of intestinal microbiota in weaned piglets, promoting the proliferation of beneficial bacteria and inhibiting the growth of harmful bacteria, thereby reducing the invasion of gastrointestinal pathogens (Xia et al., 2017; Pei et al., 2019). Research indicates that altering cell functions can affect the body’s metabolism, and ZnONPs can effectively change the levels of metabolites such as glucose and amino acids in broiler chicken plasma, especially β-glucose (Feng et al., 2017). Pearce et al. (2015) found that dietary organic zinc can improve the production of markers associated with muscle breakdown, such as reducing plasma urea nitrogen, while increasing lysozyme levels, thereby alleviating the adverse effects of heat stress on pigs. However, excessive ZnONPs can disrupt energy metabolism in mice’s kidneys, causing damage to mitochondria and cell membranes (Yan et al., 2012). Therefore, the concentration of ZnONPs in animal diets should be controlled at a specific minimum level to prevent its potential toxic effects.

Existing applications of ZnONPs are primarily focused on animal production in piglets and poultry, with fewer research findings related to ruminant production. Therefore, this experiment aims to investigate the effects of dietary supplementation with ZnONPs on the lactation, rumen microbiota, and metabolomics of dairy goats. The purpose is to provide guidelines for the application of ZnONPs in ruminant production.

Zinc has a positive effect on the production performance of ruminants. Appropriate use can enhance the digestion and absorption of feed, increase feed efficiency, and, to some extent, improve intestinal health, thereby effectively increasing milk yield (Wang et al., 2013; Oconitrillo et al., 2024). In this study, dietary supplementation with ZnONPs resulted in a significant increase in milk production. This result is consistent with previous research (Abdollahi et al., 2020), indicating that the appropriate addition of ZnONPs positively impacts milk production in ruminants. This may be related to the small particle size and high chemical activity of ZnONPs. It can enhance the integrity of the intestinal mucosal barrier, improve the balance of the gut microbiota, increase the efficiency of food digestion and absorption, and increase the supply of energy and nutrients, thereby effectively improving production performance (Liu et al., 2021). In this study, dietary supplementation with ZnONPs significantly increased milk fat content, with no significant impact on milk protein and lactose content. The lack of significant effect of ZnONPs on milk protein indicates that the addition of zinc to the diet does not have a noticeable impact on rumen protein metabolism in dairy goats. However, some studies have shown that adding zinc to the feed of Holstein cows significantly reduced milk fat content, with no significant impact on milk protein and lactose content (Nayeri et al., 2014). In addition, Hosseini-Vardanjani et al. (2020) found that adding ZnONPs to the diet of ewes had no significant effect on the content of protein, fat, and lactose in milk. This discrepancy may be related to factors such as the dosage and duration of feeding. Existing research on the effects of ZnONPs on milk quality in ruminants is limited, and further studies are needed to explore its mechanisms of action on lactation in ruminants.

Based on our research findings, dietary supplementation with ZnONPs significantly impacted the structure of the rumen microbiota in dairy goats. The primary bacterial phyla in the rumen were Bacteroidota, Firmicutes, and Synergistota, with Bacteroidota being the dominant phylum, consistent with previous studies (Pitta et al., 2016; Zhang et al., 2017). In this study, dietary supplementation with ZnONPs increased the relative abundance of Bacteroidota. Previous research has shown that Bacteroidota play a crucial role in promoting the production of volatile fatty acids, thereby enhancing the host’s energy utilization efficiency (Pandit et al., 2018; Söllinger et al., 2018). Hence, the promotion of Bacteroidota by ZnONPs could ultimately improve animal production performance by enhancing energy metabolism efficiency. Notably, our study found that Prevotella was predominant in the rumen, with its relative abundance significantly higher in the ZnONP group compared to the control group, indicating its high representation in the rumen microbiota (Bekele et al., 2010; Wang et al., 2017; Petrič et al., 2021; Betancur-Murillo et al., 2023). Prevotella effectively degrades hemicellulose through its hemicellulase, producing short-chain fatty acids that protect the gut and benefit host health (Emerson and Weimer, 2017; Schofield et al., 2018). This finding suggests that ZnONPs might enhance the fiber digestion capacity of goats, thereby improving their production efficiency. In addition, ZnONP supplementation altered the abundance of certain important bacteria, such as increasing the abundance of the Rikenellaceae_RC9_gut_group, which plays a significant role in the degradation of carbohydrates and the digestion of crude fiber (Pitta et al., 2010; Guo et al., 2023). The increase in such functional microbial groups could further promote the nutritional absorption and conversion in dairy goats to some extent. ZnONPs also exhibit potential anti-inflammatory properties. Gronbach et al. (2014) found that a low abundance of Enterobacteriaceae and a high abundance of Bacteroidota effectively alleviated colitis symptoms in mice. In this study, the increase in Bacteroidota following ZnONP supplementation may be attributed to changes in Prevotella abundance. Prevotella can produce immunosuppressive LPS, suggesting that ZnONPs have potential anti-inflammatory properties (d'Hennezel et al., 2017; Mangalam et al., 2017; Itano et al., 2023). This characteristic is of great importance for maintaining animal health, especially in dairy goat populations under high production pressure. Notably, ZnONPs may exert an inhibitory effect on pathogenic bacteria. We observed a weakening in the pentose phosphate pathway, indicating that ZnONPs might inhibit the growth of pathogens such as Staphylococcus aureus via this pathway (Kim et al., 2023). This antimicrobial effect could potentially reduce disease incidence and improve animal health. In summary, ZnONPs demonstrate the potential to enhance production performance and health in dairy goats by optimizing the structure and function of the rumen microbiota.

The supplementation of ZnONPs in the diet significantly altered the metabolite composition in the rumen of dairy goats, revealing its profound impact on metabolic pathways. These changes are significant not only at the metabolic level but also for animal production and health. Yan et al. (2012) reported significant metabolic changes in the kidneys of rats following oral administration of ZnONPs (100–1,000 mg/kg) for 14 days, with elevated levels of lactate and α-glucose. Similarly, Lee et al. (2016) observed that acute inhalation of ZnONPs altered lung metabolism in rats. While these studies have focused on metabolic changes in the kidneys and lungs, the effects of ZnONPs on the rumen metabolism of ruminants remain unclear. Purine metabolism plays a crucial role in alleviating oxidative stress damage, which is essential for maintaining the stability of rumen tissue (Manzoni et al., 2020). Xiong et al. (2022) found that purine metabolism disorders led to increased adenine content in the feces of mice with liver fibrosis, causing oxidative stress damage to the liver. In this study, we observed an upregulation of guanosine and inosine levels following the addition of ZnONPs, which had a positive impact on purine metabolism. This reduced the risk of diseases associated with oxidative stress and contributed to improved health in dairy goats. In addition, the supplementation of ZnONPs increased the levels of L-arginine, which is significantly enriched in the arginine and proline metabolism pathways. Similarly, Zhang et al. (2018) found that after 24 weeks of oral administration of ZnONPs to hens, the level of arginine in plasma metabolites increased significantly, promoting hepatic lipid metabolism and enhancing growth performance. This suggests that the upregulation of L-arginine levels following ZnONP supplementation may contribute to improved growth and immune function in dairy goats. Studies have shown that nicotinamide riboside (NR) participates in intracellular redox reactions and energy metabolism through the activation of sirtuin 1 in macrophages stimulated by alcohol, exerting anti-inflammatory and antioxidant effects (Kourtzidis et al., 2018; Kang et al., 2021). In this study, dietary supplementation with ZnONPs exhibited an upregulation of NR levels, which may help mitigate oxidative stress-induced cellular damage and positively impact the growth efficiency of dairy goats. In summary, ZnONPs significantly enhance the immunity and antioxidant capacity of dairy goats by modulating metabolites and affecting metabolic pathways, thereby contributing to the stability of metabolic and physiological functions.

Many studies indicate a close relationship between the rumen microbiota and production performance (Li et al., 2024; Liu et al., 2024). Zhou et al. (2023) found that an increase in the relative abundance of rumen coccus can effectively improve the production performance of goats. In our study, milk yield was significantly positively correlated with probiotics such as RF39 and Clostridia_vadinBB60_group and significantly negatively correlated with Succinivibrio. The RF39 is generally considered to be one of the microbial groups closely associated with the rumen fermentation process (Tilahun et al., 2024). Clostridia_vadinBB60_group contributes positively to production performance by degrading cellulose and other complex carbohydrates to promote the production of volatile fatty acids, which provide the necessary energy for ruminants (Xiao et al., 2021). In addition to milk yield, milk fat was also closely associated with rumen microbiota. Xue et al. (2018) found that milk fat was positively correlated with the relative abundance of Butyrivibrio, Pseudobutyrivibrio, and Clostridium. In this study, we found that the relative abundance of Quinella was positively correlated with milk fat content. The Quinella produces beneficial metabolites (such as lactic acid, alcohols, and volatile fatty acids) through carbohydrate fermentation, effectively improving the host digestive metabolism (Gruninger et al., 2014). Therefore, our results corroborate the finding that production performance is strongly correlated with rumen microbiota (Li et al., 2024; Liu et al., 2024). In addition, ZnONPs enhance milk yield, and milk fat might be associated with increased RF39 and Clostridia_vadinBB60 and increased Quinella, respectively.

In this study, correlation analysis also revealed the relationships between differential metabolites and microbial taxa, further emphasizing the crucial role of the rumen microbiome in animal production. This study has found that Anaeroplasma was positively correlated with most metabolites including deoxyinosine, guanine, and deoxyguanosine. Importantly, these metabolites are significantly enriched in purine metabolism, which can effectively enhance antioxidant capability (Furuhashi, 2020). As a bacterium with anti-inflammatory effects, Anaeroplasma can enhance the integrity of the intestinal mucosal barrier by inducing the anti-inflammatory cytokine TGF-β, thereby increasing the level of mucosal IgA (von Goetze et al., 2022). In addition to Anaeroplasma, Christensenellaceae_R.7_group, a common genus within Firmicutes, is considered a beneficial bacterium (Zhang et al., 2020; Jiang et al., 2022; Yan et al., 2022). In the present study, dietary supplementation with ZnONPs increased the relative abundance of Christensenellaceae_R.7_group and the levels of nicotinamide riboside. The Christensenellaceae_R.7_group was significantly positively correlated with nicotinamide riboside, which can reduce the damage caused by oxidative stress (Hong et al., 2018). Combining previous findings with this study highlights the importance of ZnONPs in improving antioxidant and anti-inflammatory effects through the regulation of Anaeroplasma and Christensenellaceae_R.7_group and related metabolites. This further emphasizes the critical role of ZnONPs in metabolic activities that enhance animal health by regulating specific rumen microbiota.

The safety assessment of ZnONPs as a novel material is crucial. Although the European Union has explicitly banned the use of zinc oxide in piglet feed since 2022 due to excessive use of zinc oxide causing zinc overload and environmental pollution, low doses of ZnONPs have a positive effect on animal production, with no potential adverse effects observed. Compared to commercial zinc oxide, the larger surface area and higher bioavailability of ZnONPs enhance absorption efficiency within the animal digestive system, which can achieve a good effect at lower doses (25–100 mg/kg diet) (Oh et al., 2022). In addition, many studies have found that low doses of ZnONPs (20–40 mg/kg DM) contributed to maintaining the health and productive performance of sheep and ewes (Alijani et al., 2020; Hosseini-Vardanjani et al., 2020). In this study, the ZnONPs used had an average particle size of approximately 30 nm and a purity of 99.9%, which effectively enhanced the biological activity of zinc oxide and reduced the adverse effects of impurities in commercial zinc oxide. This study selected 30 mg ZnONPs/kg DM as the supplementation dose, which was considered to be safe and beneficial to dairy goats and was in line with dosages used in previous research. Although the genotoxicity of ZnONPs remains controversial, low doses of ZnONPs did not induce genotoxicity, which has been demonstrated in multiple studies (Kwon et al., 2014; Mahmoud et al., 2021; Gencyilmaz et al., 2024). Therefore, the low doses (30 mg/kg DM) of ZnONPs used in this study might not have potential genotoxic effects in dairy goats; further in vivo studies and long-term toxicological assessments are needed to determine the specific risks.

The results indicated that the addition of ZnONPs to the diet can increase milk yield and milk fat content in dairy goats and enhance the richness and diversity of the rumen microbiota. The rumen metabolism of dairy goats exhibited significant changes, resulting in the identification of 261 differential metabolites. There is a significant correlation between differential bacteria and metabolites. Milk yield is significantly positively correlated with beneficial bacteria (RF39 and Clostridia vadinBB60 group), and milk fat content is significantly positively correlated with Quinella. ZnONPs enhance lactation performance and regulate the production of metabolic products by modulating the structure of the rumen microbiota in dairy goats.