Effect of differing amounts of zinc oxide supplementation on the antioxidant status and zinc metabolism in newborn dairy calves

Impact of 48-h water and feed deprivation and hydroxychloride sources of copper and zinc on the metabolism and performance of grazing Nellore cattle during the dry period

Procedures such as transport and marketing can subject animals to water and feed deprivation and impair animal health and performance. Maintaining the mineral status of animals under these conditions can bring benefits to health and performance. The use of hydroxychloride mineral sources can improve mineral status, nutrient digestibility and performance. Two studies were conducted to investigate how the supplementation of 02 trace mineral sources of Cu and Zn and 48-hour water/feed deprivation would affect the performance and metabolism of grass-fed beef cattle. In the first study, 20 castrated and rumen-canulated Nellore steers (BW = 350 ± 132 kg; 20 m) were distributed in individual pens, in a 2 × 2 factorial arrangement: supplemental Cu and Zn sources from inorganic vs hydroxychloride (HTM) and 48-hours deprivation (WFD) vs unrestricted (WFU) access to water and feed. The 57d of study was divided into two periods: (1) Adaptation from -21d to -1d and (2) evaluation from 0d to 36d. Interaction between deprivation × period was detected (P < 0.05) for digestibility of DM (DMD), organic matter (OMD), neutral detergent fiber (NDFD), and acid detergent fiber (ADFD). Deprivation increased DMD, OMD, NDFD, and ADFD immediately after the deprivation period (3-5d), but impaired digestibility at longer periods such as 11-13d and 32-34d. DM (DMI) and nutrient intake (P = 0.075), as well as NDFD were higher in HTM. Several ruminal parameters were affected by deprivation: short-chain fatty acids concentration decreased, while rumen pH increased (deprivation × time; P < 0.05); decreased propionate, butyrate and increased isobutyrate, isovalerate, and valerate in WFD (deprivation × time; P < 0.05), respectively. In the second study, eighty-four intact Nellore males (BW = 260 ± 35 kg) were blocked by BW and randomly assigned to Urochloa brizantha cv. Marandu paddocks for 131d in a 2 × 2 factorial arrangement. Liver Cu was higher in WFU/HTM animals (mineral × deprivation; P < 0.05). Interaction between deprivation × period (P < 0.05) was detected for BW and average daily gain (ADG). On 2d and 12d after deprivation, WFD increased ADG and recovered the BW lost. In conclusion, water and feed deprivation imposed in these trials were able to impact several nutrient digestibility and ruminal fermentation parameters in short- and long-term. Performance was not affected by the studied factors. Furthermore, supplementation with sources of Cu and Zn hydroxychloride increased Cu in the liver and tended to increase DMI and NDFD.

Effects of Different Galacto-Oligosaccharide Supplementation on Growth Performance, Immune Function, Serum Nutrients, and Appetite-Related Hormones in Holstein Calves

Our previous study showed that early supplementation with 10 g/(d·head) of galacto-oligosaccharides (GOS) in newborn Holstein dairy calves reduced the incidence of diarrhea and improved growth performance and mineral absorption. Since the dose of 10 g/(d·head) was the lowest by dose screening in our previous study, the present study was designed to investigate whether a lower amount of GOS has similar effects on growth performance, immune function, serum nutrients in newborn Holstein heifer calves, and to further investigate its effect on appetite-related hormones. Twenty-eight healthy newborn (1 day of age) Holstein heifers with similar average body weight (41.18 ± 1.90 kg) were randomly divided into four groups (n = 7): the control group (CON group), which received heated raw milk, and three experimental groups, which received heated raw milk supplemented with 2.5 (GOS2.5 group), 5 (GOS5 group), and 10 g/(d·head) (GOS10 group) GOS. All heifer calves were fed the same starter for 28 d. Supplementation with GOS linearly increased the final body weight, average daily gain, and feed efficiency in heifer calves (p < 0.01). Compared with the control group, the average daily gain and feed efficiency of heifer calves were significantly higher in the GOS5 and GOS10 groups than in the control group (p < 0.05). Furthermore, supplementation with GOS quadratically enhanced the starter and total average daily feed intake of the heifers (p < 0.01), especially in the GOS2.5 and GOS5 groups, (p < 0.05 vs. CON). The serum concentration of immunoglobulin A was linearly increased by GOS supplementation (p < 0.05), and the levels in the GOS5 and GOS10 groups were significantly higher than those in the CON group. Meanwhile, GOS linearly decreased serum interleukin-1β and interleukin-6 concentrations (p < 0.05). The serum concentration of triglycerides was also linearly decreased (p < 0.05), whereas total protein and blood urea nitrogen were linearly increased (p < 0.05). Supplementation with GOS linearly decreased the serum concentration of leptin (p < 0.05) but increased cholecystokinin and glucagon-like peptide-1 (p < 0.05). Increasing doses of GOS linearly improved serum calcium and copper concentrations (p < 0.01) and quadratically enhanced the concentration of magnesium, which peaked in the GOS5 group (p < 0.05). In conclusion, GOS supplementation reduced the incidence of diarrhea and improved the growth performance and immune function of Holstein heifer calves.

Early supplementation with zinc proteinate does not change rectal microbiota but increases growth performance by improving antioxidant capacity and plasma zinc concentration in preweaned dairy calves

The present study evaluated the effects of early supplementation with zinc proteinate (ZnP) or zinc oxide (ZnO) for 2 weeks on the growth performance, redox status, plasma trace element concentrations, and rectal microbiota of preweaned dairy calves. A total of 60 newborn healthy female Holstein dairy calves, with initial body weight (BW): 41.33 ± 0.62 kg, were randomly allocated to 5 groups of 12 each: a control group (CON); three groups supplemented with 261 (L-ZnP), 523 (M-ZnP), and 784 (H-ZnP) mg/day ZnP, equivalent to 40, 80, and 120 mg/day zinc, respectively; and one group supplemented with 232 mg/day ZnO, equivalent to 180 mg/day zinc (ZnO). Zinc supplements were administered on days 1-14, and the calves were followed up until day 70. Zinc supplementation increased total dry matter intake (DMI) and starter DMI compared with the CON group (p < 0.01). The final BW, average daily gain, and feed efficiency were higher in the M-ZnP, H-ZnP, and ZnO groups (p < 0.05). The incidence of diarrhea on days 1-28 was reduced by zinc administration (p < 0.01), whereas the incidence on days 29-56 was lower in the M-ZnP and ZnO groups (p < 0.05). Serum glutathione peroxidase activity, total antioxidant capacity, immunoglobulin G and plasma zinc concentrations were increased linearly (p < 0.05), while the serum concentration of malondialdehyde was decreased linearly (p < 0.01), as the dose of ZnP increased. ZnP yielding 80 mg/day zinc had similar effects as ZnO yielding 180 mg/day zinc, except that final BW was higher in the ZnO group (p < 0.05). At the phylum level, ZnO decreased the relative abundance of Firmicutes while increasing the abundance of Bacteroidetes (p < 0.05). At the genus level, ZnO increased the relative abundances of Prevotella, Subdoligranulum, and Odoribacter (p < 0.05). These findings indicated that early supplementation with ZnP did not affect the rectal microbiota of preweaned dairy calves but increased their growth performance, antioxidant capacity, and plasma zinc concentration. In summary, ZnP is an organic zinc source with greater bioavailability than ZnO for preweaned dairy calves. Early dietary supplementation with ZnP yielding 80 mg/day zinc is recommended.

The Effects of Zinc Supplementation on Growth, Diarrhea, Antioxidant Capacity, and Immune Function in Holstein Dairy Calves

The current study examined the effects of supplementary zinc proteinate (ZnPro) and zinc oxide (ZnO) on growth performance, diarrhea, antioxidant capacity, immune function, and mineral element concentrations of calves aged 1 to 28 days. A total of twenty-four newborn calves were divided randomly into 3 groups (n = 8; 2 males and 6 females per group), and each received: 0 mg/d Zn (CON), 627 mg/d ZnPro (80 mg/d Zn; ZnPro group), and 101 mg/d ZnO (80 mg/d Zn; ZnO group). The calves received the additive in their milk during the first 28 days of life. Compared with the CON group: ZnPro and ZnO improved average daily gain (ADG) and decreased the feed:gain ratio (FGR) between days 1 and 14 (p < 0.05), while the ADG increased and FGR decreased only in the ZnPro group between days 1 and 28 (p < 0.05). The incidence of diarrhea decreased (p < 0.05) in the ZnPro and ZnO groups between days 15 and 28 as well as days 1 and 28, but decreased (p < 0.05) only in the ZnPro group between days 1 and 14. The serum immunoglobulin G (IgG) concentration of the ZnPro and ZnO groups increased on days 14 and 28 (p < 0.05). ZnPro supplementation increased serum IgM concentration during the whole study, while ZnO enhanced serum IgM concentration only on day 14 (p < 0.05). In the ZnO group, the serum concentration of cytokines interleukin (IL)-10 increased on day 14, while that of IL-1β increased on day 28 (p < 0.05). In addition, ZnPro reduced the serum malondialdehyde (MDA) concentration on days 14 and 28 (p < 0.05). Both ZnPro and ZnO increased the serum concentrations of alkaline phosphatase (ALP) and metallothionein (MT) on day 14 (p < 0.05). With zinc supplementation, plasma Zn concentration increased (p < 0.05) on days 14 and 28 of age. We concluded that supplementary ZnPro and ZnO reduced incidences of diarrhea and promoted the immune function, but ZnPro improved the growth performance and antioxidant capacity of Holstein dairy calves to a greater extent.

The Dietary Supplementation of Copper and Zinc Nanoparticles Improves Health Condition of Young Dairy Calves by Reducing the Incidence of Diarrhoea and Boosting Immune Function and Antioxidant Activity

This study was conducted to evaluate the effect of nano copper (nano Cu) and nano zinc (nano Zn) supplementation on the biomarkers of immunity and antioxidant and health status attributes in young dairy calves. Twenty-four young cattle calves were randomly assigned into four groups (6 calves per group) on a body weight and age basis for a period of 120 days. The feeding regimen was the same in all the groups except that these were supplemented with 0.0 mg nano Cu and nano Zn (control), 10 mg nano Cu (_nanoCu₁₀), 32 mg nano Zn (nanoZn₃₂), and a combination of nano Cu and nano Zn (_nanoCu₁₀ + _nanoZn₃₂) per kg dry matter (DM) basis in four respective groups. Supplementation of nano Cu along with nano Zn improves immune response which was evidenced from higher immunoglobulin G (IgG), immunoglobulin M (IgM), immunoglobulin A (IgA), total immunoglobulin (TIg), and Zn sulphate turbidity (ZST) units and lower plasma concentrations of tumour necrosis factor-α (TNF-α) and cortisol in the nanoCu10 + nanoZn32 group. There was no effect of treatment on the plasma concentrations of immunoglobulin E (IgE) and interferon-gamma (IFN-γ). Antioxidant status was also better in the nanoCu10 + nanoZn32 group as evidenced by lower concentrations of malondialdehyde (MDA) and higher activity of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), ceruloplasmin (Cp), and total antioxidant status (TAS). However, treatment did not exert any effect on catalase (CAT) activity. Although the nano Cu or nano Zn supplementation, either alone or in combination, did not exert any effect on growth performance or body condition score (BCS), the frequency of diarrhoea and incidence of diarrhoea were lower, while faecal consistency score (FCS) and attitude score were better in the _nanoCu₁₀ + _nanoZn₃₂ groups. In the control group, one calf was found affected with joint illness and two calves were found affected with navel illness. During the experimental period, none of the calves in all four groups were found to be affected by pneumonia. The findings of this study revealed that dietary supplementation of nano Cu in combination with nano Zn improved the health status of young dairy calves by improving immunity and antioxidant status.

Supplementation with galacto-oligosaccharides in early life persistently facilitates the microbial colonization of the rumen and promotes growth of preweaning Holstein dairy calves

We aimed to determine the effects of dietary supplementation with galacto-oligosaccharides (GOS) on the growth performance, serum parameters, and the rumen microbial colonization and fermentation of pre-weaning dairy calves. The study comprised 2 phases of 28 and 42 d, respectively. During phase 1, 24 newborn female Holstein dairy calves were randomly allocated to consume a diet supplemented with 10 g/d GOS (GOS, n = 12) or not (CON, n = 12). Thereafter, during phase 2, the GOS group was further divided into 2 groups: one that continued to consume GOS (GOSC, n = 6) and one that no longer consumed GOS (GOSS, n = 6), alongside the CON group. Galacto-oligosaccharides increased the average daily gain (ADG), body weight, feed efficiency, and serum high-density lipoprotein-cholesterol concentration of dairy calves during phase 1 (P < 0.05). Supplementation with GOS for the entire study reduced the incidence of diarrhea and increased the serum total protein and Ca concentrations (P < 0.05) compared with the CON group. The effect of GOS supplementation persisted after it was stopped because the ADG and final body weight of the GOSS group were higher than those of the CON group (P < 0.05). Furthermore, the GOSS group showed a persistently lower incidence of diarrhea and greater colonization of the rumen with probiotics, at the expense of less beneficial bacteria, which would promote ruminal fermentation and microbial protein synthesis. These findings provide a theoretical basis for the rational application of prebiotics and have important practical implications for the design of early life dietary interventions in dairy calf rearing.

Supplementation With Zinc Proteinate Increases the Growth Performance by Reducing the Incidence of Diarrhea and Improving the Immune Function of Dairy Calves During the First Month of Life

Two experiments were conducted to identify the optimal dose of zinc proteinate (ZP) in the diet for dairy calves and then to compare early supplementation with the ZP or zinc methionine (ZM) on the growth performance, incidence of diarrhea, antioxidant status, and immune function of dairy calves during their first month of life. In Experiment 1, forty newborn female Holstein dairy calves were randomly divided into four groups (n = 10): a control group (without ZP supplementation, ZP0) or groups that received ZP supplementation at 40, 80, and 120 mg zinc/day, respectively (ZP40, ZP80, and ZP120). The experiment lasted 14 days, and the growth performance, incidence of diarrhea, and serum zinc concentration were analyzed. In Experiment 2, thirty-six newborn female Holstein dairy calves were randomly allocated to three groups (n = 12): a negative control group (without zinc supplementation, CON), a positive control group (supplemented with 80 mg zinc/day in the form of zinc methionine, ZM), and a ZP group (supplemented with 80 mg zinc/day in the form of ZP). The experiment lasted 28 days, and the growth performance, incidence of diarrhea, serum zinc concentration, serum antioxidant indicators, and concentrations of plasma immunoglobulins and cytokines were determined on days 7, 14, 21, and 28. Results showed that in Experiment 1, supplementation with ZP to yield 80 mg zinc/day increased the ADG (P &lt; 0.01) and serum zinc concentration (P &lt; 0.01), and decreased the F/G (P &lt; 0.01) and the incidence of diarrhea (P &lt; 0.05) during days 1–14. In Experiment 2, compared with the CON group, ZP increased the ADG (P &lt; 0.01), serum zinc concentration (P &lt; 0.01), and plasma immunoglobulin G (IgG; P &lt; 0.01) and IgM (P &lt; 0.01) concentrations, but reduced the incidence of diarrhea (P &lt; 0.01), serum malondialdehyde (P &lt; 0.01), and plasma interleukin-1β (P &lt; 0.01) concentrations during days 1–28. Overall, ZP supplementation to yield 80 mg zinc/day improves the growth performance and immune function, and decrease the incidence of diarrhea of dairy calves, which was comparable to the same dose of zinc in the form of ZM.

Comparative effect of zinc supplementation by hydroxy and inorganic sources on nutrient utilisation, mineral balance, growth performance and growth biomarkers in pre-ruminant calves

Zinc (Zn), an important microelement in ruminant diet, plays a critical role in various enzymes, hormones and functional proteins involved in nutrient metabolism. The present study was conducted to assess the effect of zinc hydroxychloride (Zn₅(OH)₈Cl₂·H₂O [ZnOHCl] and zinc sulphate (ZnSO₄) on nutrient utilisation, mineral metabolism and biomarkers pertaining to growth performance in pre-ruminant crossbred calves. Twenty-four crossbred calves [body weight (BW); 31.03 ± 4.30 kg; age 10 d] were randomly allocated to four treatment groups (n = 6), i.e. no supplementation of Zn (0 mg/kg dry matter [DM]), 80 mg/kg DM Zn as ZnSO₄(ZnS-80), 40 mg/kg DM Zn as ZnOHCl (ZnH-40) and 80 mg/kg DM Zn as ZnOHCl (ZnH-80) for 90 d experimental period. Results showed that dietary Zn supplementation improved (p < 0.05) feed intake, BW, average daily gain, heart girth, body length, plasma growth hormone, insulin-like growth factor and thyroxin concentration; however, nutrient digestibility remained unaffected among the groups. Addition of Zn increased (p < 0.05) Zn retention and plasma Zn concentration without affecting  retention and plasma concentration of other minerals. Retention of Zn was the highest in ZnH-80 group followed by ZnH-40, ZnS-80 and lowest in control group. Overall results of the present study indicate that regardless of sources and levels, Zn supplementation increased growth performance, plasma Zn concentration and hormones levels in pre-ruminant crossbred calves. However, supplementation of hydroxy Zn at 40 mg/kg DM had similar effect as produced by ZnSO₄ or ZnOHCl at a supplementation level of 80 mg/kg DM. Therefore, from the present study it can be concluded that ZnOHCl can be used as a Zn source for pre-ruminant calves at a lower dose compared to ZnSO₄.

Oxidative Stress in Dairy Cows: Insights into the Mechanistic Mode of Actions and Mitigating Strategies

This review examines several molecular mechanisms underpinning oxidative stress in ruminants and their effects on blood and milk oxidative traits. We also investigate strategies to alleviate or repair oxidative damages by improving animal immune functions using novel feed additives. Microbial pathogenic cells, feeding management, and body condition score were some of the studied factors, inducing oxidative stress in ruminants. The predominance of Streptococcus spp. (24.22%), Acinetobacter spp. (21.37%), Romboutsia spp. (4.99%), Turicibacter spp., (2.64%), Stenotrophomonas spp. (2.33%), and Enterococcus spp. (1.86%) was found in the microbiome of mastitis cows with a decrease of d-mannose and increase of xanthine:guanine ratio when Streptococcus increased. Diversity of energy sources favoring the growth of Fusobacterium make it a keystone taxon contributing to metritis. Ruminal volatile fatty acids rose with high-concentrate diets that decreased the ruminal pH, causing a lysis of rumen microbes and release of endotoxins. Moreover, lipopolysaccharide (LPS) concentration, malondialdehyde (MDA), and superoxide dismutase (SOD) activities increased in high concentrate cows accompanied by a reduction of total antioxidant capacity (T-AOC), glutathione peroxidase (GPx), and catalase (CAT) activity. In addition, albumin and paraoxonase concentrations were inversely related to oxidative stress and contributed to the protection of low-density and high-density lipoproteins against lipid peroxidation, protein carbonyl, and lactoperoxidase. High concentrate diets increased the expression of MAPK pro-inflammatory genes and decreased the expression of antioxidant genes and proteins in mammary epithelial tissues. The expression levels of NrF2, NQO1, MT1E, UGT1A1, MGST3, and MT1A were downregulated, whereas NF-kB was upregulated with a high-grain or high concentrate diet. Amino-acids, vitamins, trace elements, and plant extracts have shown promising results through enhancing immune functions and repairing damaged cells exposed to oxidative stress. Further studies comparing the long-term effect of synthetic feed additives and natural plant additives on animal health and physiology remain to be investigated.

Oxidative stress mitigation by antioxidants - An overview on their chemistry and influences on health status

The present review paper focuses on the chemistry of oxidative stress mitigation by antioxidants. Oxidative stress is understood as a lack of balance between the pro-oxidant and the antioxidant species. Reactive oxygen species in limited amounts are necessary for cell homeostasis and redox signaling. Excessive reactive oxygenated/nitrogenated species production, which counteracts the organism's defense systems, is known as oxidative stress. Sustained attack of endogenous and exogenous ROS results in conformational and oxidative alterations in key biomolecules. Chronic oxidative stress is associated with oxidative modifications occurring in key biomolecules: lipid peroxidation, protein carbonylation, carbonyl (aldehyde/ketone) adduct formation, nitration, sulfoxidation, DNA impairment such strand breaks or nucleobase oxidation. Oxidative stress is tightly linked to the development of cancer, diabetes, neurodegeneration, cardiovascular diseases, rheumatoid arthritis, kidney disease, eye disease. The deleterious action of reactive oxygenated species and their role in the onset and progression of pathologies are discussed. The results of oxidative attack become themselves sources of oxidative stress, becoming part of a vicious cycle that amplifies oxidative impairment. The term antioxidant refers to a compound that is able to impede or retard oxidation, acting at a lower concentration compared to that of the protected substrate. Antioxidant intervention against the radicalic lipid peroxidation can involve different mechanisms. Chain breaking antioxidants are called primary antioxidants, acting by scavenging radical species, converting them into more stable radicals or non-radical species. Secondary antioxidants quench singlet oxygen, decompose peroxides, chelate prooxidative metal ions, inhibit oxidative enzymes. Moreover, four reactivity-based lines of defense have been identified: preventative antioxidants, radical scavengers, repair antioxidants, and those relying on adaptation mechanisms. The specific mechanism of a series of endogenous and exogenous antioxidants in particular aspects of oxidative stress, is detailed. The final section resumes critical conclusions regarding antioxidant supplementation.

Effect of the Source of Zinc on the Tissue Accumulation of Zinc and Jejunal Mucosal Zinc Transporter Expression in Holstein Dairy Calves

Simple Summary

Diarrhea is the main cause of death in newborn calves and is associated with antibiotic use and economic loss for dairy farms. In this study, we evaluated the effects of different sources of the mineral zinc (zinc oxide (ZnO) and zinc methionine (Zn-Met)) on the growth, incidence of diarrhea, tissue zinc accumulation, gene expression of jejunal zinc transporters and serum concentrations of zinc-dependent proteins in newborn Holstein dairy calves. We found that Zn-Met supplementation promoted growth and reduced diarrhea from the second week after birth. It also increased the levels of zinc in the serum and liver, the level of the transporter protein ZIP4 in the jejunal mucosa, as well as the serum alkaline phosphatase and metallothionein concentrations compared to the control group. ZnO supplementation had similar but less marked effects to Zn-Met supplementation. These results suggest that Zn-Met supplementation may be an alternative to antibiotics for the treatment of newborn calf diarrhea.

Abstract

Zinc is considered to be an anti-diarrheal agent, and it may therefore reduce the incidence of diarrhea in young calves. In the present study, we aimed to compare the effect of zinc source on growth performance, the incidence of diarrhea, tissue zinc accumulation, the expression of zinc transporters, and the serum concentrations of zinc-dependent proteins in neonatal Holstein dairy calves. Eighteen male newborn Holstein dairy calves were fed milk and starter diet supplemented with or without 80 mg zinc/d in the form of Zn-Met or ZnO for 14 days, and were then euthanized. Zn-Met supplementation improved average daily gain and feed efficiency, and reduced the incidence of diarrhea, compared with control calves (p < 0.05). It also increased the serum and hepatic zinc concentrations and the mRNA expression of the ZIP4 transporter in the jejunal mucosa of the calves (p < 0.05). In addition, the serum alkaline phosphatase activity and metallothionein concentration were higher in Zn-Met-treated calves than in control calves (p < 0.05). ZnO supplementation had similar effects, but these did not reach significance. Thus, Zn-Met supplementation is an effective means of increasing tissue zinc accumulation and jejunal zinc absorption, and can be used as an anti-diarrheal strategy in neonatal calves.

Effects of different types of zinc supplement on the growth, incidence of diarrhea, immune function, and rectal microbiota of newborn dairy calves

Neonatal diarrhea in dairy calves causes huge economic and productivity losses in the dairy industry. Zinc is an effective anti-diarrheal agent, but high doses may pose a threat to the environment. Therefore, we aimed to evaluate the effects of low-dose zinc supplementation on the growth, incidence of diarrhea, immune function, and rectal microbiota of newborn Holstein dairy calves. Thirty newborn calves were allocated to either a control group (without extra zinc supplementation), or groups supplemented with either 104 mg of zinc oxide (ZnO, equivalent to 80 mg of zinc/d) or 457 mg of zinc methionine (Zn-Met, equivalent to 80 mg of zinc/d) and studied them for 14 d. The rectal contents were sampled on d 1, 3, 7, and 14, and blood samples were collected at the end of the study. Supplementation with ZnO reduced the incidence of diarrhea during the first 3 d of life, and increased serum IgG and IgM concentrations. The Zn-Met supplementation increased growth performance and reduced the incidence of diarrhea during the first 14 d after birth. The results of fecal microbiota analysis showed that Firmicutes and Proteobacteria were the predominant phyla, and Escherichia and Bacteroides were the dominant genera in the recta of the calves. As the calves grew older, rectal microbial diversity and composition significantly evolved. In addition, dietary supplementation with ZnO reduced the relative abundance of Proteobacteria in 1-d-old calves, and increased that of Bacteroidetes, Lactobacillus, and Faecalibacterium in 7-d-old calves, compared with the control group. Supplementation with Zn-Met increased the relative abundance of the phylum Actinobacteria and the genera Faecalibacterium and Collinsella on d 7, and that of the genus Ruminococcus after 2 wk, compared with the control group. Thus, the rectal microbial composition was not affected by zinc supplementation but significantly evolved during the calves' early life. Zinc supplementation reduced the incidence of diarrhea in young calves. In view of their differing effects, we recommend ZnO supplementation for dairy calves during their first 3 d of life and Zn-Met supplementation for the subsequent period. These findings suggest that zinc supplementation may be an alternative to antibacterial agents for the treatment of newborn calf diarrhea.