Digestive tract development in calves presents a uniquely organized system. Specifically, as the rumen develops and becomes colonized by microorganisms, a calf physiologically transitions from a pseudo-monogastric animal to a functioning ruminant. Importantly, the development of rumen in calves can directly affect the intake of feed, nutrient digestibility and overall growth. Even minor changes in the early feeding regime and nutrition can drastically influence rumen development, resulting in long-term effects on growth, health, and milk yields in adult cattle. Rumen development in newborn calves is one of the most important and interesting areas of calf nutrition. This paper presents a comprehensive review of recent studies of the gastrointestinal (GI) tract development in calves. Moreover, we also describe the effect of the environment in shaping the GI tract, including diet, feed additives and feeding management, as well as discuss the strategies to promote the physiological and microbiological development of rumen.

Garlic extracts have been reported to be effective in reducing methanogenesis. Related mechanisms are not well illustrated, however, and most studies have been conducted in vitro. This study investigates the effects of supplementary allicin (AL) in sheep diet on in vivo digestibility, rumen fermentation, and shifts of microbial flora. Two experiments were conducted using Dorper × thin-tailed Han crossbred ewes. In experiment 1, eighteen ewes (60.0 ± 1.73 kg BW) were randomly assigned for 29 days to either of two dietary treatments: a basal diet or the basal diet supplemented with 2.0 g AL/head·day to investigate supplementary AL on nutrient digestibility and methane emissions. In experiment 2, six ewes (65.2 ± 2.0 kg BW) with ruminal canulas were assigned to the same two dietary treatments as in experiment 1 for 42 days to investigate supplementary AL on ruminal fermentation and microbial flora. The methane emissions were determined using an open-circuit respirometry system and microbial assessment was done by qPCR of 16S rRNA genes. Supplementary AL increased the apparent digestibility of organic matter (P < 0.001), nitrogen (P = 0.006), neutral detergent fiber (P < 0.001), and acid detergent fiber (P = 0.002). Fecal nitrogen output was reduced (P = 0.001) but urinary nitrogen output was unaffected (P = 0.691), while nitrogen retention (P = 0.077) and nitrogen retention/nitrogen intake (P = 0.077) tended to increase. Supplementary AL decreased methane emissions scaled to metabolic bodyweight by 5.95 % (P = 0.007) and to digestible organic matter intake by 8.36 % (P = 0.009). Ruminal pH was unaffected (P = 0.601) while ammonia decreased (P = 0.024) and total volatile fatty acids increased (P = 0.024) in response to supplementary AL. Supplementary AL decreased the population of methanogens (P = 0.001) and tended to decrease that of protozoans (P = 0.097), but increased the populations of F. succinogenes (P < 0.001), R. flavefaciens (P = 0.001), and B. fibrisolvens (P = 0.001). Supplementation of AL at 2.0 g/head·day effectively enhanced OM, N, NDF, and ADF digestibility and reduced daily methane emissions (L/kg BW0.75) in ewes, probably by decreasing the population of ruminal protozoans and methanogens.

The rumen bacterial community plays a critical role in feeds degradation and productivity. The effects of different forage to concentrate ratios on the ruminal microbial population structure have been studied extensively; however, research into changes in the ruminal bacterial community composition in heifers fed different energy level diets, with the same forage to concentrate ratio, has been very limited. The purpose of this study was to investigate the effects of different dietary energy levels, with the same forage to concentrate ratio, on ruminal bacterial community composition of heifers. Furthermore, we also determine the relationship between rumen bacteria and ruminal fermentation parameters. The 16S rRNA gene sequencing showed that, under the same forage to concentrate ratio of 50:50, an 8% difference in dietary energy level had no significant impact on the alpha diversity and the relative abundance of the major phyla and most of the major genera in heifers. In all the treatments groups, Firmicutes, Bacteroidetes, and Proteobacteria were the dominant phyla. Spearman correlation analysis between the relative abundances of the rumen bacteria at the genus level and the fermentation parameters showed that the relative abundances of Prevotella and BF311 were positively correlated with the ammonia nitrogen and butyrate concentrations, and these two genera were negatively correlated with the propionate and isovalerate concentrations, respectively, and the genus Bifidobacterium was positively correlated with the butyrate concentration and was negatively correlated with propionate and isovalerate concentration. The total volatile fatty acid concentration was positively correlated with BF311 abundances, and was negatively correlated with Trichococcus and Facklamia abundances. Under the same forage to concentrate ratio condition of 50:50, an 8% difference in dietary energy levels had little impact on rumen bacterial community composition in heifers. The correlations between some genera of ruminal bacteria and the concentrations of volatile fatty acids and ammonia nitrogen might be indicative that the ruminal fermentation parameters are strongly influenced by the rumen bacterial community composition.

Objective: Microbial exposure is critical to neonatal and infant development, growth and immunity. However, whether a microbiome is present in the fetal gut prior to birth remains debated. In this study, lambs delivered by aseptic hysterectomy at full term were used as an animal model to investigate the presence of a microbiome in the prenatal gut using a multiomics approach. Design: Lambs were euthanised immediately after aseptic caesarean section and their cecal content and umbilical cord blood samples were aseptically acquired. Cecal content samples were assessed using metagenomic and metatranscriptomic sequencing to characterise any existing microbiome. Both sample types were analysed using metabolomics in order to detect microbial metabolites. Results: We detected a low-diversity and low-biomass microbiome in the prenatal fetal gut, which was mainly composed of bacteria belonging to the phyla Proteobacteria, Actinobacteria and Firmicutes. Escherichia coli was the most abundant species in the prenatal fetal gut. We also detected multiple microbial metabolites including short chain fatty acids, deoxynojirimycin, mitomycin and tobramycin, further indicating the presence of metabolically active microbiota. Additionally, bacteriophage phiX174 and Orf virus, as well as antibiotic resistance genes, were detected in the fetal gut, suggesting that bacteriophage, viruses and bacteria carrying antibiotic resistance genes can be transmitted from the mother to the fetus during the gestation period. Conclusions: This study provides strong evidence that the prenatal gut harbours a microbiome and that microbial colonisation of the fetal gut commences in utero.

Alternatives to antibiotics for improving productivity and maintaining the health of livestock health are urgently needed. The scope of this research was conducted to investigate the effects of two alternatives (Bacillus licheniformis and Saccharomyces cerevisiae) to monensin on growth performance, antioxidant capacity, immunity, ruminal fermentation and microbial diversity of fattening lambs. One hundred and sixty Dorper × Thin-tailed Han sheep (32 ± 3.45 kg BW) were randomly assigned into 5 treatments of n = 32 lambs/group. Lambs in the control group were fed a basal diet (NC) while the other four treatments were fed basal diets supplemented with monensin (PC), Bacillus licheniformis (BL), Saccharomyces cerevisiae (SC), and the combination of Bacillus licheniformis and Saccharomyces cerevisiae with protease (BS), respectively. The experiment lasted for 66 d. Feed intake was recorded every 2 d and lambs were weighed every 20 d. Ten lambs from each group were slaughtered at the end of the trial, and samples of serum and rumen fluid were collected. The results indicated that the dietary regimen did not affect the dry matter intake (DMI). The average daily gain (ADG) of BS treatment was significantly higher than NC group (P < 0.05). Compared with the NC treatment, the other four supplementation treatments increased the concentration of growth hormone (GH), insulin-like growth factor I (IGF-I) and insulin (INS) (P < 0.05). The malondialdehyde (MDA) and total antioxidant capacity (TAOC) showed no significant difference among the 5 treatments while the activity of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) of BS group was significantly increased (P < 0.05). The supplementation regimen decreased the concentration of ammonia Nitrogen (NH₃-N) and increased the content of microbial crude proteins (MCP) (P < 0.05). The supplementation of antibiotics and probiotics reduced the concentrations of acetate and increased the concentrations of propionate (P < 0.05). The supplementation treatments increased the relative abundance of Lentisphaerae, Fibrobacteres and Tenericutes at the phylum level, whereas at the genus level, they increased the relative abundance of Fibrobacter (P < 0.05). Overall, this study confirmed the facilitating effect of B. licheniformis, S. cerevisiae and their compounds on growth performance, improve the antioxidant capacity and immune function, and beneficially manipulate ruminal fermentation and microbial diversity of fatting lambs.