The demand for safe and healthy goat milk and meat products is increasing with the growth of population and income increases [1]. With about one billion stock and 200 breeds, goats produce milk with high nutritional value, meat with high protein and low cholesterol, and wool of high quality [2]. Accordingly, goats play an important role in agricultural economics and ecological niches [3]. Compared with large ruminants, goats have a shorter production cycle, higher development rate, and greater environmental adaptation, and can cope with different stressors, harsh environments, and diseases [4]. Compared to sheep, goats are an intermediate browser preferring trees and shrubs ^[5][6][5,6].

The rumen mucosa structure, including size and shape of papilla, has a significant role in nutrient digestion or absorption ^[7][30]. The main factors affecting rumen epithelial development are diets and age ^[8][9][10][10,31,32]. At birth, the rumen papillae growth is minimal in goat kids ^[8][10] and calves ^[11][33]. The rumen is not engaged in the digestion of plant material or milk during the suckling stage ^[12][13][14][34,35,36]. This is because a reflex mechanism closes the esophageal groove directing the colostrum and milk flow to the abomasum for enzymatic digestion ^[15][16][17][19,37,38].

The transition period starts from 3 to 8 weeks of age when the rumen gradually develops ^[18][39] and adapts to the consumption of solid feed ^[19][40]. Several studies reported the beneficial effects of solid feed in stimulating rumen development through the early stage of life in ruminants ^[10][20][32,41]. As a result of the introduction of a solid diet, the length and size of the rumen papillae in goat kids rose gradually during the transition period ^[8][21][10,42]. This is supported by Chai et al. ^[22][43], who observed that the most important factor that altered rumen microbiota and epithelial gene expression in pre-weaning ruminants was the amount of neutral detergent fiber. It may be due to the solid feed providing butyrate as well as a physical stimulus favoring rumen anatomical development ^[9][10][31,32]. Htoo et al. ^[23][44] showed that the large particle size of roughage and the content of fiber increased the rumen wall by physical stimulation, and consequently enhanced rumen motility, muscularization, and rumen volume in goat kids with free access to creep feed with roughage. In addition, Malhi et al. ^[24][45] reported that intraruminal infusion of sodium butyrate at 0.3 g/kg of body weight for 28 days in goats improved the rumen papillae size, density, and surface area. Ruminal infusion of butyrate at 0.3 g/kg of body weight in goats increased the full weight of rumen as percent of total stomach weight ^[25][46]. In addition, infusion of sodium butyrate at 0.3 g/kg of body weight can improve ruminal epithelial growth by modulating both proliferative and apoptotic genes, i.e., Bax, caspase 3, and caspase 9 ^[26][47]. Moreover, sodium butyrate supplementation at 3.2% of diet increased rumen epithelium thickness in growing rams ^[27][48]. Accordingly, the transition period should be considered by a goat nutritionist for the adaptation of rumen papillae to the dietary changes.

After sequential dynamic changes of development ^[8][10], the rumen development of a mature animal accounts for 60 to 80% of the complex stomach volume ^[17][28][38,49]. It has been reported that the microbial colonization in the rumen of goats occurred earlier and is achieved at 1 month of age, functional achievement is at 2 months, and anatomic development is achieved after 2 months ^[8][10]. Meanwhile, the rumen transforms to a fully functional fermenter with capabilities of utilizing fibrous diets ^[17][38]. At this stage, goats are able to break down and extract nutrients from tough fibrous plants through a process of microbial fermentation and regurgitation. The mature rumen serves as a fermentation chamber, where bacteria and protozoa break down cellulose and hemicellulose into simple sugars, organic acids, and gases, which allows goats to efficiently extract nutrients from low-quality and less digestible forages, and adapt to grazing on a variety of vegetations [6].

In addition, the development of rumen absorption function in goat kids after birth is a crucial process that enables animals to adapt to solid diets. Initially, the rumen is a small pouch-like structure with a thin lining and lacks the necessary capacity to perform efficient absorption of nutrients. However, the rumen gradually expands due to the induced cell proliferation and differentiation as the kid starts to consume solid feed ^[29][30][50,51]. Papillae can greatly enhance the absorptive capacity of the rumen by increasing its surface area ^[24][31][45,52]. High-fiber diets induce the expression of genes related to short-chain fatty acid absorption in the rumen epithelium of goats ^[32][53]. A similar improvement in absorption capacity of the rumen epithelium was observed in sheep ^[31][52] and cows ^[33][54], even with unchanged absorptive surface area of the rumen papillae ^[33][54]. On the other hand, low-fiber diets can cause insufficient rumen development and impede proper nutrient digestion and absorption ^[13][35]. Therefore, it is important to carefully evaluate the impact of age and diets in further studies to determine their effectiveness in enhancing the absorptive capacity of the rumen.

Daily, goats spend much time eating and ruminating ^[34][55]. Rumination is a complex process involving regurgitation and chewing of previously swallowed feed. Lickliter ^[35][56] reported that kids start mouthing soil and grasses during their first week following birth but ruminating is first observed during week 3 of age. Before weaning, kids ruminate for over 3 h/day ^[36][25]. After weaning, removal of dietary milk results in a dramatic increase in the time spent for rumination (5.2 h/day) ^[36][25], indicating the major roles of age and feeding ^[34][35][36][25,55,56]. Finally, it has been reported that the adult goats spend 7.2 h/day reaching about one third of their day for rumination ^[37][38][57,58].

Various prokaryotic (bacteria and archaea) and eukaryotic (protozoa and fungal) microorganisms live in the rumen and work together to digest and ferment feed ^[39][11]. The ruminal microbiota has a symbiotic relationship with the host and is distinguished by its high population density, diversity, and complexity of interactions ^[12][40][21,34]. Rumen microbes have a remarkable ability to ferment and transform plant feed into microbial matter, volatile fatty acids (VFAs), fermentation gases (methane and carbon dioxide), and ammonia, as well as producing heat ^[22][41][42][43,59,60]. The VFA and microbial proteins provide nutrients for the host’s maintenance and growth ^[12][43][34,61]. The rumen microorganisms, especially bacteria and their end products, are essential for regulating rumen function and nutrient digestion ^[17][38], thereby improving production efficiency and health status ^[17][44][45][38,62,63]. In addition, the rumen microbiota plays an essential role in the development of the rumen during the early stages of life ^[39][11]. Early weaning disrupts the development of the rumen microbiota, leading to a less diverse microbial community and impaired fermentation ^[46][64]. This, in turn, can result in reduced nutrient absorption and growth in the young animal. Therefore, appropriate management practices are necessary to ensure the early establishment and development of a healthy rumen microbiota in early-weaned ruminants ^[47][65].

It is debatable whether the gut microbial community colonizes prenatally or postnatally ^[19][40]. Bacterial communities found in numerous maternal-associated sources, including the colostrum, vagina, udder skin, and saliva, have been shown to colonize the gastrointestinal system of newborns within days of birth ^[19][48][49][40,66,67]. The first day, rumen bacterial communities of goat offspring may also be acquired from the intake of amniotic fluid in pregnancy ^[49][67]. Moreover, a recent study showed that the bacterial colonization of the fetal gut commences in utero ^[50][68]. In goats, colonization of the bacterial population has been shown to be age-dependent ^[18][51][39,69]. It takes 3 to 4 weeks for the bacterial community structure to stabilize, implying that this period is essential ^[52][16]. The rumen bacterial communities of goat kids exhibited remarkable alterations in three stages within the first two months after birth, according to recent longitudinal studies ^[8][49][10,67]. The three stages of rumen development and microbial colonization are the non-rumination phase, transition phase, and rumination phase ^[8][49][53][10,67,70]. From the non-ruminant to the ruminant phase, aerobic and facultative anaerobic microbial taxa are primarily replaced by anaerobic species in the gastrointestinal tract colonizers ^[54][71].

The microbiome gradually matures into a complex microbial community ^[15][55][19,72]. Most alpha diversity indices, including the Shannon index, observed bacteria, and Chao1 estimator, increase with age, suggesting that the microbiota in older age groups is more diverse than in earlier age groups ^[18][39]. This is similar to microbial colonization of the rumen contents in calves ^[56][57][14,73]. The three predominant phyla in newborn goats are Proteobacteria, Bacteroidetes and Firmicutes ^[19][48][58][27,40,66]. The abundance of Proteobacteria decreased quadratically with age at 7 days, but Bacteroidetes and Firmicutes increased ^[18][59][39,74]. Most of genera detected within Firmicutes and Bacteroidetes are anaerobes ^[59][74]. This could be related to the shift from an aerobic or facultative anaerobic environment niche occurring close to birth to an exclusively anaerobic one with the development of rumen ^[57][59][73,74]. The reduction in the phylum Proteobacteria and the increase in the phylum Bacteroidetes were found in rumen as a result of weaning ^[60][9]. As Bacteroidetes have a strong ability to degrade proteins and polysaccharides, they could enhance nutrient digestion and metabolism in the rumen ^[61][75]. Bacteroidetes were more reliant on solid diet intake than milk removal, reaching a consistent abundance after 7 weeks of age ^[62][76]. As the rumen bacterial communities are not only influenced by diet, but also by age, the fetal and newborn communities were dominated by species from the Proteobacteria while Bacteroidetes and Firmicutes were the two major phyla from weaning to adulthood ^[57][60][9,73]. However, it has been reported that after two weeks of age, the community no longer demonstrated large temporal variations at the phylum level, albeit the relative abundance of certain species remained variable ^[18][56][63][14,39,77]. Firmicutes, for example, was the dominant phylum in the rumen on the first day of life, and its members Bacillus and Lactococcus were prominent genera ^[49][67]. Following lactation, the primary bacterial phylum detected was Bacteroidetes, with extremely low amounts of Bacillus and Lactobacillus ^[49][67]. Additionally, at the genus level, the proportion of Bacteroides family was undetectable on the first day after birth but increased from 3 to 14 days of age ^[49][67]. Similarly, Jiao et al. ^[51][69] showed that Bacteroides surged in abundance during the first week. The bacterial biomarkers for goat kids during the non-rumination stage (i.e., 7 to 21 days) were mainly Bacteroidetes and its members (e.g., Bacteroidaceae, Bacteroides, and Alistipes) and several members of Firmicutes (e.g., Lactobacillaceae, Lactobacillus, and Butyricicoccus), which can be attributed to the dam’s milk-dominated feeding that is rich in lactose, protein, and fat ^[48][66]. In addition, Jiao et al. ^[51][69] indicated that the lactic acid bacteria in the phylum Firmicutes, such as Enterococcus and Lactobacillus, were found in the rumen of newborn goats. However, during the primary stages of development, Bacteroides, as the main genus within phylum Bacteroidetes, is immediately replaced by the Prevotella in the rumen after the provision of solid feed at three months of life in goats ^[18][49][60][9,39,67]. Thus, solid food disrupted the ruminal epithelial microbiota by selecting bacterial taxa that were more specific and were adapted to new substrates ^[18][39]. Irrespective of feeding type, relative abundances of ruminal Prevotella, Fibrobacter, Ruminococcus, and Butyrivibrio increased with age ^[51][69].

Methanogens are important for digestion and gas production, and understanding the colonization of methanogens is important for the healthy gut microbiota and digestion in both infants and adults. Diversity within the archaea is much lower than that of bacteria, with only a few methanogenic groups being the top three most abundant active methanogens (Methanosphaera, Methanobacteriaceae, and/or Methanobrevibacter) ^[40][41][21,59]. The age-dependent tendency of alpha diversity did not show in the archaeal community of the goat’s rumen and gut ^[40][58][21,27]. Methanogens that use hydrogen as an energy source to reduce carbon dioxide or acetate to methane have a negative relationship with the oxidative condition within the rumen ^[64][78]. It has been reported that the methanogens initially colonized rumen on the first day after birth in goats ^[8][40][10,21]. Jiao et al. ^[8][10] showed that the archaeal copy numbers increased with age in goat kids. Moreover, irrespective of feeding type, relative abundances of ruminal Methanobrevibacter increased with age in goats ^[51][69]. Other studies showed that the methanogenic archaeal populations began to increase and stabilize after the starter feed intake due to the starter’s starchy components, which promote hydrogen production ^[40][21]. After weaning 40-day-old kids, the abundance of Methanomicrobium spp. and Methanimicrococcus spp. increased, while the abundance of the genus Methanimicrococcus decreased from 50 to 60 days and lost its dominance ^[40][21].

Whereas rumen bacteria and methanogens are early rumen colonizers ^[52][16], other microbiomes such as protozoa and fungi colonize the rumen later than bacteria and methanogens do ^[65][17]. This could be attributed to protozoa being highly sensitive to oxygen and requiring direct contact between young and adult animals for effective transmission ^[66][79]. Thus, ruminants are born protozoa-free, and rumen protozoa only become established after direct and continuous nose–nose contact with adult animals ^[66][79]. Protozoa can usually be found within 15 days postpartum in the rumen of young ruminants ^[8][19][10,40]. The genera Entodinium and Epidinium are dominant protozoa ^[41][59]. In neonatal ruminants, anaerobic fungi mainly composed of Neocallimastix frontalis appear in rumen samples collected at 7 days of age ^[8][10] or between 8 and 10 days after birth ^[19][40]. However, several invasive fungal pathogens, such as Aspergillus and Candida, were observed during the first week, suggesting that fungi may also play a role in developing ruminal mucosal innate immune function ^[49][51][67,69]. However, these pathogens declined to undetectable levels from 3 days to 14 days of life and replaced several predominant microbes with the changes in diet after 14 days of life, such as Neocallimastix sp. and Orpinomyces sp., which may be involved in the digestion of feed fiber in rumen ^[49][67]. Solid feeds play an indispensable role in the fungi and protozoa colonization as their levels surged during 28 days of life ^[8][10]. Moreover, Jiao et al. ^[51][69] showed that the relative abundances of ruminal Neocallimastix and Entodinium increased with age irrespective of feeding type. Overall, the rumen colonization of bacteria, protozoa, and fungi in animals is a complex and dynamic process that plays a vital role in the digestive system of ruminants.

Kids rely on their dam’s milk in the early weeks of life because the rumen is physically and metabolically immature ^[67][80]. At this stage, the ruminal milieu does not form VFA and lacks activities of enzymes such as amylase, urease, protease, and xylanase, as well as a very low ammonia nitrogen production, implying a deficiency in fermentation ability and enzyme activities in the newborn goat kids ^[8][10]. Meanwhile, the intestinal microbiota can use milk carbohydrates such as lactose and oligosaccharides to produce a variety of metabolites, including VFA. As a result, a slight increase in acetate molar proportion in kids was observed at 14 days of age ^[8][10], indicating the slight increase in fermentation capability during the non-rumination period by the consumption of only milk ^[68][81]. Thus, the varied nutrition sources and hormonal signals during rumen development cause irreversible alterations in body composition (protein, fat, carbohydrates, minerals, vitamins, and water) and metabolic function of newborn ruminants ^[65][69][17,82]. Metabolic hormones from the adipose (leptin), liver (Insulin-Like Growth Factor 1), and gut (Ghrelin) act as signaling factors that regulate the activity of the gonadotropin-releasing hormone in the hypothalamus, which control appetite and feeding behavior ^[70][71][83,84]. Thus, animals with high levels of dietary protein and energy have greater concentrations of leptin and Insulin-Like Growth Factor 1 ^[72][85].

During growth, the goats’ feed supply is substantially altered from a high-fat milk diet to a forage- and concentrate-based diet ^[8][10]. The pattern of nutrient absorption shifts from glucose, fatty acids, and milk-derived amino acids to substances from both feed and microbial sources ^[13][35]. Rumen microorganisms ferment carbohydrates to produce VFA such acetate, propionate, and butyrate, all of which are used as energy sources in the ruminant body ^[73][86]. VFA and ketone bodies are considered the most reliable indicators of a completely functional rumen ^[13][67][35,80]. Thus, a significant increase was observed in VFA followed by a decline in acetate-to-propionate ratio due to the rise in starch digestion and amylolytic microbes degrading starch in weaned goats compared to goat kids fed on milk ^[8][10]. Furthermore, ammonia nitrogen concentration increased to reach its levels in adult goats along with the microbiota colonization processes ^[8][10]. The blood urea nitrogen increased in 30-days-old kids with increasing ruminal degradation of protein and ammonia production due to the increasing microbial activity ^[67][80]. Several enzymes such as amylase, xylanase, and carboxymethyl cellulose increased in 14-days-old kids even before the introduction of solid diets due to the significant role of microbial colonization, which occurs before the functional changes ^[8][10]. Overall, the rumen functional development occurs at 2 months in goats ^[8][10].