A functional rumen allows young ruminants to transition from dependence on milk toward microbial fermentation, VFA absorption and efficient use of solid feeds, making rumen maturity a key factor in weaning success and subsequent growth.

Why rumen development matters

In newborn ruminants, the rumen is small and functionally immature. Milk feeding largely bypasses the rumen via the esophageal groove, so the young animal depends on the abomasum and small intestine for digestion.

As solid feed intake increases, the rumen:

• Expands in volume and surface area
• Develops papillae that absorb volatile fatty acids (VFAs) produced by microbial fermentation
• Establishes a complex microbiota capable of fermenting fiber and starch
• Becomes the primary site of energy supply via VFAs, particularly acetate, propionate and butyrate

The review emphasizes that rumen development is pivotal for overall growth performance and production efficiency in dairy calves, beef cattle, sheep and goats.

Animals with earlier and more complete rumen maturation typically handle weaning better, maintain growth during diet transitions and achieve target weights sooner.

How the rumen develops: stages and key changes

The authors outline rumen development as a progressive process.

1. Pre-natal and neonatal phase

• Rumen structures are present at birth but remain underdeveloped.
• Microbial colonization begins during and immediately after birth, influenced by the dam, environment and early feeding.

2. Transition phase: the first weeks of life

• Young animals begin nibbling hay, forage and concentrates.
• Mechanical stimulation from fibrous feed and chemical stimulation from VFAs—especially butyrate and propionate—drive papillae growth and rumen wall thickening.
• Microbial populations shift from facultative anaerobes toward strict anaerobes specialized in fiber and starch fermentation.

3. Functional rumen phase: by weaning and beyond

• Rumen fermentation supplies a growing share of energy and microbial protein.
• Papillae become well developed, enabling efficient VFA absorption.
• Rumen pH buffering, motility and microbial balance become critical to maintain health and performance.

The review stresses that solid feed intake is the primary driver of rumen development. Calves, lambs and kids that begin consuming starter feed and forage earlier tend to develop larger and more functional rumens at a younger age.

Main factors influencing rumen development

Yu and Wu group the key influencers into nutritional, management and biological factors.

1. Diet composition and feeding program

Starter feed and forage: Early access to high-quality starter concentrates and some effective fiber such as hay or chopped forage stimulates rumen motility, papillae development and microbial colonization.

Diets too low in physically effective fiber can impair rumination and rumen muscle development.

Fermentable carbohydrates: Starch and soluble fibers increase production of propionate and butyrate, which act as local signals for papillae growth and epithelial cell proliferation.

However, excessive rapidly fermentable starch without adequate fiber can cause subacute ruminal acidosis and damage the developing epithelium.

Protein level and source: Adequate rumen-degradable and undegradable protein supports microbial growth and host tissue development. Imbalances can limit microbial protein synthesis or lead to excess ammonia.

Additives and supplements: Buffers such as sodium bicarbonate, yeast products, probiotics and certain feed additives can help stabilize rumen pH and microbiota during the transition.

2. Management practices

Age and criteria for weaning: The review supports weaning based on digestive maturity—steady solid feed intake, visible rumination and good growth—rather than age alone.

Animals weaned before the rumen is functionally ready often suffer growth checks and higher morbidity.

Feeding frequency and form: Frequent, consistent access to fresh starter and forage encourages steady intake. Pelleted versus textured starters, particle size and palatability all influence how quickly young ruminants adopt solid feed.

Housing and environment: Clean, low-stress environments with adequate space and social contact promote normal feeding behavior and rumen development. Overcrowding, poor ventilation and high pathogen load can depress intake and disrupt microbial colonization.

3. Host factors

Species and breed: Dairy calves, beef calves, lambs and kids differ in growth patterns and rumen development rates.

Sex and birth weight: Heavier, well-grown neonates often begin eating solid feed earlier and develop the rumen faster than small or compromised newborns.

Health status: Early-life diseases such as scours and pneumonia reduce feed intake and divert nutrients away from growth and rumen development, potentially causing long-term setbacks.

Hormonal regulation: the GH/IGF axis

A major focus of the review is the role of the growth hormone (GH) / insulin-like growth factor (IGF) axis in rumen development.

• GH is produced by the pituitary and stimulates production of IGF-1 primarily in the liver and locally in tissues, including the rumen epithelium.
• IGF-1 promotes cell proliferation, differentiation and survival in many tissues, and evidence suggests it plays a similar role in the developing rumen.

Key points from the review include:

• Nutritional status strongly influences the GH/IGF system. Better energy and protein intake generally increase IGF-1 concentrations, which are associated with faster growth and tissue development.
• Local IGF-1 signaling in the rumen wall may mediate some of the effects of diet and VFAs on epithelial cell growth and papillae development.
• The GH/IGF axis interacts with other hormonal systems, including insulin, thyroid hormones and glucocorticoids, creating an integrated network that links overall growth with rumen maturation.

The authors propose that nutritional strategies which optimize the GH/IGF axis—through adequate energy, protein and micronutrients, while minimizing stress and disease—can indirectly support more rapid and complete rumen development.

Cellular signaling: RAP1A/GPR41–PPAR pathways

Beyond systemic hormones, the review highlights emerging evidence on local cellular signaling pathways that respond to microbial metabolites, particularly VFAs.

GPR41 (FFAR3) and GPR43 (FFAR2) are G-protein-coupled receptors expressed in various tissues, including the gastrointestinal tract, that recognize VFAs such as acetate, propionate and butyrate.

Activation of these receptors can influence:

• Epithelial cell proliferation and differentiation
• Inflammatory and immune responses
• Energy metabolism
• Barrier function

The review focuses on a proposed RAP1A/GPR41–PPAR signaling cascade:

• RAP1A, a small GTPase, is implicated in regulating cell adhesion, proliferation and differentiation.
• GPR41 activation by VFAs may modulate RAP1A activity and downstream effectors.
• PPARs (peroxisome proliferator-activated receptors), especially PPAR-γ, regulate genes involved in lipid metabolism, inflammation and cell differentiation. VFA signaling through GPR41 and related pathways can influence PPAR activity in the gut epithelium.

VFAs as signals—not just energy

In the context of the rumen, VFAs produced by microbial fermentation—particularly butyrate and propionate—may act not only as energy sources but also as signaling molecules.

These metabolites may activate GPR41/43 and influence RAP1A and PPAR pathways in rumen epithelial cells.

This could help explain how increased solid feed intake and VFA production directly stimulate papillae growth, epithelial thickening and metabolic maturation of the rumen wall.

The proposed pathway provides a mechanistic link between:

Diet → microbial fermentation → VFA production → local signaling → structural and functional rumen development.

Microbiota–host interactions

Although the review focuses on hormonal and cellular pathways, it acknowledges that the rumen microbiome is central to the whole process.

• Early colonizers shape the environment for later, more specialized microbes.
• Microbial fermentation produces VFAs, B-vitamins and microbial protein, all of which support host growth and rumen epithelial development.
• Microbial metabolites—including VFAs and potentially other compounds such as indoles and secondary bile acids—act as signals to host receptors such as GPR41/43 and PPARs, influencing gene expression in the rumen wall.

Future work integrating metagenomics, metabolomics and host transcriptomics is likely to clarify how specific microbial communities and metabolites interact with the GH/IGF axis and RAP1A/GPR41–PPAR pathways to drive rumen development.

Practical implications for feeding and management

The review translates these mechanistic insights into several practical recommendations.

1. Start solid feed early

• Offer high-quality starter concentrate and some effective forage from the first 1–2 weeks of life.
• Ensure feed is fresh, palatable and available ad libitum to encourage consistent intake.

2. Balance fermentable carbohydrates and fiber

• Provide enough starch and soluble fiber to generate propionate and butyrate, while including sufficient physically effective fiber to support rumination, buffering and muscle development.
• Avoid abrupt increases in highly fermentable carbohydrates that could cause acidosis in young, still-adapting rumens.

3. Support microbial establishment

• Consider probiotics, yeast products or targeted additives that help stabilize pH and promote beneficial microbial populations during the transition.
• Minimize unnecessary antibiotic use that could disrupt normal microbial colonization unless clearly indicated for health reasons.

4. Wean based on rumen readiness, not just age

• Use indicators such as consistent starter/forage intake, visible rumination, good growth rates and overall health to decide when to reduce or stop milk feeding.
• Gradual weaning programs that maintain nutrient intake while shifting reliance to the rumen can reduce stress and growth checks.

5. Optimize overall nutrition and health

• Ensure adequate energy, protein, minerals and vitamins to support both systemic growth through the GH/IGF axis and local rumen development.
• Prevent and promptly treat diseases that depress intake or cause systemic inflammation, as these can impair both growth and rumen maturation.

The review supports moving beyond age alone and considering solid feed intake, rumination, growth and general health as indicators that the young animal is physiologically prepared for the transition away from milk.

Research gaps and future directions

Yu and Wu identify several areas where more work is needed:

• Detailed mapping of signaling pathways: More direct evidence is needed on how GH/IGF, RAP1A, GPR41/43 and PPARs interact in the rumen epithelium of young ruminants under different diets and management conditions.
• Microbiome–host crosstalk: Integrating multi-omics data to link specific microbial communities and metabolites with host gene expression and rumen structural changes.
• Nutritional interventions: Testing targeted feeding strategies, including specific VFA precursors and additives that modulate GPR41/PPAR signaling, to accelerate healthy rumen development without compromising welfare.
• Long-term outcomes: More studies tracking animals from early life through production are needed to quantify how early rumen development influences lifetime productivity, health and efficiency.

Bottom line

The review by Yu and Wu provides a concise but comprehensive picture of rumen development in young ruminants, linking:

• Nutritional and management drivers — early solid feed, balanced diets and good health
• Systemic hormonal regulation — the GH/IGF axis
• Local cellular signaling — RAP1A/GPR41–PPAR pathways responsive to VFAs

Source

Yu G, Wu D. Research advances in rumen development and regulatory mechanisms in young ruminants. PeerJ. 2026.

Open-access review summarizing rumen development stages, influencing factors, and regulatory mechanisms via the GH/IGF hormonal axis and RAP1A/GPR41–PPAR-mediated signaling pathways in calves, lambs and kids.