From Monogastric to Polygastric
The development of the digestive tract in calves presents a uniquely organized system. In particular, due to the rumen and its colonization by microorganisms, the calf physiologically transitions from a pseudo-monogastric animal to a functioning ruminant.
The progression of the rumen in calves can directly influence feed intake, nutrient digestibility, and overall growth. Even small changes in early feeding regimes and nutrition can drastically affect this process, with long-term effects on growth, health, and milk production in adult cattle.
The development of the rumen in newborn calves is one of the most important and interesting areas of calf nutrition.
In newborn calves, the abomasum is the only fully developed and functional stomach and is also the most important digestive organ for calves at birth. The digestion of fats, carbohydrates, and proteins relies primarily on digestive enzymes secreted by the abomasum and small intestine, which is similar to the digestive system of monogastric animals. Over time, with increasing intake of solid foods, the rumen begins to develop and take on more significant digestive roles.
Why is it important to know that the rumen is not developed during the early stages of the animal’s life?
To avoid feeding hay too early!
This practice leads to the formation, at the ruminal intestinal level, of “hay balls” that occupy space, reducing the intake capacity of concentrates and slowing rumen development.
Guide to Rumen Development
Rumen Microbiota Composition
At birth, the gastrointestinal tract of young ruminants is sterile. During the first hours of life, the forestomach is rapidly colonized by an abundant microbial population.
Newborns acquire bacteria from the dam, from peers, from feed, from housing, and from the environment. Within one day of age, a high concentration of bacteria, primarily aerobic (or oxygen-utilizing), can be found.
Subsequently, the total number of bacteria/ml of ruminal fluid does not change drastically, but as the calf begins to consume dry feed, the type changes. With increasing intake of dry feed, the bacterial population shifts from aerobic to anaerobic and facultative anaerobic, which gradually increases the production of fermentation products, among which, of particular interest for rumen development, are volatile fatty acids (propionic, butyric, and acetic).
Fluid in the Rumen
Bacteria present in the ruminal environment require an adequate supply of water to sustain their vital activities. Bacterial growth and the development of the rumen are intrinsically linked to its presence. Water supply is primarily derived from drinking water.
The benefit of providing access to water for calves is well established. Availability from the early stages of their life in the diet promotes increased body weight, improved starter feed intake, and reduced gastrointestinal issues. It is a misconception that water from milk alone is sufficient for the calf.
Rumen Functionality
The rumen wall is composed of two distinct layers: the epithelium and the muscle. Each of these ruminal components performs specific functions and develops in response to different stimuli.
The epithelium is the absorptive lining that borders the interior of the rumen and interacts with its contents. Characterized by a thin tissue membrane, this layer is adorned with numerous small finger-like projections called papillae, which constitute the rumen’s absorption surface.
The ruminal mucosa performs many important functions and plays a key role in rumen development, including absorption, transport, short-chain fatty acid metabolism, and wall protection.
The proliferation and growth of the ruminal squamous epithelium promote the length and width growth of papillae and increase the thickness of the rumen’s inner wall. Newborn calves have a smooth floor without prominent papillae. It has been shown that calves fed exclusively with milk have limited ruminal development. In contrast, greater intake of solid foods contributes to rapid development of ruminal fermentation with positive effects on rumen weight, papilla growth, keratinization degree, and muscle development.
As calves consume more starter feed, the pH of the ruminal digesta decreases, while the concentration of volatile fatty acids (VFA) gradually increases. The presence and absorption of VFAs in the rumen provide the necessary chemical stimuli for ruminal epithelial proliferation. The development of ruminal papillae is associated with an increase in blood flow through the rumen wall and a direct effect of butyrate and propionate on gene expression.
Acetic fermentation promoted by forages has a poor stimulating effect on epithelial growth. Therefore, the feeds used in weaning, such as milk, concentrates, and forages, each affect the rate and extent of ruminal epithelial growth differently.
The second layer, the muscular layer, covers the outer surface of the rumen and supports the inner component (epithelial layer). Its primary function is muscle contraction, which facilitates the movement of ruminal contents within the rumen itself, regurgitation, and transfer of digested material to the omasum.
The rumen musculature can be stimulated by feeds such as forages. When calves are fed a combination of milk, hay, and grains shortly after birth, normal ruminal contractions can be detected as early as 3 weeks of age. However, if calves are fed exclusively with milk, there may be a prolonged delay in the manifestation of normal ruminal movements.
If feed and water are provided from the early days, even in the case of so-called “intensive” feeding, characterized by a high milk administration to the weaning calf, there is no negative interference with ruminal growth as one might think. On the contrary, this technique, if properly managed in terms of timing and dosage, acts as an additional stimulus to development. Epigenetics likely plays a fundamental role.
Starter Feed and Rumen Development
Which form promotes optimal rumen development?
The availability and intake of starter feed is crucial for calves before weaning. Among protein sources, soybean has shown the best results. Among cereals, corn in its various forms (grain, flake meal, extruded) is the most commonly used in calf starters. Other ingredients, such as cane or beet molasses, also used as a carbohydrate source, increase palatability, reduce particle separation, and decrease dustiness.
However, high concentrations of molasses have been shown to reduce DMI, may cause palatability issues, reduce growth, and increase the incidence of diarrhea.
To improve energy intake, the starter can also be supplemented with lipid sources, but in minimal quantities to avoid depressing intake. Additionally, a high-quality starter feed should contain a source of easily digestible fiber to prevent parakeratosis, or an accumulation of dead cells on the papillae that block nutrient absorption.
Recent studies have found that supplementing starter feed with short-ground forage improves intake, growth, and digestibility of the diet. Forages with structured fiber stimulate saliva production during chewing and rumination, which provide urea and minerals, such as sodium bicarbonate, that help maintain normal rumen growth and microbial development. Coarse forages are important for promoting the growth of the rumen muscle layer and maintaining epithelial health.
Physical Form of Starter Feed
A calf’s interest in feed is regulated by several factors: odor, taste, and physical form.
Regarding physical form, pellets have been shown to perform better than mash. The debate remains open when comparing pellets with so-called “multiparticle” feeds, which present the ingredients mostly “visible” and with varying particle sizes. The latter has shown particularly interesting results both in terms of intake and growth, but especially in ruminal stimulation and thus in papillae development, VFA composition, and pH.
The practice of introducing forage during weaning has long been discouraged due to its low energy value and its property of shifting fermentation towards acetate, thus limiting papillae development. However, it is also true that feeding with low-fiber and high-fermentable-cereal feeds can lower pH and favor reduced rumen motility, which can lead to hyperkeratosis and parakeratosis of the ruminal epithelium.
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Multiparticle Starter
