(PhD in Animal Nutrition & Physiology)

(PhD in Animal Nutrition & Physiology)
By: A. Riasi (PhD in Animal Nutrition & Physiology) Advanced Digestive Physiology (part 2) Isfahan University of Technology Isfahan, Iran

The properties of esophagus
It is the least complex section of the digestive tube. Its role in digestion is simple: To convey boluses of food from the pharynx to the stomach. Absorption in the esophagus is virtually nil. Anatomically and functionally, the esophagus is the least complex section of the digestive tube. Its role in digestion is simple: to convey boluses of food from the pharynx to the stomach. Absorption in the esophagus is virtually nil, but the mucosa layer contains a few mucous glands that, as you might expect, secrete mucus to aid in lubrication. The architecture is that of a typical hollow organ with four layers: mucosa, submucosa, muscularis externa, and serosa/adventitia. In the carnivores an internal annular fold is found at the junction of the laryngopharynx and esophagus. 3 3

The mucosa does contain a few mucous glands.
Esophagus The mucosa does contain a few mucous glands. The architecture is that of a typical hollow organ with four layers. The lamina propria contains a relatively dense connective tissue, with the elastic fibers. The surface epithelium is of the stratified squamous type. The cells are keratinized to varying degrees depending on the type of food being ingested. This layer serves as a barrier that separates the lumen of the digestive tract from the rest of the animal. The stratified layers of epithelial cells provide protection from physical abrasion by ingested food. The lamina propria contains a relatively dense connective tissue, which includes an abundant amount of elastic fibers. Immune response cells are scattered in the connective tissue. The muscularis mucosa consists of smooth muscle bundles that, although basically longitudinal, may appear to be spiraling. The submucosa is made up of loose connective tissue and contains arteries, veins, lymphatics, ganglia and nerves. Many seromucous glands are present in the submucosa. Ducts from the glands open to the luminal surface. The secretion of these glands helps the movements of the boluses of ingesta. The distribution of the glands vary in different species. In ruminants, glands are present in the cranial third (cervical region) of the esophagus. Submucosal plexus (Meissner’s) are present but may be quite small. The muscularies externa is made up of two muscle layers, an inner circular and an outer longitudinal layer. Depending on the location in the esophagus and the species involved, the musculature may be skeletal muscle, smooth muscle, or a mix of the two in transition. Myenteric plexus (Auerbach’s) can be found between the muscular layers. Contraction of the muscle cells (peristalsis) help to propel the boluses of ingesta toward the stomach. In ruminants the skeletal muscle fibers are throughout the length of the esophagus. In horse the cranial two thirds is skeletal and caudal one-third is smooth muscle. In pig the cranial one third is skeletal, middle one-third is mixed, and caudal one third is smooth muscle. The inner circular muscle layers become thick and form a cardiac sphincter muscle at the cardiac ostium of the stomach in all domestic animals. In ruminants the skeletal muscle, which runs throughout the esophagus, extend into the wall of the reticularis sulcus (groove). The outermost layer of the esophagus is the tunica serosa in the region where the esophagus is covered by the pleura or peritoneum. This consists of loose connective tissue, blood vessels and a covering of mesothelial cells. In the region of the esophagus, which is not covered by the pleura or peritoneum (usually the cervical part), the outermost layer of the esophagus is the tunica adventitia. This contains blood vessels and loose connective tissue that blends with the surrounding tissue. The body of the esophagus is bounded by physiologic sphincters known as the upper and lower esophageal sphincters. Both the upper and lower sphincters are closed except during swallowing, which prevents constant entry of air from the oral cavity or reflux of stomach contents. In ruminants, a nasopharyngeal sphincter is present and the sphincter help to remove the ruminal gases via the nasal cavities. 4 4

Many seromucous glands are present in the submucosa.
Esophagus Many seromucous glands are present in the submucosa. In ruminants, glands are present in the cranial third of the esophagus. Submucosal plexus (Meissner’s) are present but may be quite small. 5 5

The musculature may be:
Esophagus The musculature may be: Skeletal muscle Smooth muscle A mixture of smooth and skeletal muscles 6 6

Esophagus Contraction of the muscle cells (peristalsis) help to propel the boluses of ingesta toward the stomach. 7 7

The ruminant stomach and its development
The ruminant stomach has four parts. The first three parts form the forestomachs and are histologically quite different from the abomasum that is similar to the glandular stomach. The forestomachs serves as a large fermentation vat. With the aid of microorganisms, otherwise unavailable food material is broken down and absorbed through this modified epithelium. Other functions of the forestomachs include mixing, regurgitation and eructation (belching). 8 8

The wall of stomach is made of 4 layers: T. Mucosa T. Submucosa T. Mascularis T. Serosa Stratified squamous epithelium lines the internal forestomachs. This epithelium has absorptive capabilities and is structurally different from the stratified squamous epithelium in other organs, because the cells have abundant mitochondria which allows active transport of nutrients. Beneath the epithelium is an elaborate network of capillaries. Digestive glands are not present in the forestomachs. The mucosa is usually thrown into tongue-like projections, which increase the surface area of the organ. The lamina propria contains connective tissue and capillary network. Muscularis mucosa may or may not be present. The muscularis externa contains two layers of muscles typical of a hollow organ. In contrast to the ruminant esophagus, which has striated muscle throughout its entire length, the stomach has only smooth muscle, distributed in layers oriented in different direction. At birth, the forestomachs is underdeveloped and, not until the ingestion of solid food or roughage, does the forestomachs become stimulated to develop. From birth to about 2 weeks of age, the calf is a monogastric, or simple-stomached animal. The abomasums is the only stomach compartment actively involved in digestion, and milk or milk replacers provides nutrients. As the calf begins to eat dry feeds, particularly grains containing readily fermentable carbohydrates, the rumen takes on a more important role. The stomach compartments grow and change as the calf develops into a ruminant animal. The fascinating differences between calves and mature ruminants create unique nutritional needs for preweaned calves. 9 9

1 2 3 4 5 Solid lines: internal oblique fiber (ruminal pillars, lips of reticular groove, omasal pillar); broken lines: longituidal fibers; wave lines: circular fibers. At any given place, there are only two muscle layers in the stomach wall. 1= cardia; 2= reticulum; 3= rumen 4= omasum; 5= abomasum. 11 11

Preruminant stomach and food digestion
The ruminant stomach and its development Preruminant stomach and food digestion The calf is a monogastric (birth - about 2 weeks). The abomasums is actively involved in digestion. Readily fermentable carbohydrates are important for the rumen development. 12 12

Size of ruminant stomach compartment
The abomasum constitutes 60 percent of the young calf’s stomach capacity. In contrast, it makes up only 8 percent of the stomach capacity in a mature cow. At birth, the reticulum and rumen make up 30 percent of the stomach capacity, and the omasum makes up approximately 10 percent. By 4 weeks of age, the reticulum and rumen comprise roughly 58 percent of the stomach, the omasum remains the same at 12 percent, and the abomasum falls to about 30 percent. By 12 weeks of age, the reticulum and rumen will make up more than two-thirds of the total stomach capacity. The omasum still makes up about the same proportion at 10 percent. In contrast, the abomasums comprises only 20 percent. The abomasum continues to function as it did at birth, and it actually grows in size. However, the reticulum and rumen grow in size and in function; they become the most important parts of the stomach system. As the stomach develops more fully, the calf begins functioning as a mature ruminant. Actually at birth the young ruminant becomes abruptly dependent on the extra-uterine environment with respect to food supply. During the first months of life, the calf has to deal with one of situations, requiring physiological and digestive adaptations to extra-uterine life, to maintain the preruminant stage over a long period and up to weaning. 13 13

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Pre-ruminant period Coming from esophagus Leading to omasum

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Transition from pre-ruminant to ruminant
A calf can not be successfully weaned unless it has sufficient rumen development to enable it to function as a ruminant rather than a monogastric. Functioning as a ruminant means that a calf is able to efficiently use grains and forages to derive sufficient protein and energy for the optimal rate of growth. Calves can appear to be thriving in the first few weeks of life but if rumen development is not being fostered, the calf will be severely set back at weaning and may develop stress related diseases such as pneumonia or ringworm. Calves without adequate rumen function are likely to have restricted growth for a month or more after weaning while the rumen develops. The cross-section of the rumen wall consists of three main layers: A mucosal layer, which is the layer on the inside A muscle layer, which helps to churn the contents of the rumen and is necessary to keep food particles in suspension, to expel gasses produced during digestion and to move digested material into the abomasums A tough, fibrous exterior layer to protect the whole organ. Changing the rumen from a small inactive organ to one which functions efficiently enough to supply the calf with the nutrients for maintenance and growth involves many changes: 1- The rumen must enlarge 2- The walls must thicken 3- The papillae, which are the tiny finger-like projections which become part of the mucosal layer, must form, develop and elongate. Proliferation and growth of squamous epithelial cells causes increases in papillae length, papillae width, and thickness of the interior rumen wall (Church, 1988). Prior to transitioning from a pre-ruminant to a ruminant, growth and development of the ruminal absorptive surface area (papillae), is necessary to enable absorption and utilization of microbial digestion end products, specifically rumen volatile fatty acids (Warner et al., 1956).

Absorptive surface area is enhanced by increasing:
Transition from pre-ruminant to ruminant Absorptive surface area is enhanced by increasing: Papillae length Papillae width Papillae density

Two important factors for stimulating papillae growth:
Transition from pre-ruminant to ruminant Two important factors for stimulating papillae growth: Presence and absorption of volatile fatty acids (VFAs) in rumen Stimulatory effect of different VFAs is not equal Rumen epithelial ketogenesis (BHBA production) Presence and absorption of volatile fatty acids is indicated to stimulate rumen epithelial metabolism and may be key in initiating rumen epithelial development (Baldwin and McLeod, 2000). However, it has been suggested that rumen epithelial ketogenesis, indicating metabolic activity, may occur independently of volatile fatty acid production. Nevertheless, numerous researchers have indicated that ingestion of dry feeds and the resultant microbial end products sufficiently stimulate rumen epithelial development. However, the stimulatory effects of different volatile fatty acids are not equal, with butyrate being most stimulatory, followed by propionate. This volatile fatty acid is an end product of the digestion of grain by microorganisms in the rumen. It is not produced by the digestion of forages such as straw or hay, which are commonly fed to young calves in the mistaken belief that they stimulate rumen development. Butyrate metabolism by the epithelium appears to increase concomitantly with decreasing rumen pH and increasing butyrate concentrations (Baldwin and McLeod, 2000). There is some reports that butyrate had no effect on papillae growth. Zitnan et al. (2005) reported that the papillae growth in female calves was not correlated with the molar proportion of butyrate, but the molar propionate proportion in the rumen and the plasma IGF-1 had significant effect.

A: caudal portion of the caudal ventral blind sac; RB: right side and LB: left side caudal dorsal sac; RC: right side and LC: left side cranial dorsal sac; RD: right side and LD: left side cranial ventral sac; and RE: right side and LE: left side ventral portion of caudal ventral blind sac (Lesmeister et al. (2004)

Undeveloped Rumen Developed Rumen

Importance of diet to rumen development (6 weeks of age)
Transition from pre-ruminant to ruminant Milk only Milk and grain Milk and hay It is reported that a continuous presence of volatile fatty acids maintains rumen papillae growth, size, and function. Therefore, it is likely that diets composed of milk, concentrates, or forages affect the rate and extent of rumen epithelial growth differently. Although papillae length and width are the most obvious factors influencing absorptive surface area, but changes in papillae density also should be considered. Dietary and age differences have been found to alter papillae density of the developing rumen; however, significant differences due to dietary treatment are seldom reported for papillae density in calves (Lesmeister et al., 2004). Papillae density is commonly reported as the number of papillae in a fixed area (usually 1 cm2), regardless of rumen volume, and rumen volume has been shown to increase with age. Lesmeister et al. (2004) demonstrated a procedure for sampling rumen tissue that was capable of detecting treatment differences for papillae length and width and moderately capable of detecting treatment differences for rumen wall thickness. Minimal treatment influence on papillae density may be explained by a confounding effect of rumen volume. In addition, McGavin and Morrill (1976) and Lesmeister et al. (2004) reported intra-rumen variation for papillae measurements, demonstrating that papillae growth is not universal in all rumen areas. After development, the inside of the rumen turns from a smooth pale surface to a convoluted, dark coloured surface with greatly increased surface area (papillae) and rich blood supply. This increases the ability of the rumen to absorb nutrients. There are five "ingredients" that are required to cause ruminal development: 1-establishment of bacteria in the rumen, 2- liquid in the rumen, 3.outflow of material from the rumen (muscular action), 4- absorptive ability of the tissue, and 5-substrate available in the rumen. A number of other metabolic changes occur during ruminal development in the rumen and other tissues, but we will consider the above "ingredients" as requisite for the rumen to begin to function. dies. Importance of diet to rumen development (6 weeks of age)

Milk and hay Milk, hay and grain

Five factors affect the rumen development:
Transition from pre-ruminant to ruminant Five factors affect the rumen development: Establishment of bacteria in the rumen Liquid in the rumen Outflow of material from the rumen Absorptive ability of the tissue Substrate available in the rumen. 25

At birth day the rumen is sterile
Establishment of bacteria in the rumen At birth day the rumen is sterile Aerobic bacteria Change of bacteria population Establishment of bacteria in the rumen: When the calf is first born, the rumen is sterile and there are no bacteria present. However, by one day of age, a large concentration of bacteria can be found mostly aerobic (or oxygen-using) bacteria. Thereafter, the numbers and types of bacteria change as dry feed intake occurs and the substrate available for fermentation changes. Pounden and Hibbs (1948) observed that grain and roughage diets stimulated different microflora in calves. Bryant and Small (1960) reported that the bacterial flora of isolated calves was considerably different from that of nonisolated calves. Prolonged milk feeding was found to delay the onset of typical ruminal microflora (Lengemann and Allen, 1959) and establishment of protozoa (Singh, 1972). Other researchers found that the lactobacillus population in the rumen was directly affected by diet (Eadie et al., 1959; Ziolecki and Briggs, 1961). In addition to feeds, the environment, bedding, and hair provide microorganisms that inoculate the calf’s rumen. The types of rumen microbes that proliferate are those that best digest and utilize the feeds eaten by the calf. The numbers of total bacteria (per ml of rumen fluid) do not change dramatically, but the types of bacteria change as the calf begins to consume dry feed. This results in a dramatic loss of aerobes and predominance of anaerobes with increasing dry feed intake. Many methanogens, proteolytic and cellulolytic become established. The number of "typical" rumen bacteria, those found in adults, become established by about two weeks after dry feed intake commences. In an assay the protozoal population was zero in weaned calves, probably because the calves were completely isolated from mature ruminants (Bryant et al., 1958; Ziolecki and Briggs, 1961). In addition, the low ruminal pH of the calves, especially in the early-weaned group, would have been unfavorable for establishment of protozoa (Eadie, 1962).

Prolonged milk feeding may retard:
Establishment of bacteria in the rumen Prolonged milk feeding may retard: Typical ruminal microflora Establishment of protozoa

Factors may affect calf’s rumen microflora
Establishment of bacteria in the rumen Factors may affect calf’s rumen microflora Feeds Environment Bedding Hair

The numbers of total bacteria
Establishment of bacteria in the rumen The numbers of total bacteria Change in types of bacteria by feeding DM: Decreasing aerobic bacteria Increasing anaerobic bacteria

Establishing a rumen microflora

Establishment of bacteria in the rumen

Milk does not help rumen development at all
Liquids in the rumen Milk does not help rumen development at all Water is essential for rumen development Without sufficient water, bacteria cannot grow, and ruminal development is slowed. Liquids in the rumen: Milk or milk replacer is initially the primary diet of neonatal dairy calves; however, its chemical composition and the shunting effect of the esophageal groove limit its ability to stimulate rumen development (Warner et al., 1956). Numerous researchers have reported minimal rumen development in calves receiving solely milk or milk replacer even up to 12 weeks of age (Tamate et al., 1962), and others have reported a regression, or stasis, of rumen development when calves were switched from a dry to milk/milk replacer diet (Harrison et al., 1960). In addition, calves receiving only milk/milk replacer exhibit minimal rumen epithelial metabolic activity and volatile fatty acid absorption, which once again does not increase with age. However, ruminal size of the milk-fed calf, regardless of rumen development, has been shown to increase proportionately with body size (Vazquez-Anon et al., 1993). Therefore, while a milk/milk replacer diet can result in rapid and efficient growth, it does little to prepare the pre-ruminant calf for weaning or utilization of grain and forage based diets. Water is essential for development of the rumen. Calves should have fresh clean water available at all times; this means cleaned out regularly and frequently. The rumen cannot function without a supply of fresh water. To ferment substrate (grain and hay), rumen bacteria must live in a water environment. Without sufficient water, bacteria cannot grow, and ruminal development is slowed. Most of the water that enters the rumen comes from free water intake. If water is offered to calves from an early age, this is not usually a problem. It is reported that feeding water can increase body weight gain and starter intake. Water consumed in other feeds, including milk or milk replacer, is not readily available to rumen microbes because it enters the abomasum. Water may enters abomasums if consumed immediately after milk or water added to a bucket that still contains some milk or milk replacer may stimulate reformation of the esophageal groove. For this reason a minimum of 10 minutes has been suggested as the necessary waiting period before offering water after milk feeding. A 90 kg heifer will drink about 12-15< on a 25°C day and will increase significantly in very hot weather.

Measures of ruminal activity:
Outflow of material from the rumen Measures of ruminal activity: Rumen contractions Rumen pressure Regurgitation (cud chewing) Little muscular activity at birth. Outflow of material from the rumen: Proper ruminal development requires that material entering the rumen must be able to leave it. Measures of ruminal activity include rumen contractions, rumen pressure, and regurgitation (cud chewing). At birth, the rumen has little muscular activity, and few rumen contractions can be measured. Similarly, no regurgitation occurs in the first week or so of life. With increasing intake of dry feed, rumen contractions begin. When calves are fed milk, hay, and grain from shortly after birth, normal rumen contractions can be measured as early as 3 weeks of age. However, when calves are fed only milk, normal rumen contractions may not be measurable for extended periods. Cud chewing has been observed as early as 7 days of age, and may not be related to ruminal development per se. However, calves will ruminate for increasing periods when dry feed (particularly hay) is fed. Solid feeds, unlike liquid feeds, are preferentially directed to the reticulo-rumen for digestion (Church, 1988). The esophageal groove does not function when the calf eats dry feeds; they enter the rumen, where they must be digested by microbes or chewed further by rumination. Solid feed intake stimulates rumen microbial proliferation and production of microbial end products, volatile fatty acids, which have been shown to initiate rumen epithelial development. However, solid feeds differ in their efficacy to stimulate rumen development. Chemical composition of feeds and the resultant microbial digestion end products have the greatest influence on epithelial development (Nocek et al., 1984). Concentrates and diets containing casein, starch, cellulose, and minerals have increased the rate of rumen development when compared to forage sources. When introduced into the rumen as purified sodium salts, sodium butyrate had the greatest influence on rumen epithelial development, followed by sodium propionate; sodium acetate and glucose had minimal effects. Furthermore, the chemical composition of concentrates causes a shift in the microbial population, subsequently increasing butyrate and propionate production at the expense of acetate. Increased microbial production of stronger acids, i.e. lactate, butyrate, and propionate, also decreases rumen pH. Forages, on the other hand, have an increased ability to maintain a higher ruminal pH, due to a larger particle size and increased fiber content (Zitnan et al., 1998). Maintenance of a higher ruminal pH supports microbial populations typically associated with forages, which in turn shifts volatile fatty acid production from butyrate and propionate to acetate. Whether the actual stimulant for epithelial development is increased butyrate and propionate production, a decreased ruminal pH concomitant with stronger ruminal acid production, or a combination; concentrates appear to result in greater rumen epithelial development than forages. The works are powerful illustrations that forage – hay or straw, is not helpful in the early stage of rumen development. In the pre-weaning phase grain alone is necessary to initiate the development process. After weaning, when the calf is able to digest enough grain to survive and grow on, forage is necessary to develop the musculature of the rumen and to assist its functioning. Calves have a high energy requirement relative to their ability to consume dry feed. If calves consume significant amounts of hay, their intake of grain or pellets will be limited and growth and rumen development will be slowed down. Recent studies have looked at dietary alterations or additions and their effect on rumen development and its subsequent effects on rumen microbial end products. While addition of yeast culture increased calf grain intake, it did not appear to significantly affect rumen development in young calves when added at 2% of the diet (Lesmeister et al., 2004). Papillae length and rumen wall thickness were significantly greater in 4 week old calves fed calf starters containing steam-flaked corn over those fed dry-rolled and whole corn when these corn supplements made up 33% of the calf starter (Lesmeister and Heinrichs, 2004). This study showed that the type of grain processing can influence rumen development in young calves.

Solid feed intake stimulates:
Outflow of material from the rumen Solid feed intake stimulates: Rumen microbial proliferation Production of microbial end products

Effect of chemical composition of concentrates:
Outflow of material from the rumen Effect of chemical composition of concentrates: A shift in the microbial population Increasing butyrate and propionate production at the expense of acetate.

Outflow of material from the rumen
Forages, have an increased ability to maintain a higher ruminal pH, due to: A larger particle size An increased fiber content

Outflow of material from the rumen
A research was performed to determine the effects of different levels of alfalfa meal feeding on ruminal fermentation and development of Holestin suckling milk calves. For this purpose thirty calves were randomly assigned to different treatments consisting of control (without forage), and experimental groups (contain 8 and 16 % alfalfa meal in starter). Calves were slaughtered at 72 day to measure rumen development parameters. Results showed that starter intake, average daily gain and feed efficiency during the pre and post-weaning periods were not different among treatments. Ruminal pH significantly increased at 50 and 72 days with increasing alfalfa level. Acetate to propionate ratio and acetate concentration of rumen fluid increased at d 50 and 72. Propionate and total volatile fatty acids concentrations linearly decreased at 50 and 72 days, respectively. Omasum weight with and without digest matter were greater with increasing forage level. Reticulo-rumen weight without digest matter increased with forage feeding. Alfalfa addition decreased corneum and epithelial layer thickness, but decreased papillae length. In overall, results of this study indicated that alfalfa meal feeding had no negative effect on performance of suckling milk calves, also improved ruminal pH, digestive organ weights and rumen development in suckling milk calves (Mirzaee et al. 2014) برگرفته از میرزایی و همکاران، 1393

The rumen wall consists of two layers:
Absorptive ability of the rumen tissue The rumen wall consists of two layers: The epithelial The muscular Absorptive ability of the rumen tissue: The absorption of end-products of fermentation is an important criterion of ruminal development. The rumen wall consists of two layers, the epithelial and muscular layers. The muscle layer lies on the exterior of the rumen and provides support for the interior (epithelial layer). The epithelial layer is the absorptive layer of tissue that is inside the rumen and is in contact with the rumen contents. It is composed of a very thin film of tissue holding many small finger-like projections called papillae. These papillae provide the absorptive surface for the rumen. As mentioned before, at birth, the papillae are small and non-functional. They absorb very little and do not metabolize significant VFA. Gradually, research has identified butyrate and propionate as the volatile fatty acids most readily absorbed by rumen epithelium, especially when present at physiological concentrations (Baldwin and McLeod, 2000). The VFA or end products of metabolism (lactate and ß-hydroxybutyrate) are transported to the blood for use as energy substrates. However, there is little or no absorption or metabolism of VFA in neonatal calves. Therefore, the rumen must develop this ability prior to weaning. Increased absorption and utilization of butyrate and propionate over acetate provides further evidence that the former volatile fatty acids stimulate epithelial development (Baldwin and McLeod, 2000). Many researchers have evaluated the effect of various compounds on the development of the epithelial tissue in relation to size and number of papillae. Milk, hay, and grain added to the rumen are all fermented by the resident bacteria to these organisms; therefore, they contribute VFA for epithelial development. Plastic sponges and inert particles, both added to the rumen to provide "scratch", did not promote development of the epithelium. These objects could not be fermented to VFA, and thus did not contribute any VFA to the rumen environment. Therefore, rumen development (defined as the development of the epithelium) is primarily controlled by chemical, not physical means. This is further support for the hypothesis that ruminal development is primarily driven by the availability of dry feed, but particularly starter, in the rumen.

The end-products of fermentation.
Absorptive ability of the rumen tissue The end-products of fermentation. Butyrate and propionate most readily absorbed by rumen epithelium.

The primary factor determining ruminal development is dry feed intake.
Availability of substrate The primary factor determining ruminal development is dry feed intake. Starter Proper stimulation for rumen development Availability of substrate: We have seen that bacteria, liquid, rumen motility, and absorptive ability are established prior to rumen development, or develop rapidly when the calf begins to consume dry feed. Thus, the primary factor determining ruminal development is dry feed intake. To promote early rumen development and allow early weaning, the key factor is early consumption of a diet to promote growth of the ruminal epithelium and ruminal motility. Because grains provide fermentable carbohydrates that are fermented to propionate and butyrate, they are a good choice to ensure early rumen development. On the other hand, the structural carbohydrate of forages tend to be fermented to a greater extent to acetate, which is less stimulatory to ruminal development. Early and aggressive intake of calf starter is the key to good ruminal development. Offer starter from 3 days of age and keep it fresh, clean, and available. This will help provide the proper stimulation for rumen development and allow early weaning.

Parakeratosis have some adverse effects:
Rumen parakeratosis Parakeratosis have some adverse effects: Creating a physical barrier. Restricting absorptive surface area and volatile fatty acid absorption. Reducing epithelial blood flow and rumen motility Causing papillae degeneration and sloughing in extreme cases. Rumen epithelial development cannot be thoroughly discussed without covering the influence of parakeratosis on papillae development and absorptive ability. Parakeratosis occurs when epithelial squamous cells develop a hardened keratin layer due to a diet’s inability to continuously remove degenerating epithelial cells (Hinders and Owen, 1965). Parakeratosis creates a physical barrier, restricting absorptive surface area and volatile fatty acid absorption, reducing epithelial blood flow and rumen motility, and causing papillae degeneration and sloughing in extreme cases (Beharka et al., 1998). Initial evidence of parakeratosis is papillae clumping and branching, followed by papillae degeneration and sloughing (Anderson et al., 1982; Zitnan et al., 1998). Concentrate diets with small particle size and low abrasive value (Greenwood et al., 1997) increased volatile fatty acid production, decreased rumen buffering capacity, and subsequently decreased rumen pH (Anderson et al., 1982) are factors commonly associated with occurrences of parakeratosis. Abrasive value has been defined as a feed’s efficacy in physically removing keratin and/or dead epithelial cells from the rumen epithelium (Greenwood et al., 1997). Therefore, increased feed particle size, especially with forages or coarsely-ground concentrates, maintains epithelial and papillae integrity and absorptive ability via physical removal of the keratin layer, increased rumination and rumen motility, increased salivary flow and buffering capacity, and development of mature rumen function and environment. However, factors such as individual animal susceptibility, intake differences, passage rate, rumination rate, and salivary production may also contribute to occurrences of parakeratosis (Zitnan et al., 1998).

Initial evidence of parakeratosis is papillae clumping and branching.
Rumen parakeratosis Initial evidence of parakeratosis is papillae clumping and branching. Followed by papillae degeneration and sloughing.

Concentrate diets: Rumen parakeratosis
Increased volatile fatty acid production Decreased rumen buffering capacity Subsequently decreased rumen pH

Increased feed particle size:
Rumen parakeratosis Increased feed particle size: Maintains epithelial and papillae integrity and absorptive ability. Increased rumination and rumen motility Increased salivary flow and buffering capacity Development of mature rumen function and environment.

Feed physical structure:
Changes in rumen muscularization Feed physical structure: Development of rumen muscularization Development of rumen volume Stimulation of rumen motility Changes in rumen muscularization and volume Feed physical structure likely has the greatest influence on development of rumen muscularization and volume. Stimulation of rumen motility is governed by the same factors, particle size and effective fiber, in the neonatal ruminant as in the adult ruminant (Beauchemin and Rode, 1997). In contrast to concentrate’s advantages for epithelial development, forages appear to be the primary stimulators of rumen muscularization development and increased rumen volume (Zitnan et al., 1998). Large particle size, high effective fiber content, and increased bulk of forages or high fiber sources physically increase rumen wall stimulation, subsequently increasing rumen motility, muscularization, and volume (Vazquez-Anon et al., 1993; Warner et al., 1956; Zitnan et al., 1998). As discussed earlier, increases in rumen muscularization and volume have occurred independently of epithelial development. Supporting evidence for independent muscle and epithelial growth is found in studies determining the effects of inert material (sponges, toothbrush bristles, or bedding) on rumen epithelial, muscular, and capacity development. Inert materials were found ineffective for stimulating papillae growth, but capable of significantly increasing rumen capacity and muscularization (Harrison et al., 1960). However, solid feeds other than forages or bulky feedstuffs can be effective in influencing rumen capacity and muscularization. Coarsely or moderately ground concentrate diets have been shown to increase rumen capacity and muscularization more than finely ground or pelleted concentrate diets, indicating that extent of processing and/or concentrate particle size affects the ability of concentrates to stimulate rumen capacity and muscularization (Beharka et al., 1998; Greenwood et al., 1997). Therefore, concentrate diets with increased particle size may be the most desirable feedstuff for overall rumen development, due to their ability to stimulate epithelial development, rumen capacity, and rumen muscularization. While the basics of rumen development have been published in the literature, current rumen development research focuses on dietary manipulation, attempting to optimize the rate and extent of rumen development. Increased availability of feed by-products, development of new feed additives, and differences in calf starter particle size all provide areas for future rumen development research. Understanding the cellular biology and physiological changes that occur during rumen development, clarifying neonatal calf digestion kinetics, and development of low-impact or non-invasive research procedures could be instrumental in advancing this area further. While much is known related to rumen development, several areas require additional study. The adoption of newer technologies to stimulate the rate of rumen development may have important economic consequences for dairy and beef producers and warrant further applied research studies.

Changes in rumen muscularization
Understanding the cellular biology and physiological changes of rumen development: Neonatal calf digestion kinetics Development of low-impact or non-invasive research procedures could be instrumental in advancing this area further.

Two important aspects for development of rumen:
Physiology and ontogeny of rumen development Two important aspects for development of rumen: Ruminal growth and cellular differentiation A major shift in the pattern of nutrients being delivered to the intestine and liver Thus nutrient delivered to peripheral tissues Physiology and ontogeny of rumen development in neonatal young ruminants Among of the most dramatic physiological challenges to young ruminants are the events surrounding the development of the rumen. This not only entails growth and cellular differentiation by the rumen, but also results in a major shift in the pattern of nutrients being delivered to the intestine and liver, and thus the peripheral tissues of the animal.

Control of ruminal epithelial cell proliferation
In vivo and in vitro studies using mitotic indices for ruminal epithelial cell proliferation. Butyrate may induce a mitotic proliferation Propionate and acetate have been shown to stimulate mitotic indices Control of proliferation. Ruminal epithelial cell proliferation has been studied both in vivo and in vitro by measuring mitotic indices. In an assay butyric acid infused directly into the rumen of sheep resulted in a stimulation of mitotic indices (number of basal cell nuclei showing mitotic figures/total basal cell nuclei counted). However, only 2 animals were observed in this study, and data are confounded by the fact that both animals were fasted throughout the 5 and 7 d, and the effect of butyrate did not appear until 3 or 4 d subsequent to the initiation of the fast. Thus, from this experiment it is difficult to determine whether butyrate is stimulatory or permissive in promoting mitosis. In a series of similar trials, a single pulse dose of sodium butyrate followed by continuous saline infusion stimulated mitotic indices to a greater extent than a pulse-chase regime with sodium butyrate. Thus, a rapid but unsustained increase in butyrate in the rumen, which is not observed physiologically, stimulates cell proliferation, as indicated by greater mitotic indices. To a lesser extent than with butyrate, both propionate and acetate have been shown to stimulate mitotic indices when administered as a single dose (Sakata and Tamate, 1979).

Contradiction in response to VFAs by in vivo and in vitro.
Control of ruminal epithelial cell proliferation Contradiction in response to VFAs by in vivo and in vitro. The differences may be attributed to indirect pathways during in vivo condition.

Some hormones and growth factors may have mediator effect:
Control of ruminal epithelial cell proliferation Some hormones and growth factors may have mediator effect: Insulin, Pentagastrin, Glucagon IGF-1, Epidermal growth factor Cortisol Contrary to results in vivo, 3H-thymidine incorporation into cellular DNA by isolated ruminal epithelial cells was inhibited over a 24-h period in a dose-dependent manner by the presence of sodium butyrate (2 and 10mM) in the medium. Further studies with butyrate in cultures of tissue explants indicate that butyrate tends to arrest cell division of cells in the basal lamina, while increasing the keratinization and protein expression of the other cell types. Moreover, Baldwin (1999) used isolated rumen epithelial cells in 48-h cultures to demonstrate decreased cell proliferation, as determined by MTT assay, in the presence of butyrate or propionate alone, with half-maximal inhibition occurring at concentrations of 0.52 mM butyrate and 0.93 mM propionate. The different responses in vivo and in vitro, and the seemingly contradictory nature of the in vivo reports, is suggestive of an indirect pathway of cell stimulation. Ruminal epithelial mitotic indices have been shown to be stimulated by intravenous insulin infusions (Sakata et al., 1980). Because propionate has been shown to be a stimulator of insulin release in vivo (Sakata et al., 1980), it is possible that insulin could be a mediator in the stimulation of mitosis in the ruminal epithelium. In fact, administration of pentagastrin, insulin, and glucagon to isolated ruminal cells all resulted in stimulation of cell proliferation (as detected by 3H-thymidine incorporation), while responses to cortisol were inconsistent (Sakata and Tamate, 1978, 1979). Of the additions tested, only insulin was able to overcome the inhibitory effect of butyrate (Ga´ lfi and Neogrady, 1989). Similar results were observed by Baldwin (1999), where insulin, epidermal growth factor, and IGF-I stimulated cell proliferation.

Neonatal ruminal epithelial metabolism
In neonatal ruminant primary source of energetic substrates are blood borne, derived from intestinally absorbed nutrients. Difference between neonate and mature ruminant for uptake of oxidizable substrates by ruminal cells Neonatal ruminal epithelial metabolism. In the milk-fed ruminant, due to the reflexive closure of the reticular groove and the lack of SCFA in the ruminal lumen, the primary source of energetic substrates are blood borne, derived from intestinally absorbed nutrients. Fatty acids and glucose absorbed in the small intestine must first pass through the liver; thus it has been assumed that glucose is a primary energy substrate of the immature tissue, as is the case with other neonatal tissues (White and Leng, 1980). Early experiments in vitro evaluated rates of oxygen uptake by rumen slices from 14-d-old calf ruminal epithelium (undeveloped) or mature ruminal papillae in the presence of various oxidizable substrates such as glucose, butyrate, and lactate (Giesecke et al., 1979). Oxygen uptake by neonatal rumen was greatest when glucose was present as the oxidizable substrate; however, oxygen consumption by mature rumen papillae increased above basal oxygen uptake when glucose was added, but did not respond as dramatically as neonatal rumen slices (Giesecke et al., 1979). Lactate addition to the media induced a similar pattern of oxygen uptake as that observed with glucose (Giesecke et al., 1979). In contrast, the addition of butyrate stimulated oxygen uptake to a greater degree in mature rumen papillae than in the neonatal rumen. Thus, although the ruminal epithelium can use glucose and butyrate as its energetic substrate, neither appears to be the favored substrate. In the same experiments, ketogenic capacity of the isolated cells was apparent by 4 d of age and did not increase until weaning when an 8-fold increase in BHBA production was observed (Baldwin and Jesse, 1992). Lane et al. (2000) assessed rumen metabolic development of ruminal epithelium in the absence of solid feed intake by maintaining lambs solely on milk replacer and observed the characteristic and marked increase in ketogenic capacity at 42 d regardless of dietary regimen.

Neonatal ruminal epithelial metabolism
Ontogenic control of some of the critical development changes of rumen: Increase in gene transcripts for 3-hydroxy-3-methylglutaryyl-CoA synthase. Using rumen epithelial RNA isolated from these same experimental animals, Lane et al. (2002) were able to demonstrate an increase in gene transcripts for 3-hydroxy-3-methylglutaryyl-CoA synthase (E. C ), despite the lack of significant SCFA production in the rumen. Thus, ontogenic control of some of the critical development changes occurring in the developing ruminant can not be eliminated as a causative factor, despite the large volume of evidence implicating butyrate as the putative trigger for development. Moreover, these processes need not be mutually exclusive events.

Liver metabolism & rumen development
The liver undergoes a maturation process of its own in response to ruminal development The most notable of changes is the shift from a glycolytic to glucogenic liver. In the preruminant, the liver serves as the primary site of ketogenesis and is glycolytic, and gluconeogenesis is highly regulated. Conversely, in the ruminating animal, the ketogenic function of the liver is decreased, and gluconeogensis is more constant and refractory to glycolytic and glucogenic hormonal inputs. Even though the neonatal and preruminant liver represents a greater percentage of empty BW than that of the adult ruminant (Moulton, 1922). In developing animals, the peri-weaning digestive adaptation from a preruminant to functional ruminant coincides with a shift from primarily intestinally absorbed glucose, long-chain fatty acids, and milk-derived amino acids to SCFA, ketones, amino acids from feed and microbial sources, and other dietary compounds. Consequently, this change in dietary nutrient pattern and supply causes substantial alterations in hepatic function and energy requiring processes such as glucose and urea synthesis, protein synthesis, maintenance of ion gradients, substrate cycling, and detoxification of compounds (Seal and Reynolds, 1993). Most research directed at elucidating hepatic mechanisms involved with this period of transition have in effect attempted to determine whether the preruminant liver is always equipped to accommodate the digestive adaptations of the foregut or whether the liver undergoes a maturation process of its own in response to ruminal development. The most notable of changes in principal metabolic processes during ruminal development is the shift froma glycolytic to glucogenic liver. As microbial fermentation increases, less carbohydrate is available for postruminal digestion, and the dietary supply of glucose diminishes. These changes in glucose metabolism were reviewed in detail by Leat (1970).

Liver adaptation in the developing animals:
Liver metabolism & rumen development Liver adaptation in the developing animals: Shift from primarily intestinally absorbed glucose, long-chain fatty acids, and milk-derived amino acids to SCFA, ketones, amino acids from feed and microbial sources, and other dietary compounds.

Liver metabolism & rumen development
A basic reduction in enzyme capacity for hepatic glucose oxidation via glycolytic and hexose monophosphate pathways: Glucose-6- phosphate dehydrogenase 6-phosphogluconate dehydrogenase Fructose 1,6-bisphosphate aldolase Glyceraldehyde 3- phosphate dehydrogenase There is a basic reduction in enzyme capacity for hepatic glucose oxidation via glycolytic and hexose monophosphate pathways (Bartley et al., 1966) caused by decreased activities of hepatic glucose-6- phosphate dehydrogenase (E.C ), 6-phosphogluconate dehydrogenase (E.C ), fructose 1,6-bisphosphate aldolase (E.C ), and glyceraldehyde 3- phosphate dehydrogenase (E.C ; Goetsch, 1966; Leat, 1970). Hepatic tissue must now support a larger portion of the animal’s glucose requirements. Concurrent with decreased importance of glycolytic pathways is the rapid increase in activity of hepatic gluconeogenic enzymes with ruminal development, glucose 6-phosphatase (E.C ) activity having been shown to double during this period (Bartley et al., 1966). Donkin and Armentano (1995) demonstrated that gluconeogenesis from lactate was approximately 10-fold greater in preruminating than ruminating calves. Ortigues et al. (1996) determined the maximum contribution of lactate to hepatic glucose production was 20% in preruminant calves fitted with chronic indwelling catheters in portal, hepatic, and mesenteric vein and artery. Interestingly, Vander Walt et al. (1983) attributed 20% of hepatic uptake of lactate being released as glucose in adult sheep.

A rapid increase in activity of hepatic gluconeogenic enzymes:
Liver metabolism & rumen development A rapid increase in activity of hepatic gluconeogenic enzymes: Glucose 6-phosphatase activity having been shown to double during this period

Bloat can affect either:
Bloat in young ruminant animals Bloat can affect either: Abomasum Rumen Abomasal bloat is often rapidly progressive and life threatening. Bloat in young ruminant animals Bloat can affect either the abomasum or the rumen. Since the nature of abomasal bloat is quite different from that of ruminal bloat, it is important to have some understanding of what’s happening inside the calf’s stomach as the calf grows from a pre-ruminant into a ruminant animal. Abomasal bloat in calves and other pre-ruminant livestock is often rapidly progressive and life threatening. The processes involved in abomasal bloat are not completely understood. A rapid growth or proliferation of organisms results in the production of an excessive quantity of gas that cannot escape the abomasum. This causes severe distention that compresses the abdominal and thoracic organs (heart, lungs) and the blood vessels that lead to them. The result is asphyxiation and heart failure. The abomasum of an affected calf usually becomes grossly distended within 1hour after feeding with death occurring within a few minutes after the distention becomes clinically obvious. At necropsy, the abomasum is grossly distended with gas, fluid and milk or milk replacer. Treatment of abomasal bloat is very difficult. Attempts to release the gas with a stomach tube will not likely be successful since the esophageal groove is not present to guide tube movement. Limited success has been achieved by inserting a needle into the abomasum through the distended right flank of the animal to release some of the gas. Factors contributing to abomasal bloat include overfeeding milk or feeding milk too fast. In the presence of fermenting bacteria, a large quantity of milk or milk replacer arriving at the abomasum can provide an excellent substrate for these bacteria to grow rapidly and ferment sugars. Excessive gas is produced as a result of this rapid fermentation. The pH of the abomasum becomes more acidic as these sugars are processed, resulting in a detrimental effect on other bacteria. The end result is overproduction of gas that cannot escape. The rapid growth of certain pathogens, such as Clostridium, can also lead to abomasal bloat. Clostridium perfringens types A, B, C are commonly found in young calves with types B and C being the most common cause of disease. Clostridia cause enterotoxemia, an acute intestinal infection, and kill through the production of a systemic toxin. Clostridia are normally found in the intestine of cattle and can survive for months in the soil. Overeating or abrupt diet changes tend to produce indigestion that slows gut movement, providing the sugars, proteins and lack of oxygen needed for rapid growth of Clostridia. Wet conditions also seem to favor this organism. Clostridial infections of the intestines are uncommon in young calves. There are many more cases of clostridial infection that involve the abomasum, usually in calves between two and five weeks of age. Affected calves may stop eating, show uneasiness and strain or kick at their abdomen. Calves are often found dead without having shown any previous symptoms. Moderate bloating of the abomasum is often found. There are a variety of other factors that can contribute to abomasal bloat. These include impaction of the abomasum or intestines with non-feed substances such as bedding or hairballs. An animal may even have structural or physiological problems with the abomasum that lead to improper functioning and bloat. Ruminal bloat in young calves takes place within the context of the developing rumen. The population of rumen microbes that exists at any point in time is determined by the types of feed consumed as well as other substances commonly ingested such as bedding and hair. The right set of circumstances can allow one or more of these microbes to produce excessive gas and bloat in the developing rumen. When milk persistently flows into the rumen, calves may develop a number of symptoms including unthriftness, growth retardation, poor appetite, abdominal distention, recurrent bloat, hard feces and a long dry hair coat. Calves that are restricted to a liquid diet, such as veal calves, are more likely to develop these symptoms. In addition to dry feed and water management there are a variety of other factors that can contribute to the occurrence of ruminal bloat in pre-ruminant animals. As a matter of fact, many of these factors are the same ones that contribute to abomasal bloat. Management practices to consider include:

Factors contributing to abomasal bloat:
Bloat in young ruminant animals Factors contributing to abomasal bloat: Overfeeding milk Feeding milk too fast Pathogens, such as Clostridium Colostrum Management: Ensuring a newborn calf receives an adequate supply of high quality colostrum at the right time is the single most important factor in preparing the animal to withstand disease challenges during the first few weeks of life. Feeding Time: Feeding at the same time each day is important. Variable feeding times can cause calves to become very hungry. Hungry calves eat and drink quickly and often over-eat, leading to changes in digestion. Feed volume and feed types should also be consistent with any changes being made gradually. Milk Temperature: Under normal circumstances, milk and milk replacer should be fed at body temperature, about C. Cold temperatures may alter milk intake as well as the rate of feed passage. Feeding Equipment: Feeding equipment, especially equipment used to feed milk should be cleaned and sterilized before each use. Dirty, contaminated equipment is a sure-fired way to introduce and spread unwanted microbes to calves. Ensure feeding equipment is in good condition. Do not cut the ends of nipples so calves drink milk faster or use deteriorated nipples that have developed large holes. Antibiotics: Milk replacers containing an antibiotic such as Oxytetracyline/Neomycin Base have been reported to reduce the incidence of bloat on some farms. A 200/400 gram per ton (Oxy/Neo) inclusion rate can affect the bacterial population in the digestive tract and may produce a positive effect on the incidence of bloat. Medications that target coccidia would not be effective against bacteria. Oxytetracyline/Neomycin Base is not, however, approved for incorporation into milk replacers for lambs and kids. Feed Ingredients: The importance of dry starter feed and water in rumen development has already been discussed. The starter must be palatable. It should not be something the calf grudgingly eats to keep from starving to death. The minimum crude protein level for calf starters is 18%. Research conducted by the University of Wisconsin and the University of Minnesota found that 19-20% crude protein is the optimal protein level for calf starters. Other nutrients need to be properly balanced and particle size should be sufficient to help stimulate rumen function and development. The fiber level of milk replacer is a key measure of milk replacer quality. Ingredients such as vegetable proteins tend to increase the fiber content from 0.15% to 0.5% or higher. Milk replacer cost goes down, but so does calf performance to some degree. The most commonly used vegetable protein is soy flour. Although a large volume of soy flour milk replacer is manufactured and used without incidence, soy flour contains anti-nutritional factors that can cause intestinal inflammation. Such a stress can alter digestive function and cause digestive disorders that can lead to bloat. These anti-nutritional factors are reduced with additional processing of the soy flour into soy protein concentrate and soy protein isolate. Stress: Stress causes physiological and behavioral responses. Management and environmental changes often lead to stress. Vaccination, dehorning, feed and housing changes are routine stressors that most calves are subjected to within a very short period of time. Careful attention needs to be paid to calves as they undergo these changes. Subjecting the calf to several of these stressors at once can have significant health effects. Changes in weather, both short term and seasonal can lead to major nutritional and health challenges. Efforts must be made to minimize the impact these conditions have on calf health. Health Status: The overall health status of the calf can certainly affect its predisposition to bloat. A calf that succumbs to repeated bouts of scours or respiratory problems, perhaps from inadequate colostrum intake, may be far more likely to develop the digestive conditions most conducive to bloat. On the other hand, an aggressive, fast growing calf with only restricted access to water for part of each day, may develop eating and drinking patterns that facilitate the rapid growth of gas producing bacteria. Treatments such as antibiotic therapy may diminish bacterial populations in the digestive tract. The same may be true for calves that receive large amounts of oral electrolyte solutions via an esophageal feeder. The large volume of water entering the rumen may wash away a portion of the rumen microbes. Although the treatment may have been effective, these calves may be more susceptible to digestive challenges until microbial populations are re-established.

Clostridium perfringens types A, B, C
Bloat in young ruminant animals Clostridium perfringens types A, B, C Clostridia are normally found in the intestine of cattle and can survive for months in the soil.

Overeating or abrupt diet changes tend to:
Bloat in young ruminant animals Overeating or abrupt diet changes tend to: Produce indigestion that slows gut movement Providing the sugars, proteins and lack of oxygen needed for rapid growth of Clostridia Wet conditions also seem to favor this organism

The other factors: Bloat in young ruminant animals
Impaction of the abomasum or intestines with non-feed substances such as bedding or hairballs Structural or physiological problems with the abomasum

Management practices to consider include:
Bloat in young ruminant animals Management practices to consider include: Colostrum management Feeding time Milk temperature Feeding equipment Antibiotics Feed ingredients Stress Health status

(PhD in Animal Nutrition & Physiology)

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