Introduction
Cows are ruminants, as are goats, buffaloes, giraffes, camels and antelopes. Ruminants have the ability to digest large amounts of roughage containing high amounts of (crude) fibre and cell wall materials (cellulose, lignin). Their alimentary tract is specially adapted, and they have the following main characteristics:
Absence of front teeth (incisors) in upper jaw, which facilitates rumination and/or mastication of fibrous material.
A complex stomach specially “designed” to break-down large amounts of roughage (rumen reticulum as a microbial “fermentation barrel”).Digestion means the breaking-down of different food components into simpler compounds. Hence, they can pass through the mucous membrane (wall) of the gastro-intestinal tract into blood and lymph (absorption) and be transported to those places in the body where needed. In cattle, the process of digestion can be divided into 3 groups
especially Mechanical digestion, to reduce the size of food-particles by chewing, mastication 1(rumination) and muscular contractions of the gastro intestinal tract, the rumen reticulum and omasum.Microbial digestion, brought about by rumen micro organisms (RMO’s), consisting of
: degradation + synthesis in rumen/reticulumChemical digestion through enzymes, secreted by the animal in the various digestive juices in the abomasum and intestines.
2.1 Process of Digestion in Cattle
2.1.1 The Mouth
The mouth is used for:
Eating/cutting, chewing and mixing food with saliva and formation of boluses/cuds and swallowing.
Rumination/mastication
The saliva plays a very important role in digestion and is very rich in the following minerals:
NaHCO3 (sodiumbicarbonade), K and P.
These are the so called base minerals, which are recycled through the blood. They provide the buffering-capacity to keep pH in the rumen at a desired level (control acidity).
• ca,C S, Mg and urea
The saliva also is rich in agents that prevent the formation of foam in the rumen (bloat). The amount of saliva produced can reach up to 150 litres per day, partly depending on the type of food. On average a cow needs per day about 8 hours for eating and 8 hours (max. 10-11 hours) for rumination! Each bolus of food is normally ruminated for about 40-50 times (sign of health).
2.1.2 Stomach Complex
The stomach of a cow is divided into 4 compartments, as shown in figure 2.1:
Rumen[ Pre-stomachs]
Rumen
Rumen/honeycomb
Omasum
Abomasum
The abomasum (true stomach) is similar to the stomach of non-ruminants (mono-gastrics). The other 3 pre-stomachs are specific for ruminants.
Just after birth, the pre-stomachs of a calf are still relatively undeveloped. The milk, which a calf drinks, is channelled directly through a groove (tube-like-fold) to the omasum and abomasum. However, the pre-stomachs develop rapidly if stimulated by feeding good quality roughage and concentrates. This should start at about one week after the birth. In adult cows the volume of the three pre-stomachs is about 14 times larger than the abomasum. A well developed rumen has a volume of 100-150 litres. The four stomachs together fill about 3/4 of the abdominal cavity. A well developed rumen is essential for the intake of high amounts of roughage and concentrates, resulting in a high (milk) production. During calf rearing and young stock rearing, special attention should be paid to the development of the rumen. The size of the rumen is a main factor in the potential intake of DM, and thus production
2.1.2.1 Rumen and Pre-stomachs
The rumen is basically a large barrel for digestion/fermentation of food by rumen micro organisms (micro bacterial digestion). These RMO’s are mainly bacteria and protozoa.
Rumen Contents
The rumen contents is normally made up of three layers:
A top layer of methane gas (CH4) and carbondioxide (CO2), produced by RMO’s as by-products from breaking-down (fermentation) carbohydrates. The gas is partly absorbed directly through the rumen wall into the blood and expelled through the lungs by breathing and partly expelled through eruption/burping. Failure to eruct causes bloat
A middle layer of recently ingested coarse materials (solid mass). In this layer, fermentation takes place. Particles size is reduced through mechanical action (contraction of the rumen) and microbial action and fibres become water soaked. Absence of this layer as a result of high quality energy diets supplied by concentrates (low roughage intake) causes (severe) problems. For a proper functioning the rumen, a minimum amount of fibre is required. As rule of thumb, a minimum of 30 % of the total DM ration should be supplied by roughage. In a healthy cow it is possible to feel the contractions by pushing your fist deeply into the rumen. The rumen contracts and expands about 10-12 times per 5 minutes (sign of health). From this layer, food is ruminated 40-50 times per cud and swallowed again.
A bottom layer consisting of liquid mass
Rumen Climate and Rumen Micro Organisms (RMO’s)
RMO’s can either be bacteria, the active digesters and fermenters (16,000 x 10-6 in number), or protozoa, of which the role is less clear (34 x 10-6 in number). The total mass of RMO’s (microbes) in the rumen is over 5 kg, “producing” several 100’s of grams microbial protein per day and fermenting carbohydrates into volatile fatty acids (VFA’s).
In an adult cow, the size of rumen and reticulum is 60-150 litres. The rumen has a specific
climate:
Basically anaerobic (no oxygen). Small amounts of oxygen enter the rumen with food and are quickly oxidized.
A pH of 6-7. This is the ideal climate for microbial growth and activities to break-down roughage. Concentrate diets, low fibre and high in energy, may cause the rumen pH to decrease to levels below 6. This has in general a negative effect (lower butterfat percentage, depressed appetite, metabolic disorders, and possibly death). A higher pH (>7) may be caused by urea toxicity (alkalosis) and possibly be followed by death.
Note: monogastrics have a stomach pH 2.
Rumen Fermentation
Rumen fermentation consists of two processes:
Microbial degradation of food components, mainly carbohydrates and proteins. Food enters the rumen partly in a degradable form, and partly in an undegradable form. If the undegradable food particles are sufficiently reduced in size, the particles move to the abomasum and small intestines for digestion and absorption.
Synthesis of organic macromolecules into microbial biomass, mainly proteins, nucleic acids and lipids. (Tropical) forages in a late stage of maturity (hay, straw) usually have a high fibre contents and can be highly lignified and usually have a low protein contents. Utilization of energy from such roughage increases heat production, lowering the feed intake, which was probably already low due to the slow rates of degradation and slow rate of passage of food (full stomach, thus feeling less hungry).
2.1.3 Abomasum and Small Intestines
In the abomasum and small intestines the “normal” chemical digestion (enzymes) takes place of the food as in monogastric animals. This digestion does not affect the management of ruminant nutrition and is consequently not further discussed in this paper.
2.2 Digestion of Food Components in Rumen
2.2.1 Fermentation of Carbohydrates
All carbohydrates entering the rumen are “attacked” by RMO’s, except lignin. Generally, 90% of the carbohydrates are broken-down (degraded, fermented, digested) into three types of Volatile Fatty Acids (VFA’s). In a ration with mainly roughage, VFA’s are normally proportioned as follows:
– Acetic acid (acetate) 65-70%
– Propionic acid (propionate) 20-25%
– Butyric acid (butyrate) 10%
Also carbondioxide (CO2) and methane (CH4) are released in the process. Quite some body heat is produced from energy required to break-down carbohydrates. Poorer quality roughage require more time and energy from RMO’s. This slows down digestion of roughage and increases body heat production. Ensuing, this leads to a lower food intake due to lower turn- over rates (passage rates of food in the rumen). Increase in heat production by the body may also depress appetite, especially in warm climates/seasons and/or during hotter parts of the day. The production of body heat and gas is at its peak immediately after a meal. Gas production can reach over 30 litres of gas per hour. Regular feeding or continuous access to food will reduce the gas- and heat production peaks, while night feeding of roughage will increase appetite (DMI). The latter should especially be considered for the warmer climates and seasons.
The amount of VFA’s produced can be as high as 4 kg/cow/day. Most of the acids are directly absorbed into the bloodstream through the walls of the pre-stomachs (mainly rumen). Some VFA’s enter into the abomasum and small intestines and some VFA’s are used by the RMO’s for the development of their own microbial tissues.
In rations with substantial amounts of roughage, acetic acid will exceed the amount of propionic acid. Acetic acid is formed mainly from cellulose and has a very positive effect on the butterfat contents of milk. A sufficient amount of cellulose (fibre) in a ration is also essential for a proper functioning of the rumen and to keep the desired optimum range of the rumen pH level between 6-7.
However, propionic acid production may exceed acetic acid production in diets containing high levels (over 70% of the total ration DM) of energy rich concentrates. Starches and sugars are very quickly fermented into propionic acid. This results in lowering the rumen pH level. Also less saliva will be produced and consequently less base-minerals, with an acid buffering capacity, will enter the rumen. The consequences depend on how much the rumen pH will be lowered:
– At pH 5, the appetite will decrease as the first RMO’s get killed. The lower amount of acetic acid and higher amount of propionic acid will results in a lower butterfat content in the milk: the so called low butterfat-syndrome
– At pH levels below 4½, the animal may suffer from acidosis. This can lead to laminitis (hoof problems) and ketosis (fat cow syndrome). The normal RMO’s in the rumen are getting destroyed, as the more acid loving lacto-bacilli (lacto-acid) will start to prevail. Symptoms indicating acidosis are: panting, distress, diarrhoea and anorexia. In prolonged cases, the rumen wall lining may be affected, destroyed and shed.
– At pH level below 3½, the cow may experience shock and die of toxaemia.
In order to prevent the diseases and to keep the rumen functioning at an optimum, with a sufficient level of butterfat in the milk, it is advised to feed a maximum of 70% DM concentrates, and a minimum of 30% DM roughage.
Note: Monogastric animals that can eat large quantities of roughage, such as horses, donkeys, rabbits and pigs to a certain extend, have bacterial protozoal fermenta- tion of carbohydrates (fibre, cellulose etc.) in specific parts of the hindgut (intestines after the stomach), like the caecum and/or colon. These are generally less efficient than the rumen.
2.2.2 Digestion of Lipids/Fats (Ether Extract)
Ruminants have evolved as plant-eaters and the rumen is not adapted to diets that contain a high amount of lipids/fats. The capacity of RMO’s to digest lipids/fats is strictly limited. Fat/lipid contents of ruminant diets is normally low (< 50 gr/kg DM). If fat/lipid content is increased above 100 gr/kg DM (= 10%) the RMO's reduce their activity. This leads to:
Decreased fermentation of carbohydrates Reduced intake of DM
Stearic acid is the predominant fatty acid of ruminant fat deposits due to RMO's activities. Recent efforts to include undegradable by-pass fat in concentrates to add cheap energy to rations have not (yet) produced any significant results.
Deficiencies of fat are not likely to occur, since the available fatty acids are efficiently used by the metabolic system of the animals.
2.2.3 Protein Degradation and Synthesis
2.2.3.1 True Proteins
Most of the true proteins entering the rumen are degraded by RMO's into amino-acids. Subsequently, ammonia (NH3) is produced (degradation). RMO's can utilize both amino- acids and NH3 to be synthesized into proteins. These are used as building stones for their own new bodies: the microbial protein! The ruminant does not depend on the protein quality of the diets for its survival (maintenance), although the quality of proteins becomes an important factor for good milk production. When RMO's die, they will be washed into the abomasum and small intestines, where the microbial protein is digested in the normal way (chemical digestion) and absorbed.
With most diets, majority of protein reaching the small intestine of a cow will be microbial protein of reasonable constant composition. Not all the true protein in food is degradable into ammonia. Some of the undegradable true proteins will escape the rumen degradation and will be digested in the small intestines. In highly productive dairy this is essential, since the capacity of the RMO's is too low to synthesize all the protein needed at the high milk production levels. This undegradable protein sometimes is, misleadingly, called by pass protein. This protein does not by pass the rumen, and is therefore not degraded by RMO's.
Proteins of different feedstuffs have a different percentage of by pass protein. The rumen degradability of some proteins from different foods varies between 40-90%. E.g. for young grass and good grass silage, degradability is indicated at 85%, while degradability of protein from meat/bonemeal and white fish meal is respectively 50% and 40%. Degradability of a food is however influenced by particle size and feed intake level (speed at passage through the rumen). A separate list indicating the degradability of certain foods is given in Appendix 1.
If a diet is deficient in protein (negative N balance), or if protein is largely undegradable and not available to RMO's in the rumen, concentration of ammonia will be (too) low. Growth of RMO's will slow down. This results in a longer fermentation time in the rumen and consequently in a lower food intake and loss of bodyweight! (slower digestion, food stays longer in the rumen, cow feels less hungry, "dying with full stomach"). The minimum level of required ammonia for a proper functioning of RMO's in the rumen is reached when a diet is fed with a minimum of 7% CP (= 4.55% DCP)! A protein or N deficient diet may lead to cannibalism among the RMO's.
On the other hand, if protein degradation proceeds more rapidly than the synthesis of microbial protein, ammonia will accumulate in the rumen liquid. The optimum concentration level will be exceeded. This optimum level is reached at a CP level in the diet of 13% (= 8½% DCP). Above this level, bacteria can not utilize all the NH3. If the required level of CP in the diet is higher for a certain production level, the protein should be made available to the animal in the form of undegradable protein. Otherwise, the excess ammonia in the rumen will be absorbed by the rumen wall, taken into the blood, carried to the liver and converted into urea. Some of this urea may return to the rumen via the saliva and/or directly through the rumen wall. However, the majority will be excreted through kidneys in the urine, and thus wasted! An overall diagram of protein digestion in cattle is presented in
2.2.3.2 Utilization of NPN (Non Protein Nitrogen Compounds)
The ammonia pool in the rumen is not supplied only by degradation of true protein. As much as 30% of nitrogen in ruminant foods may be in the form of simple organic compounds, such as amino-acids and/or inorganic compounds.
2.2.3.3 Urea (NH2)2CO as a Protein Replacer
If food is short in protein, urea can be used as a supplement in order to improve the nitrogen balance of the animal. Urea is rapidly converted into ammonia in the rumen by the action of water
(NH2)2CO + H2O ---> 2NH3 + CO2
urea ammonia
However, one has to be careful with urea as a supplement. High amounts of ammonia in the rumen and in the blood may lead to toxicity and possibly death (urea toxicity). In practice urea is only supplemented to rations with a rather low energy and protein value (poor roughage quality). The supplementation of urea to dairy with a high production potential is not recommended, as results have been disappointing.
This training course supports management of intensive/high dairy production systems with feeding rather high amounts of concentrates. These concentrates should contain sufficient amounts of proteins to meet the need of degradable proteins. Therefore, the subject is not further elaborated upon, as urea does not play a role in these systems. Treatment of straw with urea may offer some scope for certain production systems.
2.3 Practical Implications for Ruminant Management
The rumen plays a very important and specific role in the digestion of food by dairy. In order to exploit the high (genetic) potential of a cow to an economic maximum, a manager has to consider some important aspects in the feeding. In fact, one must know exactly how to manage and manipulate the RMO’s in the rumen. The farm manager thus has to be a Rumen Management Officer.
Some aspects to consider in feeding management are:
A. Composition of the ration.
It is seen, that the RMO’s play a very important role in the digestion of food. RMO’s have to adopt themselves to certain rumen climates as created by the different types of food given to them. Changes in the diet and in the composition of the ration will disturb and/or change the rumen climate to which the RMO’s have adopted themselves. Therefore such changes should be as much as possible limited and only introduced very gradually.
B. Frequent feeding will reduce the peaks in heat-and gas production.
This peaks may result in lower food intake. For a high milk production a high food intake is essential and it is therefore advisable to allow the dairy cattle to have continuous access (24 hours per day) to food and water. During warmer seasons roughage should be offered during the cooler nights. If outside feeding is practised (in yards) during the hotter parts of the day (between 10 am and 4.30 pm) it is advised to provide shade over the feeding place and feed-trough. Shade protects animals from direct sunlight and also may create some extra natural ventilation, reducing the heat load.
C. Sufficient (ad lib) amounts of water should be available to support food intake. Water plays an important role in the digestion of food (saliva).
D. Sufficient minerals P, Ca and Na have to be offered.
Those are the most important minerals excreted in the saliva to regulate the pH level of the rumen (acid-buffering capacity) to create an optimum environment for the RMO’s.
E. A minimum amount of (good quality) roughage has to be offered.
A minimum 30 % of the total DM allows the rumen to function properly. This will avoid rumen and metabolic disorders due to a lowered rumen pH and guarantees a high butterfat content in the milk. If the available roughage is ground finely or chopped less than 1 cm, arising problems may be similar to lack of fibre structure. One has to keep in mind that the rumen (ruminant) evolved in order to digest large amounts of roughage (nature!).
F. Poor quality roughage with low digestibility, such as straw, stover, chaff and mature stalky hay takes a longer time to be digested in the rumen and increase the heat-load in the animal (body-heat). This reduces the capacity to eat large amounts of roughage and either results in a higher demand for concentrates. This is probably more expensive, or reduces production.
G. The total diet may not contain more than 10% fats/lipids (EE).
H. For the proper functioning of the RMO’s, a minimum CP content of 7% is required in the diet (survival diet). The degradable part of the CP can be utilized up to a maximum level of 13% CP. Protein requirements over 13% CP (protein requirements) are to be fed as undegradable protein. The degradable proteins with a CP contents of over 13% will be excreted as urea in the urine, and therefore lost.
I. NPN supplement (urea) for (high yielding) dairy is usually not suitable as the NPN will be quickly degraded and probably excreted (see previous point).
J. Signs of health are:
– a good appetite
– a rumination of 40-50 times per bolus, and
– rumen contractions of 10-12 times per 5 minutes
K. High standards of feeding are required for calves and young stock. The rumen need a good development to ensure maximum intake of DM in order to reach a high production level (a cow only converts!).