A healthy digestive system for ruminants

The first factor we discuss is the diet. Composition and physical form of the diet affect growth of the internal layer lining the rumen wall by which absorption of the digestion products takes place. This was shown in a study, in which calves were fed on diets containing forages and concentrates in addition to the milk (Group A) and the anatomical features of the rumen were compared with other group of calves fed only on concentrates and milk (Group B). The rumen papillae in group A were long and well organised, while in group B the papillae were short and some of them were atrophied in parts of the internal rumen wall. These findings may be explained on the basis of the physical effect of the roughages on the rumen papillae in group A in addition to the chemical effect of the concentrates, which have both improved rumen papillae in group A. In group B, however, there was no physical effect due to the absence of roughages in the diet, and the chemical effect attributed to the concentrates seems to be inadequate for promoting growth of papillae. In this latter case, the growth of papillae can be recovered and the absorptive functions thereof can be improved by feeding the calves on graded amounts of roughages besides the concentrates and milk for a period of at least three months. The diet composition also affects the amount of saliva produced at advanced stages of rumination.

The increased ratio of roughages-to-concentrates helps increase the amount of saliva due to the prolonged time spent in chewing. With the high roughage-to-concentrate ratio, however, the amount of saliva produced may vary depending on the moisture level of the feed, i.e. the increased moisture level will decrease the amount of saliva. This may explain the occurrence of bloating in grazing animals fed on spring grasses containing large amounts of water (up to 80%). In this case, the production of saliva will be inadequate and the amount of the anti-foaming agents present herein will not be sufficient enough to prevent bloating. There is also a strong relationship between diet composition and the rumen pH which determines the microbial activity in the rumen and digestibility of some nutrients such as fibre. With the increased roughage-to-concentrate ratio in the diet, the rumen pH will increase due to the decreased amounts of VFA resulting from fermentation.

This ratio also affects the bacterial species dominating fermentation of carbohydrates in the rumen (Table 1) and hence affects the end products of carbohydrate digestion. With the increased ratio of roughages-to-concentrates in the diet, the ratio of acetic acid-to-propionic acid in the rumen will increase, and the contrary will happen when roughages-to-concentrates ratio decreases. Such changes may be useful when planning the feeding programmes for different classes of ruminants to better suit production targets in each case. For lactating cows, for example, it is better to have a higher level of acetic acid in the rumen by increasing the roughage-to-concentrate ratio. This is because acetic acid is a basic precursor of milk fat synthesis which is considered as a positive sign of milk production. The increased milk fat is often associated with an increase in milk protein due to the high correlation (R2= 40 or more) between the two elements. Also, with a high level of milk fat there will be increased amount of fat-corrected milk (FCM) which is used as a standard for comparing milk production between cows. For beef cattle and fattening animals in general, attempts should be made to increase the level of propionic acid in the rumen by increasing the concentrate-to-roughage ratio in the diet. This is because of the high-energy value of propionic acid relative to acetic acid, which promotes faster growth of animal and hence achieves the target weight at shorter periods of time.

Factor 2: Frequency of feeding

When feeding animals for 5-6 times per day, there will be a stable pH in the rumen at levels ranging for about 5.5 to 5.8, but when feeding for only 1-2 times per day, the pH value will in this case vary from about 5.1 to 7.1 within the same day. With a stable pH value in the rumen, digestibility of dietary fibre will be increased due to the increased microbial activity in the rumen which results from the increased energy level needed for such an activity (the ruminal ATP concentration is 2.5 times more under high-frequency feeding compared to the low-frequency feeding). Also, the high-frequency feeding decreases amount of ammonia produced in the rumen following digestion of protein, indicating low rates degradable protein formation and high rates of non-degradable protein which is used for productive purpose. The increased ratio of non-degradable protein relative to the degradable protein in the rumen is probably attributed to the increased rate of passage of digester from the rumen with high-frequency feeding thereby allowing insufficient time for degradation.

Factor 3: Drinking water

The salinity of drinking water is an important factor affecting feed intake, which is reduced by about 15-20% with saline water. Salinity of water also affects the digestive process in the ruminants by decreasing the number of protozoa to only 20% of their normal population, which-in turn-affects production of VFA and other useful digestion products. Further, there is a relationship between water salinity and absorption of the digestion end-products due to the change of the rumen osmotic pressure which affects utilisation of energy and other dietary nutrients for productive purposes. This effect, however, varies depending on the animal species, i.e. camels are more tolerant to water salinity, followed by goats and sheep, with the cattle being the least tolerant to salinity. Generally, drinking water should be analysed for concentration of mineral compounds such as sodium chloride, which in no case should exceed 1-1.5%. The testing of minerals in water is particularly important in arid areas where their concentration is often high either in the artesian wells or in other water sources. It is also important to test water salinity in areas where production of salinity-susceptible animals such as cattle is most prevalent.

Factor 4: Parasitism

The tropical and subtropical areas are more prone to the internal parasite infestation of animals compared to the temperate or cold areas. The climatic conditions in the tropics and subtropics allow for survival and proliferation of parasites during the growth cycles occurring outside the host animal body. The low nutritive value of the tropical and subtropical feeds, together with the shortage of feed supply, are also important factors contributing to the low resistance of animals to the parasitic infestation.The internal parasites affect feed intake by animals to a varying extent depending on the magnitude of infestation (Table 2). Such a relationship may be attributed to the effect of the parasites on rumen motility, rumen pH, and the hormonal system regulating the appetite and feed intake. The internal parasite residing in the true stomach (abomasum) such as H. contortus and O. ostertagia may cause damages to the HCL-secreting glands. As a result, the pH level in the gastric liquors increases to about seven or more, thereby leading to diarrhoea and weight loss of animals. This is due to the fact that the increased pH will lead to an increased production of gastrin hormone in the stomach, thereby increasing secretion of water from the liver, pancreas, and the distal part of the small intestines which in turn causes the diarrhoea and weight loss.

The internal parasites residing the small intestines such as T. colubriformis cause atrophy of the intestinal villi and hence reduces their ability to increase the surface area available for absorption and utilisation of feeds. In extreme cases, these parasites may cause damages to the intestinal wall itself and hence allow for passage of the immunoglobulins from the blood plasma to the gastric cavity with an eventual loss of these immunoglobulins through faecal excretion and a decreased resistance of animals to the microbial diseases. The internal parasites also affect digestion and utilisation of dietary proteins, due to the decreased feed intake and the need of animal to use feed proteins as a compensatory source of energy. Utilisation of other nutrient elements such as calcium and phosphorus may also be affected by parasite infestation (Table 3), which adversely affects bone formation and causes rickets and other disease problems. The parasite infestation should, therefore, be controlled or at least be alleviated by means of chemical and biological methods, and by adopting proper grazing systems which allow for an ideal balance of plants and animals and hence compensate for the inadequate feed supply, the low nutritive value, and the other factors contributing to parasitism, particularly in the tropical and subtropical areas, as indicated earlier. The control of other disease factors (microbial and non-microbial) should equally be considered when attempting to improve GI functions and hence achieve better health and production of animals.

Factor 5: Temperature

The rate of rumen motility decreases under high environmental temperature, reaching about 1.4 times per minutes compared to a normal rate of 2.2 times per minute. The decreased rate of motility arises from some biological changes in the internal body systems such as the decreased activity of the thyroid gland and the low rumen pH. In addition, there are decreased levels of nutrients which are dissolved in the blood and are needed for stimulation of rumen motility. This is due to the fact that a great deal of blood circulation is directed under high temperature conditions to the peripheral parts of the body to feed the sweat glands instead of being directed to the rumen to stimulate motility. With the lower rate of rumen motility, there will be only small amounts of feed passing towards the lower part of the GI tract, thereby making the animals feel full and have no desire to eat more for long periods of time. The heat-stressed animals also tend to drink large amounts of water as a means for alleviating the heat load. In one study, the water-to-feed dry matter in the rumen was 3:1 at temperature of 5 ^oC, and was increased to 8:1 at 30 ^oC. This may also be a factor contributing to the low feed intake due to the effect of the large amounts of water on the capacity of the rumen to accommodate further amounts of feed. In other study, it was found that the high environmental temperature also affects the pancreatic activity and its capacity to produce the enzymes needed for the digestion of carbohydrates and proteins when reaching the intestinal area, thereby reducing utilisation of such nutrients for productive purposes.

Factor 6: Radiation

The animals may sometimes be exposed to radiation in cases where air, water, feed, or soil is contaminated with bioactive elements such as Radon (Rn), Uranium (U), and Radium (Ra) which are produced by nuclear reactors or when running nuclear experiments. The radiation intensities is measured in RAD units, and the animals are severely affected when exposed to radiation levels ranging from 500 to 2,000 RADs over a time period ranging from 5 to 15 days. The exposure to radiation causes a number of changes in the GI functions, especially in the true stomach and the intestinal regions, in addition to changes in the metabolism of proteins, carbohydrates, and fats. In the true stomach region, the radiation inhibits division of the mucosal cells and lowers their ability to secrete sufficient amounts of mucus, thereby exposing the true stomach to abrasion and oedema, in addition to other problems relating to reduced secretion of gastric HCL and enzyme pepsin. In the intestinal region, the radiation acts on depletion of the villi at certain locations of the intestinal wall, thereby affecting absorption of nutrients at these locations. It also acts on irritation of the intestinal wall resulting from the reduced capacity thereof to absorb the bile salts after having the intestinal villi depleted. The radiation also affects the chemical structure of the protein molecules and hence affecting their ability to act as enzymes and hormones which regulate the digestive and metabolic processes. The same holds true with carbohydrates, the molecules of which are disintegrated under the influence of radiation and hence become unable to serve as a feed energy source. The functions of fats are also retarded with radiation, as they are converted in this case to organic oxides through ionisation of the cell water, thereby becoming unable to provide the energy needed for performing specific body functions.

Figure 1 – Relative capacities of the uterus and the rumen during the advanced stage of pregnancy.

Factor 7: Pregnancy

There are the changes of the relative capacities of the uterus and the rumen during the advanced stages of pregnancy (Figure 1). In this case, the ability of the rumen to accommodate the feed material is reduced by 15-20%, and its capacity to accommodate the liquids is reduced by 10-12% compared to the respective values in non-pregnant animals. It is, therefore, important to increase the frequency of feeding of the pregnant animal with limited amounts of feed at each time so as to match with the new capacity emerging from pregnancy and to also meet the feed requirements for maintenance and growth of the foetus. During pregnancy, there is also an increased rate of passage of digests towards the distal part of the GI tract, thereby affecting the retention time thereof in the rumen (Table 4). The increased rate of passage may result either from the pressure exerted by the filled uterus on the rumen, or by the increased level of the pregnancy hormones in the blood serum which stimulates rumen motility and hence increases passage of feed from the rumen downwards. Such changes, however, do not have a significant effect on the digestibility of feed nutrients. Therefore, it may not be necessary here to change the diet composition so as to make it more digestible, but the diet should be offered frequently with little amounts of feed each time to cope with the reduced capacity of the rumen, as indicated earlier.

References are available on request (drsalah31@hotmail.com)