Bentonite and Sheep and Cattle Production Explained
Professor Ronald A Leng AO B.Sc., Ph.D., D.Rur.Sc., FASAP
The first priority in ruminant nutrition must always be to establish the efficient fermentative digestion of the feed the animal consumes.
When an animal has a rumen microbial ecosystem that is functioning as an efficient fermentative system, the animal optimizes its forage intake and maximizes the digestion (fermentative) of the feed. The ratio of protein to energy in the nutrients absorbed is also maximized, and on most natural diets this means that the rate of growth of microbes in the rumen is at an optimal level. When this is achieved it leads to large quantities of microbial cells flowing to the lower digestive tract where they are digested by the enzymes secreted by the animal to produce the essential amino acids required by ruminants for growth, milk production and production of wool.
In general the total availability of nutrients and the balance of protein to energy are the critical issues that determine the levels of production by the animal. However, protein mainly comes from microbes that grow on the forage in the rumen, while the energy substances come from the acids (volatile fatty acids) that are produced by the microbes. The microbes obtain "building blocks" for their own growth and the chemical energy to stick these "building blocks" together into their own cell growth from the carbohydrates they ferment.
Efficient microbial growth is achieved when the forage fed or the forage fed with added supplements provides (in the rumen) all the minerals and ammonia constantly and at levels that meet the requirements for growth of microbes.
The rumen of cattle and sheep on balanced feeds which provides all the essential nutrients for optimum microbial growth will contain a mixed population of:
Amongst these mixed populations are numerous strains of each group of microbes which work as a consortium to degrade the forage that the animal consumes. Forage is firstly mixed with saliva before being delivered into the fermentative sac of the ruminant. Soluble plant materials are often released into the fluid in the rumen and are directly absorbed and digested by bacteria and protozoa that swim freely in the rumen contents.
Fungi are a relatively small proportion of the biomass of microbes present, but are the first organisms to produce "seeds" that invade forage. Fungi grow a network of roots (a rodii) through the tissue that secrete enzymes along their length and cause the solid fibrous plants to "shatter". Bacteria then have the large surface area on the finely broken-up fibre to attach to and begin the process of breaking down the carbohydrates to "building blocks" for their own synthesis and providing all the energy that allows them to grow. The byproducts of this are the acids that supply the animal with its energy resources.
Most of the bacteria (up to 70%) are on the plant fibres. But as the bacteria achieve their needs for energy the fibre particles decrease in size and some bacteria are released to attach again to new, bigger particles in the rumen or they are washed out of the rumen and provide the animal with protein to digest.
The protozoa can also digest fibre or the more soluble plant components but science has shown that because they are unicellular animals, and have a primitive digestive tract, they largely take up particles from the rumen to degrade for their own energy supply. Although it has not been proven while "grazing" on particles in the rumen, protozoa graze both feed particles and bacteria and do not appear to discriminate between them. Where the feed particles are high in protein or the protozoa take in bacteria (which are high in protein) then both sources of protein are degraded with a loss of protein to the animal.
When an animal swallows food that contains sugars, then the protozoa can use this and small feed particles and bacteria as food for growth. When the soluble sugars are used up the protozoa cease grazing and attach to fibre to get their feed supply. The ingested particles are slowly degraded for nutrients and the protein in both particles and bacteria ingested are degraded. Any protein in excess of the protozoa’s needs is degraded to acids and ammonia. This leads to a considerable proportion of the bacteria that grow in the rumen being broken down in the rumen. It is these bacteria that would have provided the animal with protein when they were washed from the rumen, and are now not available to the animal.
In addition, the amount of high protein feed particles could have reached the lower gut for use by the animal is also reduced. This reduction in flow of protein from the rumen in normal animals is extremely important because protein is often the first limiting factor for production.
In feed such as pasture, protein is very soluble and the bacteria quickly adsorb the protein molecules and degrade them to ammonia and acids. However, proteins that are insoluble and therefore embedded in particle form are slowly degraded because bacteria can only attack it from the surface of the particle. Some of this protein thus escapes and moves to the lower tract.
Thus, pasture protein that escapes the degradative processes of the rumen and therefore available directly to the animal depends on:
When cattle and sheep are consuming forage that provides an "ideal" growth medium in the rumen, then the protein available for digestion by the animal is mostly or only from bacteria. The level of bacteria available for digestion is reduced by that amount consumed by the protozoa.
When cattle and sheep are consuming forage deficient in some nutrients (and therefore provide a less than ideal growth medium in the rumen), then the medium can be returned to an ideal microbial growth medium by supplementing the animal with the deficient nutrients which might be a source of ammonia or minerals, or both. In this case they are provided from urea and minerals. But even in an ideal rumen, supplemented with multi-nutritional blocks, the available protein is limited by the extent of activity of the protozoa.
Protein availability to the animal can be increased by feeding a protein meal that because of its chemical and physical structure is not degraded quickly in the rumen, but escapes the activity of bacteria. However, again the amount reaching the lower digestive tract depends on the activity of the protozoa.
Scientific research has clearly shown that where protozoa have been excluded from the rumen, up to 25% more bacterial and up to 25% more dietary protein reaches the lower digestive tract.
Associated with exclusion of protozoa from the rumen is an increased population of bacteria in the rumen. The effect of increasing the availability of protein in the intestines by exclusion of protozoa has been associated with:
Improving Protein Nutrition
In grazing ruminants, improving protein nutrition may be increased by a number of techniques either applied individually or in combination.
On green pastures high in protein above say 12% crude protein and with a well-balanced array of minerals, protein nutrition is improved by:
On dry pastures low in protein but containing a complete array of minerals and protein is improved by:
On poor quality forages low in protein and which are also deficient in minerals depending on the extent of weathering, protein nutrition is increased by:
Effect of Supplements on Protein Nutrition
The way each supplement effects protein nutrition is as follows. On green pastures with the rumen microbial growth optimized, excluding protozoa allows bacteria to increase in numbers in the rumen and more reach the part of the intestinal tract where they are digested. Unfortunately there are no practical and economic methods for excluding protozoa from the rumen and it is only feasible under research conditions.
Giving some bentonite mimics the effects of excluding protozoa, without affecting their population density.
Where a protein meal is provided and sheep and cattle consume it, then the extent to which the protein escapes the rumen and provides protein to the animal depends firstly on the form of the protein. In general a proportion of protein that is insolubilised will escape. However, protozoa play a big role in solubilising protein in particles when they consume them and digest them, so more dietary protein reaches the areas of the intestinal tract where they make more protein available to the animal.
In summary, excluding the protozoa from the rumen improves the protein flow to the intestines of sheep and cattle and thus improves their protein nutrition by:
Supplementing with Urea
On dry feeds only deficient in a source of ammonia then providing urea improves the growth rate of microorganisms increasing the numbers of bacteria in the rumen.
Low Levels of Rumen Ammonia: At low levels (probably below 50mg ammonia/litre) the numbers of bacteria are too low to achieve maximum breakdown of the forage and so digestibility is decreased (less of the forage is broken down in the rumen and relatively more goes out in the faeces). This also results in lowered forage intake.
Moderate Levels of Rumen Ammonia: At moderate levels (above 80mg ammonia/litre) the numbers increase and digestibility is optimized and forage intake is increased...
High Levels of Rumen Ammonia: At still higher levels (about 200mg ammonia/litre) the numbers of bacteria probably remain constant but the proportion of their cells as protein is increased from 30% protein to 60% as the ammonia levels increase from low through moderate to high levels. This allows the same maximum digestibility but allows under some conditions a further increase in forage intake.
Supplementing with Minerals
On dry feeds deficient in both a source of ammonia and minerals, then adding minerals may slightly improve the growth of bacteria. But where phosphorus and sulphur are highly deficient, the rumen fungi will be in very low numbers. Correcting this allows the fungi to multiply. This then increases the rate of breakdown of feed particles to provide the large surface area to increase the effects of bacteria on degradation.
Once the increased fracture of forage into small particles has been encourage by allowing the fungi to function (and they do not have a big demand for ammonia) then providing a source of ammonia achieves the same effects as described above for forages only deficient in a source of ammonia.
So, how does bentonite fit in?
Protozoa grow slowly in the rumen and have low requirements for protein building blocks. They remain in the rumen for a much longer period than bacteria. Although they detach from feed particles to "soak up" the soluble feed components on entry into the rumen, this appears to be the only time that they are washed from the rumen at the same rate as bacteria free in the rumen. They attach to feed particles and as these reduce in size through the action of both bacteria and protozoa, the protozoa move on to larger particles. Protozoa can store energy and protein and when soluble feeds are exhausted in the rumen they also "flock together" and attach to the wall of the rumen. In this way only a small proportion are actually washed out of the rumen.
Probably the amount of bacteria and small particles of protein meals that the protozoa "swallow" is dependent on their swimming speed when they are free in the solution in the rumen.
Bentonite may have two effects on protozoa:
The net result is that the actual capture rate of small feed particles and bacteria is reduced to a small fraction and more protein reaches the intestines when a minimum amount of bentonite is present in the rumen. However, protozoa levels in the rumen are unaffected by feeding bentonite and therefore it must be assumed that the protozoa always capture more bacteria and protein particles than they actually need for their own growth and wastefully use them. This might be likened to a dingo that kills 50 sheep a night but consumes only the liver and the heart of the sheep and the rest, although dead, is degraded by other organisms present.
As I have indicated in previous lectures, improving protein supply to ruminants, particularly on dry feed, or on any feed that does not provide an efficient rumen for the animal will:
Olsson Industries is the first company to apply science to control the digestive function of ruminants in order to maximize the use of pastures and crop residues by sheep and cattle.
This transcript was written by:
Emeritus Professor Ronald A Leng AO, B.Sc., Ph.D., D.Rur.Sc., FASAP