CHAPTER  5

MANIPULATION OF FEEDING & THE RUMEN ECOSYSTEM

The three major objectives when manipulating rumen fermentation to increase productivity of ruminants are to: 

• Increase the digestibility of refractory carbohydrates in the rumen

•  Increase the proportion of propionic acid in the total VFAs produced (G/E ratio)

• Change the protein to energy ratio (P/E ratio) in the end-products of digestion.

5.1 CHEMICAL TREATMENTS OF ROUGHAGE

 A variety of physical and chemical treatments can be used to increase the potential rate and extent of degradability of fibrous feeds. The principal methods use alkalis, of which the most widely studied is sodium hydroxide. While this chemical is highly effective in increasing digestibility, the disadvantages (high cost, pollution through accumulation of sodium ions and the dangers to people and animals due to its corrosive nature) have led to it being almost completely discarded, particularly in developing countries.

5.1.1 Ammoniation 

There has been much greater acceptance of the use of ammonia, than of other chemicals, as an agent to treat straw. Ammonia can be used as gas, as ammonium hydroxide solution or by generation from urea by ensiling straw at high moisture content. For the urea ensiling method to be almost as effective as using ammonia gas, high temperatures seem to be necessary (see Mason and Owen 1985).

5.1.2 Acid gas treatments 

Hydrochloric acid (especially in gaseous form) and sulphuric acid (usually as sulphur dioxide) have also proved to be effective for treating straws and other fibrous residues. Sulphur dioxide combined with ammonia appears to be the treatment that has most potential for increasing digestibility of straw. However, feed intake is reduced by excessive intake of sulphur. Using gaseous ammonia and sulphur dioxide could be useful where high feed intakes are not desirable, for example where cattle and sheep are to be fed at sub-maintenance or in drought feeding.

5.1.3 Steam treatment 

A novel method of using acid hydrolysis is through high-pressure steam treatment, especially with agro-industrial by-products such as bagasse. The effect of the steam at high temperature is to liberate acetic acid, which hydrolyses the lignin-carbohydrate linkages (Wong You Cheong et al. 1974). The technology has only limited application, for example in sugar mills, where high pressure steam is readily available at little cost and where the bagasse is produced.

5.2 INCREASING FIBRE DIGESTIBILITY IN THE RUMEN

 A deficiency of a nutrient needed by the micro-organisms in the rumen will, in general, reduce the microbial biomass and therefore reduce feed digestibility, particularly that of fibrous feeds. The first criterion in manipulating the rumen ecosystem on any diet must be to provide the essential substrates for microbial growth. 

5.2.1 Rumen ammonia 

On most diets based on agro-industrial byproducts and low-digestibility forages, the primary limitation to the growth of rumen micro-organisms is probably the concentration of ammonia in rumen fluid. This must be above a critical level for a considerable period of the day. The level of ammonia that supports maximum digestibility in the rumen, and therefore the largest population of micro-organisms, will vary among diets. The critical level for ammonia has been variously reported as being 50 to 250 mg of ammonia nitrogen /litre of rumen liquor (see Chapter 3). Alvarez et al. (1983) pointed out that with forages containing considerable amounts of crude protein it is highly likely that organisms adhering to the fibre depend on the nitrogen within the plant cell wall. The efficiency of growth of these organisms may be less affected by the level of ammonia in the rumen fluid. With fibrous diets low in N or diets in which the carbohydrate is largely soluble (eg. sugarcane) the critical ammonia concentration must be higher than for protein-rich feeds.

It is essential that ammonia concentrations are continuously high in rumen fluid. For example, urea fed as a single meal with a straw-based diet is used inefficiently in the rumen. The urea is converted rapidly to ammonia and the concentration peaks soon after feeding but is below the critical level for the rest of the day (Figure 5.1). Fibre digestion by rumen micro-organisms reaches a peak rate 5 to 6 hours after a meal. Similarly, with grazing ruminants feeding urea once a day is often ineffective for the same reasons. It is well recognised that feeding the same quantity of urea in many meals during the day is much more effective than feeding the total daily requirement in a single meal (Figure 5.2).

Figure 5.1  Rumen fluid ammonia nitrogen levels after one dose of urea, two doses of urea and no urea (Source: Falvey 1982)

Figure 5.2  The effects of increasing the frequency of urea feeding on the intake of wheat straw by sheep. The same quantity of urea (10g in 24h) was given in 1, 2, 4, or 10 portions or was infused continuously (R A Leng unpublished observations)

A deficiency of ammonia results in an inefficient rumen microbial system. When changing from a diet that promotes high concentrations of ammonia in the rumen fluid to one in which the concentration of ammonia is below the critical level, the effect on fermentative efficiencies will only become apparent when the pool size of micro-organisms has decreased significantly. There will be a lag phase as the efficiency decreases from a YATP of about 14 down to 8, changing the protein-to-energy ratios from 23 g protein/MJ of VFA energy to 12g protein/MJ of VFA energy (see Table 3.11).

Once the pool size of micro-organisms has decreased, digestibility of fibrous feeds will decrease and feed intake will then fall. Thus, animals fed fibrous diets with inadequate levels of nitrogen will exhibit low levels of feed digestibility, but the reduction in digestibility of the diet may take some time to develop following a change from a high-N to a low-N diet.

The pool size of ammonia nitrogen in the rumen is small and large variations in ammonia concentration can result from small changes in N-nutrition even on diets with moderate to high N content. It is important that the ammonia concentration in rumen fluid should be above the critical level while the energy substrate is being fermented. For instance, if urea is incorporated into a concentrate supplement and given to cattle on a straw-based diet early in the morning the concentration of ammonia in the rumen will be high for a short time and may fall below the critical level when the largest proportion of the straw-carbohydrate is consumed or available for fermentation.

It cannot be assumed from the level of protein in a diet that rumen ammonia levels will be adequate. If the protein is heavily protected from rumen fermentation (eg. solvent-extracted cottonseed cake that has been toasted and pelleted) rumen ammonia levels can be low (Figure 5.3). Under these conditions the source of ammonia in the rumen is urea from plasma. The mechanisms for this are as follows: amino acids from the supplement protein are absorbed from the lower tract, deaminated in the liver and the ammonia produced is converted to urea which may be secreted into the rumen (in saliva or across the rumen wall).

Figure 5.3  Rumen ammonia levels in sheep fed liquid molasses and straw were less than the minimum needed for efficient microbial growth when cottonseed cake was used as a protein supplement. Urea was much more effective for this purpose (Source: Sambrook and Rowe 1982)

 5.2.2 Availability of peptides and amino acids

There is evidence that the availability of amino acids and peptides may at times limit the efficiency of microbial growth and therefore digestibility of feed. Micro-organisms appear to obtain only 70%of their nitrogen from the ammonia pool in the rumen, which suggests that bacteria take up peptides and amino acids. Oldham et al. (1985) observed with growing heifers that the digestibility of a diet based on cereal grain, alkali-treated and pelleted straw and maize silage increased when a fish-meal supplement was fed, suggesting that some compound(s) (perhaps amino acids but a range of other compounds is possible) in the fish meal was supporting increased microbial activity. However, Coombe (1985) showed that the digestibility of oat straw was higher when it was supplemented with protein meals than when it was supplemented with urea alone (see Table 8.6). Similar effects were observed with grass-silage diets supplemented with fish meal or groundnut meal but the effects have always been small (Table 5.1).

5..2.3 Other nutrients

There is considerable evidence that there are compounds in feeds that, at times, increase the efficiency of microbial growth in the rumen. An example of the effects of these (unknown) microbial growth promoters is shown in Table 5.2. The inclusion of chicken litter in a diet of molasses/urea and elephant grass forage fed to cattle increased the proportion of propionate and reduced the proportion of butyrate produced in rumen fermentation. Similar results were reported by Fernandez and Hughes-Jones (1981).

5.2.4 Maintenance of a large `free-floating' pool of cellulolytic organisms

 Insoluble feed materials entering the rumen must be colonised by organisms before fermentation can begin. Good-quality forage provides cellulose which is readily accessible to micro-organisms (ie. highly digestible or soluble). With straws and ligno-cellulose crop residues the fibre components are insoluble. There is strong evidence that providing a readily-digestible cellulose source just prior to feeding these refractive feeds increases the number of cellulolytic microbes floating free in the rumen liquor. In this way the supplement increases the rate at which the feed particles are colonised and increases the rate of fermentation of fibre. The evidence for this originates from in vitro studies that showed the effect of sugarbeet pulp (80% digestible) on the digestion of straws (Juul-Nielson 1981; Silva and Orskov 1985).

These observations are supported by studies with cannulated sheep fed diets based on sisal pulp, in which providing fresh grass significantly increased the degradation of pure cellulose in nylon bags suspended in the rumen (Figure 5.4). In addition, it has been demonstrated that straw incubated in the rumen of sheep on a diet based on ammoniated straw disappeared more rapidly than when it was incubated in the rumen of sheep on a diet based on untreated straw (Table 5.3).

Figure 5.4  Supplementing a diet of sisal pulp (including urea and minerals) with freshly harvested African Star grass improved the rumen ecosystem in sheep as evidenced by the 50% increase in the rate of cellulose digestion (in nylon bags in the rumen). This in turn led to an 80% increase in feed intake (Source: Guttierrez and Elliott 1984)

When high-quality green forage was added to a straw-based diet, the number of sporing bodies of fungi on the straw increased enormously and there was a concomitant increase in the number of motile zoospores in rumen fluid (Table 3.6 and Table 3.7).  Thus, part of the effect of a small amount of high-quality forage appears to be due to stimulation of fungal growth. The overall effect is to increase the digestibility of low-digestibility straws or hay.

The effect of high-quality forage or soluble cellulose in increasing the free floating pool of bacteria was observed in vitro. When methyl cellulose (which is soluble) is mixed with fibre from the rumen, the adherent bacteria detach, which allows samples from this important pool of rumen microbes to be obtained (Orpin and Letcher 1984).

The beneficial effect of ``quality" forage on rumen function on straw-based diets explains the practice of smallholder farmers in Asia of giving small amounts of green feed, which they believe stimulates the utilisation of dry forages by cattle. 

5.2.5 Protozoa and digestibility of fibrous feeds 

Unfaunated animals (ie. without protozoa in the rumen) have shown higher productivity than faunated animals on a wide range of diets. This raises a number of questions since in some of the comparisons the population of protozoa in the rumen of the faunated animals could not have been more than 5 to 10% of the total microbial biomass. In the original work carried out by Bird and Leng (1978), diets were chosen that contained considerable amounts of sugar and which promoted a large population of protozoa in the rumen.

On dry pasture the productivity of unfaunated ewes and lambs was considerably higher than that of faunated sheep (see Table 5.4, Table 5.5 and Table 5.6). In one trial in Belgium (see Table 5.7), in which alkali-treated straw was the basal diet, defaunation increased growth rate by 37%. More recently, in Australia, Bird and Leng (1984b) reported that defaunation increased wool growth by 35% in sheep fed ammoniated straw alone and by 24% when the ammoniated straw was supplemented with alfalfa hay and cottonseed meal (see Figure 8.5).

Table 5.6 Birth weight of lambs and live weight gain of unweaned lambs born to faunated (+P) and unfaunated (-P) ewes at pasture (see Table 5.5 for details of pastures).

The population of protozoa in the rumen fluid is low in animals fed these diets and it appears therefore that the causes of the increased productivity are different to those in sheep on diets that support a high population of protozoa. In the latter case the main effect of the absence of protozoa in the rumen is to increase the P/E ratio in the end-products of fermentation.

In sheep on diets of straw supplemented with urea and minerals, removal of protozoa from the rumen resulted in large increases in the number of fungal zoospores in the rumen (see Table 3.6) and concomitant increases in both the rate of and total dry-matter digestibility of straw in nylon bags in the rumen. There was also an increase in colonisation of the straw by fungi, as indicated by sporangial counts on straw leaf blades.

The rate of digestion and comminution of particles in the rumen are primary limitations to feed intake in ruminants on fibrous diets. If the extent and/or rate of digestibility is increased by the unfaunated state this should result in large increases in productivity of ruminants on these feeds. However, digestibility in sheep without protozoa (see Veira 1986), but fed well-balanced diets, has generally been decreased.

5.3 ENHANCING RUMEN PROPIONIC ACID PRODUCTION

A number of chemicals, when included in a diet for ruminants, alter the relative proportions of the VFAs produced in the rumen (see Chalupa 1980). The chemicals appear to inhibit the growth of methanogenic bacteria, which reduces methanogenesis and increases hydrogen tension within the rumen. This suppresses the growth of the bacteria that produce hydrogen (largely acetic-acid-producing bacteria) but stimulates the growth of the bacteria that produce propionic acid. There appears to be also a concomitant increase in fat synthesis by the rumen microbes (J O'Kelley, personal communication). This is beneficial to the animal because: 

• There is an increase in total energy available from VFA (provided that there is no decrease in VFA production) and from microbial lipids

• Propionic acid is used more efficiently by the animal than acetic acid (ie. more ATP is available per mole)

• More glucogenic energy is available and this may increase the productivity of ruminants on diets requiring fermentative digestion.

The inclusion of small proportions (10% of diet) of poultry litter in diets based on molasses has been shown to increase the production of propionate (Marrufo 1984; Fernandez and Hughes-Jones 1981).

One of the objectives of supplementation strategies should be to increase propionate production in the rumen without depressing rumen function.

5.4 ALTERING THE P/E RATIO

5.4.1 Chemicals inhibiting proteolysis or amino acid deamination

One approach to increasing the availability of protein to ruminants is to reduce the degradation of dietary amino acids in the rumen by using chemicals (eg. diaryliodenuum compounds, see Chalupa 1980) that inhibit proteolysis or deaminase activity or both. When added to high-protein diets these chemicals may increase the quantity of amino acid that reaches and can be absorbed from the small intestine and also increase the concentrations of amino acids in the rumen fluid. This may increase the efficiency of microbial growth (Oldham 1980). Both effects potentially increase the efficiency with which the end-products of digestion are used in the animal (Oldham 1980; Gordon 1980).

In general the responses of animal production to chemical manipulation of the rumen have been relatively small, often being merely an increase in efficiency of feed utilisation rather than an actual increase in productivity.

Often these chemicals reduce rumen fermentation (depending on their level of application), allowing a greater proportion of the diet to pass to the lower tract. This is beneficial when the diet contains starch but may be detrimental if the diet consists of refractory plant cell wall materials. 

5.4.2 Dilution rate 

Isaacson et al. (1975) showed that the YATP for a continuous culture of ruminal bacteria could be altered from 8 to about 17 by increasing the dilution rate of the medium, representing an increase in the P/E ratio from 12 to 27g of protein per MJ of VFA energy (Figure 3.11). This indicates that a large pool of micro-organisms growing at a slow rate uses ATP less efficiently than a smaller pool of microbes growing rapidly. These data have been used to suggest that an increased turnover rate of rumen digesta should increase microbial cell yield and YATP, and the results of experiments in which fluid turnover of the rumen was increased by feeding or infusing salts appear to support this suggestion. However, changing the rate at which digesta pass through the rumen by changing level of feeding had no effect on the efficiency of microbial protein production in the rumen (Hagemeister et al. 1980; see also Figure 7.5). Unpublished work from the authors' laboratory has indicated that increasing the intake of feed by supplementation had little or no effect on the efficiency of microbial cell production in the rumen of cattle fed sugarcane pith.

The theory that increasing the rate of turnover of rumen fluid should increase microbial cell yield and YATP is perhaps too simple, since the organisms that might respond to an increase in dilution rate are only those that are in the fluid phase. On fibrous feeds the only microbes in the fluid are those that are in transit between digesta particles, and washing them out of the rumen may be detrimental. 

5.4.3 Protozoa - defaunation 

It seems to be indisputable that the presence of a large population of protozoa in the rumen alters the protein-to-energy ratio in the products of digestion. In Chapter 3 evidence was presented that protozoa are retained in the rumen. It is well recognised that protozoa have high maintenance energy requirements and that they also consume bacteria in large numbers (Coleman 1975) and increase the intraruminal recycling of nitrogen (Nolan and Leng 1972) through ammonia N to bacterial N to ammonia N. This must result in inefficient utilisation of ATP (ie. YATP is decreased, which reduces the protein-to-energy ratio in the nutrients available to the animal).

Bird and Leng (1985) summarised recent research on the productivity of defaunated and faunated sheep. The data indicate substantial benefits from defaunating the rumen, which increased growth rates of sheep and wool growth substantially (Table 5.4, Table 5.5 and Table 5.6). Their more recent work with basal diets of untreated and ammoniated wheat straw with a range of supplements (Figure 8.5) provides further evidence of the advantages of defaunation. The data in Table 5.7 indicate that the greatest responses to defaunation were with sheep given diets based on alkali-treated straw or molasses, on which liveweight gains were increased by 37 and 54%, respectively.

Few studies have compared productivity in faunated and defaunated cattle. Results of two experiments with cattle on molasses-based diets and grazing green temperate pasture are shown in Table 5.8. On the molasses/urea diet with less than optimum bypass protein supplementation, growth rates of young cattle were increased by 43% by removing the protozoa from the rumen. The growth rate of the grazing cattle was low and was 50% higher in the unfaunated than in the faunated animals.

Recent data on protein flow from the rumen in defaunated and faunated sheep are shown in Table 3.9 (see Chapter 3). The protein-to-energy ratio in the products of fermentation is lower in sheep that have an abundance of protozoa in the rumen than in unfaunated sheep (see also Veira 1986). Therefore it can be concluded that reducing the population of protozoa in the rumen optimises animal production at lower dietary protein levels or will increase productivity on low-protein diets. The practical application of this awaits the development of systems for controlling protozoa in the rumen.

5.5 MANIPULATING DIETARY FAT 

The majority of diets based on agro-industrial byproducts, crop residues or dry tropical grasses are extremely low in lipids compared with diets used in temperate countries. For instance, cereal straws have 1 to 2% fat in their dry matter, molasses virtually none and dry tropical pastures only 2 to 3%. In comparison, temperate forages have about 5 to 10% lipids in their dry matter. Thus a cow consuming 15kg DM of temperate forage ingests between 750 and 1500g of lipid per day (see Moore and Christie 1984).

The lipids of forage plants are mainly in the chloroplasts and linolenic acid (53%), linoleic acid (13%) and oleic acid (10%) are the dominant long chain fatty acids. The intake of LCFAs by lactating cows on cereal-based diets may be lower than in grass-fed animals but is rarely less than about 500g daily. In the developing countries cereal straws are the basis of livestock feeding and, in general, only small amounts of supplements (oilseed cakes or brans containing some oils) are provided.  Under these conditions the intake of lipid is low. A buffalo (500kg liveweight) consuming 12kg of cereal straw (say 1.0% lipid) and supplemented with a small amount of grass (1kg dry matter per day containing 5% lipid) and 1kg of oilseed cake (3% lipid) consumes 200g of lipid, which is considerably less than that consumed by the cow on temperate grass pastures or hay/grain diets.

Dietary lipids are hydrolysed in the rumen and the components, other than LCFAs, are fermented. The LCFAs are hydrogenated by the micro-organisms but are otherwise unaffected.  LCFAs are highly digestible in the intestines (probably in excess of 80%) (see Thornton and Tume 1984). Absorption of LCFAs from the intestines is linearly related to intake.  However, more fatty acids arrive from the rumen than are consumed in the diet, indicating a small contribution from microbial (bacteria contain 10-15% lipid) or endogenous secretions (Figure 5.5).

Figure 5. 5 Effect of increasing proportion of calcium soaps on cell wall digestibility in vitro. Control: 30% timothy hay: 70% concentrate; Experimental: 25% concentrate: 65% timothy hay: 10% soya oil fatty acids as oil or calcium soaps (Source: Palmquist and Jenkins 1982)

It may be beneficial to add fat to a diet to increase its energy density. However, fat is absorbed on particulate matter in the rumen and appears to ``protect" fibre from fermentation (Harfoot et al. 1974) or is toxic to cellulolytic organisms (El Hag and Miller 1972). Both effects reduce the digestibility of fibre in the rumen.

The reduction in in vitro cellulose digestion caused by maize oil (Brooks et al. 1954) was apparently reversed by applying high levels of calcium (White et al. 1958), which led Palmquist and Jenkins (1982) to examine the effects of fatty acids versus soap on digestibility of fibre. The effect on the in vitro digestibility of fibre of replacing soya oil fatty acids with calcium soaps of the same oil is shown in Figure 5.5.

In order to use lipids in supplements for ruminants they must be either protected from rumen fermentation or converted to soaps. In addition, the balance of other nutrients for production (particularly amino acids and glucose precursors) should be adjusted. Under these conditions dietary fat is efficiently deposited in sheep on straw-based diets (van Houtert and Leng 1987).

In dairy cows under temperate-country management, the partial feed efficiency for lactation ranges from 55% at pasture to 60-70% on high-grain diets and up to 85% when protected fat is fed (Kronfeld 1976; Kronfeld et al. 1980) (for description of protected fat see Ferguson 1975). The increase in efficiency of utilisation of feed when supplemented with lipid appears to be due to a direct incorporation of C4 to C16 fatty acids into milk fat, which reduces the incorporation of acetate into the milk fat and therefore reduces the amount of glucose oxidised for NADPH production (Kronfeld 1965; Story 1970; Steele and Moore 1968).  Evidence for the direct incorporation of dietary fatty acids into milk fat is given in Table 5.9.

Kronfeld (1976) calculated that milk production was most efficient when 16% of the metabolisable energy of the diet was in the form of LCFAs.

Figure 5.6 Relationship between fatty acids reaching the small intestine and fatty acid intake (Source: Thornton and Tume 1984)

The manipulation of dietary fat (either as soaps, insoluble prills of high molecular weight LCFAs, or protected with protein-formaldehyde) is an area of research that is particularly appropriate to the dairy industries in developing countries because of the predominantly low-fat diets used.

 Back to top