S.M. Miller
Introduction
Value of Mulga
Supplements for mulga
New Directions
Summary and Conclusions
References
Approximately 13 million sheep are grazed in Queensland. Of these, six million are in the southwest region of the state, producing 42% of the state's wool clip. These figures indicate the important contribution of this area to the Queensland wool industry. The major limitation to stable wool production is frequent droughts which characterise the region (Pritchard and Mills 1986). During extended dry periods, the dominant native grasses are unable to fully support animal production. Drought management consequently focuses on progressive destocking, supplementary feeding of retained animals and, in some circumstances, not joining ewes during the drought period.
The major drought forage available in southwest Queensland is the leaf of the mulga tree (Acacia aneura). Mulga was first reported to have been successfully used to maintain a flock of 60,000 sheep through a drought of two and a half years in 1886 (Everist 1949). It has both tree- and shrub-like habits and can provide a maintenance diet for sheep and cattle.
The mulga lands occupy approximately 150 million hectares across Australia and represent a large reserve of usable supplementary feed (McMeniman and Little 1974). Confined to areas of rainfall between 200 and 500 mm (Gartner and Anson 1966), mulga grows predominantly in brown and red/brown fine sands and sandy loams (Everist 1949). These soils become hard and dusty when dry and are characterised by a hardpan 30-120 cm below the surface. This feature facilitates water retention in the subsoil for long periods, a major reason for the successful adaptation and colonisation by mulga of these dry areas. The additional feature of highly sclerophilised leaves aids in water retention. Many other plant species grow in association with mulga stands including other native trees, shrubs and grasses (Everist 1949).
Four types of mulga are present in this region. Umbrella shaped mulga is most commonly used for drought feeding. Its high leaf yield and low tree density make it suitable for continuous lopping without destruction of the tree. Whipstick mulga is the name given to dense stands of immature thin trees. Utilisation is best achieved by pulling the stand with chains between tractors. Tall mulga cannot be lopped due to its tall bare trunk. Consequently, felling is the only practical means of utilisation (Goodchild and McMeniman 1986, (Figure 4.6.1). Low shrubby mulga provides useful fodder despite its relatively low leaf yield. In dense stands, lopping of low mulga is uneconomical. Consumption of fallen leaves and pods is mainly restricted to good stands of tall mulga.
Phosphorus and energy
Although a successful drought forage, a number of problems have been highlighted during sustained feeding of mulga. Based on chemical composition determinations, Harvey (1952) and Everist et al. (1958) established mulga as a maintenance feed only, despite its apparent high crude protein content (10-14%). Rohan-Jones et al. (1972) concluded from digestible energy, CO2 entry rate and rumen fluid volatile fatty acid determinations, that mulga-fed animals were in a negative energy balance.
For these reasons much of the early work concentrated on energy and mineral supplementation to lift the value of mulga above that of a maintenance diet. Everist et al. (1958) and Gartner and Anson (1966) established that phosphorus supplementation was essential to maintain a positive phosphorus balance in grazing animals since low levels resulted in mobilisation of skeletal phosphorus reserves. Little and McMeniman (1973) subsequently used the measure of bone phosphorus deposition in the ribs of sheep as an indicator of nutrient status and made recommendations regarding the need for additional mineral supplementation. McMeniman and Little (1974) showed that phosphorus supplementation of sheep increased dry matter intake (DMI), wool growth and liveweight Supplementation with molasses overcame the energy deficiencies of mulga and an additive effect was observed when phosphorus and molasses were fed together (McMeniman 1976).
Subsequently, Entwistle and Baird (1976) demonstrated that it was primarily the mineral component of molasses that resulted in increased production. They found that 66% of the response obtained when feeding 200 g/day molasses was achieved with the first 50 g/day. This appeared to eliminate energy as the major contributing factor although there were still significant differences between liveweight gains on 200 g/day and on 50 g/day of molasses.
Sulphur and other minerals
Hoey et al. (1976) established that sulphur in molasses was responsible for approximately 50% of the response gained from the first 50 g/day. This conclusion was supported by Gartner and Niven (1978), who found that sulphur supplementation increased DMI (38%) and wool growth (45%). Sulphur in the form of CaSO4 produced the most significant (Hoey et al. 1976) and calcium was shown not to be involved (Gartner and Niven 1978). The other minerals present in molasses (calcium, copper, zinc, manganese, sodium, iron, magnesium and potassium) had no significant effect on production (Hoey et al. 1976). The primary mode of action of the supplements in increasing production was through increased DMI and not increased digestibility (Harvey 1952, Niven 1983).
Fig. 4.6.1. Sheep grazing felled mulga in the Charleville region.
Nitrogen
McMeniman et al. (1981) measured rumen ammonia levels of 3.94.7 mg/100 ml in sheep on high mulga intakes, levels below the 5 mg/100 ml necessary for efficient rumen function (Satter and Slyter 1974), suggesting that the supply of intestinally available protein was inadequate. Nitrogen, made available as cotton seed meal, increased DMI and dry matter digestibility of mulga with subsequent increases in liveweight and wool growth. However, a urea supplement produced no response in mature sheep (Entwistle and Baird 1976) and a limited response in young sheep (McMeniman et al. 1981), thus suggesting that a true protein source is necessary to maintain the condition of animals on a mulga diet (Niven and McMeniman 1983). Protein supplementation has the additional benefits of increasing reproductivity and lamb survival.
Tannins
Chemical analyses of mulga showed that nitrogen and sulphur were present at adequate levels. Gartner and Hurwood (1976) subsequently reported that the presence of tannin (7-10%) was reducing mineral availability by complexing with leaf protein following maceration of the leaf tissue. Tannins are secondary metabolites with a high capacity to form complexes with proteins. These complexes are stable at rumen pH and remain undegraded in the rumen, resulting in reduced protein availability, thus limiting animal production.
There are two types of soluble tannins present in a large number of plant species. These are the hydrolysable tannins (HTs) and the non-hydrolysable or condensed tannins (CTs). HTs are characterised by a central carbohydrate core with a number of phenolic carboxylic acids bound by ester linkages. As a direct consequence of their polyester structure, the molecules can be hydrolysed into simpler fragments (McLeod 1974). HTs are present in many plants and have been shown to be responsible for growth inhibition of agriculturally important animal species. Unlike HTs, CTs have no carbohydrate core, but rather they are derived from the condensation of flavonoid precursors without participation of enzymes. CTs are more widely distributed in higher plant species than the hydrolysable variety and are thought to be more active in precipitating proteins. Since mulga contains mainly CTs, the remainder of this review will refer to this class of tannins.
Reversible associations are formed between CTs and many other substances (for example, alkaloids and proteins), with an affinity determined by both molecular mass and molecular configuration (McManus et al. 1981, 1985). The affinity for substances increases with increasing molecular weight. Such bonding is dynamic with individual linkages being broken and reformed randomly and continuously (Barry and Manley 1986, McLeod 1974). Griffiths (1982) demonstrated that interactions are pH dependent with complexes becoming unstable at both high and low pH.
Maceration of plant material results in the formation of tannin:protein complexes and the creation of a dynamic pool of free CTs. Tannins in this pool are responsible for the inhibition of enzymes, microbes and proteins in the digestive tract. Barry and Forss (1983) define the pool of free CTs as that which has exceeded the binding capacity of plant proteins. This pool is interchangeable with the tannin:protein complexes. These tannins influence plant matter digestibility, acting in a number of ways to achieve this (McLeod 1974, Schaffert et al. 1974, Barry 1989, Horigome et al. 1988):
· by interfering with digestive through the formation of tannin:enzyme complexes (Feeney 1969),· by interfering with digestive enzymes through the formation of substrate protein:tannin complexes (Makkar et al. 1988),
· by combining with proteins in the gut wall and preventing nutrient uptake (McLeod 1974), and
· by inhibiting the growth and enzyme activity of rumen fungi, protozoa and bacteria (Akin 1982, Akin and Rigsby 1985, Makkar et al. 1988).
Each of these mechanisms contributes to reduced animal production.
Energy and minerals
Dry mixes are a cheap and easy way to provide supplements of nitrogen, phosphorus and sulphur. One to two grams of each mineral is required per sheep daily. Dicalcium phosphate, sulphate of ammonia and double superphosphate are used as the sources of minerals in a dry mix. Salt should be fed for the first 10 days as a safety precaution. If the animals have a salt craving, premature introduction to the dry mix may result in accidental death due to over-consumption of the lick with resultant nitrogen toxicity. The dry mix is fed in open ended troughs and made available to the animals at all times.
Intake of the mineral mix will vary among individual animals, paddocks and class of sheep. If intake is not optimal, consumption can be increased by the addition of molasses or water, or reducing the amount of phosphorus supplement. Reducing the salt content will reduce intake. The dry mix and associated feeding techniques are also suitable for cattle. Pregnant and lactating ewes, and sheep in poor condition may require additional energy and protein supplements. Molasses and a suitable protein meal are commonly used for this purpose.
The main effect of these dry mix supplements is to increase intake of mulga thus increasing the supply of nutrients to the sheep. Death rates are considerably reduced in supplemented compared with non-supplemented animals.
Polyethylene glycol
While feeding polyethylene glycol (PEG) as a marker substance in tannin feeding studies, Jones and Mangan (1977) observed that DMI and nitrogen digestibility were significantly increased in both sheep and cattle. Rowan and Lawrence (1986) also measured increases in the growth rate of pigs when fed soybean meal supplemented with PEG. PEG was found to bind tannins and preferentially displace protein from existing tannin:protein complexes. The net effect was an increase in the available digestible protein, resulting in an increase in nitrogen digestibility. The PEG:tannin complex is very stable and is unlikely to be broken down during passage through the digestive tract.
The dramatic increase in intake observed during supplementation with PEG is likely to be due to two factors. The immediacy of the increase may be an effect of PEG on the taste sense of the animal. PEG may, upon entering the mouth as a consequence of rumination, bind the tannins, breaking the tannin:receptor bond similarly to tannin:protein complex displacements. This would reduce the astringent nature of the fodder thus encouraging a rapid increase in intake. The second, longer term factor is the increase in microbial activity produced as a result of the increased levels of available protein in the rumen. This would lead to increased rumen digesta flow and hence intake. Despite the increases in intake, nitrogen digestibility and wool growth attributable to PEG, increases in liveweight have not been as great as those achieved with molasses supplementation (Entwistle and Baird 1976). Supplementation with both PEG and molasses maximises all of these factors (Pritchard et al. 1988, Eady et al. 1989).
Due to the cost of PEG it has not been used as a commercial supplement for sheep consuming mulga. Consequently, there is a need for cheaper alternative chemicals that will mimic all or some of the actions of PEG. Studies conducted at Charleville during 1990 were unable to identify an alternative compound. These studies are continuing as more chemicals become available.
Microbial alternatives
The diverse and dynamic population of microorganisms in the rumen presents an alternative, long-term solution to the problems associated with tannins. The detoxifying capabilities of rumen bacteria have long been exploited by animals to allow grazing on otherwise unproductive pastures. The ability of goats to survive on a mimosine-rich diet in Hawaii, one considered toxic in Australia, was recognised and has led to the isolation of DHP degrading bacteria. Inoculation of cattle with these organisms has enabled the utilisation of mimosine-rich leucaena pastures in northern Queensland (Jones and Megarrity 1986) (Section 4.4). The possibility therefore exists for suitable tannin degrading microorganisms to be isolated from adapted animals and transferred to the rumen of sheep and cattle. Bacteria capable of breaking down tannin:protein complexes have only recently been reported (Osawa 1990). Current studies at the Charleville Pastoral Laboratory have identified tannin degrading bacteria in the rumen of feral goats (Matthew et al. 1991). Further investigations are under way to assess production effects attributable to these bacteria when introduced into the rumen of sheep.
A second, longer term alternative is genetic engineering of rumen bacteria to degrade tannins. This may be achieved through the modification of pre-existing anaerobic degradative pathways or introduction of genes from anaerobic microorganisms incapable of effectively the rumen. Alternatively, genes for tannin degradation may be found in aerobic microorganisms such as soil-borne organisms inhabiting mulga leaf fall areas.
Mulga represents a significant reserve of forage for sheep and cattle, particularly during droughts. Mulga is deficient in nitrogen, phosphorus, sulphur and energy, and supplementation is necessary to maintain animal condition during prolonged feeding. The presence of tannin in the mulga leaf exacerbates the mineral deficiencies.
The tannin binding properties of PEG offer an opportunity to obtain large increases in animal production with minimal supplementation. However, cost is a limiting factor, as PEG is too expensive to be used at the levels necessary to obtain significant production increases. Therefore, economic alternatives must be found.
Long-term approaches include the isolation of suitable tannin degrading microorganisms from successfully adapted native and feral animal species and colonisation of these microorganisms in the rumen of sheep and cattle. Alternatively, genes c engineering of existing rumen bacteria to degrade tannins may be achieved by modifying existing degradative pathways or through identification and transfer of suitable genes enabling tannin degradation. These approaches will minimise the need to supplement animals on mulga diets while maximising survival and wool production.
Further research into the area of tannin:protein interactions in animals fed mulga may therefore lead to increases in feeding efficiency at minimal cost to the grazier.
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