Chapter 9

PASTURE-BASED FEEDING SYSTEMS

9.1      PASTURE AS A FEED RESOURCE

There are a number of situations in developing coun­tries in which pasture plays a significant role in the animal production system as follows:

• Traditional grazing systems that include:

  • grazing on non-arable land

  • communal grazing close to villages

  • communal and traditional grazing of nomadic routes and pastures

• Large and extensive ranches

In this chapter, the application of basic principles of feeding ruminants is discussed in relation to the different pasture-based systems that have evolved.

9.1.1 Extensive ranching

While large ranches account for considerable areas in certain countries, especially in Latin America and in parts of Africa and Indonesia, the number of people that benefit from these activities is relatively small. A typical example is Brazil, where large areas of the Amazonian region have been converted from forest into grazing land; individual farms often reach one million hectares. The economic advantages of intro­ducing technological innovations are extremely lim­ited as it is usually more profitable to farm large ar­eas extensively than to intensify production within the area.

 

The most important issue is that in most devel­oping countries this kind of exploitation involves a relatively small proportion of both the animals and people in the country. In Asia and Africa 70 to 90% of the ruminant livestock is kept by small farmers who own one to five animals. This equates to some 120 million cattle and 180 million sheep and goats in Africa and 400 million cattle and 500 million sheep and goats in Asia. There are twice as many ruminant livestock in Africa or Asia as in Europe. The own­ers of these livestock represent the major target for development aid and yet these people have probably benefited least from technical innovations.

 

Pastures in the tropics and subtropics grow rapidly during periods of heavy rainfall and high tempera­tures, leading to mature pasture plants containing high levels of cell wall constituents. Pastures are young and green (ie. high in N, soluble carbohydrates and fats and of high digestibility) for only short pe­riods. The nutritive value of the pastures decreases rapidly with maturity and, during the dry seasons, the available feed is of low digestibility and low in total N. The major factor that limits animal produc­tion from these pastures is the fact that the animals lose weight in the dry season due to the nutritional imbalance in the feed available (see Figure 9.1).

 

In most developing countries pasture management is restricted largely to grazing or burning or a com­bination of the two. The pastures usually· consist of indigenous species which, in general, are low in pro­tein, have relative low digestibility and, depending on soil type, low concentrations of minerals.

 

Stobbs (1969) suggested that the lower animal pro­ductivity from tropical pastures than from temperate pastures may be related to the more erect growth habits of most tropical grasses and legumes. The low proportion ofleafy material in these pastures appears to limit harvest ability and intake of the plants by grazing animals. As the grasses flower and mature, forage quality declines because of translocation of sol­uble carbohydrates from the stem and leaves to the inflorescence, thereby increasing the relative propor­tion oflignified cell walls in the leaves and stems. The digesti bili ty of tropical pasture species declines much more rapidly than that of temperate species. Flow­ering in legumes is not usually associated with such large changes in nutritive value as in grasses despite leaf loss due to senescence.

 

There are several other reasons why legumes may increase animal productivity. Legumes increase the dry matter production of the associated grasses due to higher soil N. Legumes have higher mineral con­centrations than grasses and there is the potential contribution of seed retained on the plant. The seeds of many legume species contain up to 45% protein, up to 0.8% phosphorus, 0.3 to 0.5% sulphur and have a digestibility of 50-80%. They have also a high lipid content which may be of considerable impor­tance where the grasses are mature and dry. Legumes also tend to remain green into the dry season, thus prolonging the grazing period.

9.1.3      Management of legumes In pastures

Introducing legumes into pastures is one option for increasing animal productivity (see Table 9.1).

 

 

How­ever, the method of introduction, the species intro­duced and the management needed to retain the legumes in the pasture differ according to soil and climate. For these reasons the transfer of technol­ogy has often failed even within a country (Mannetje 1982).

 

Management is a major constraint in the humid tropics. For instance, in research reported from In­donesia (Blair et al. 1985) marked changes in legume content in sown pastures were observed with differ­ent management systems. Grazing progressively re­duced the proportion of legumes in the pasture from 35 to 5%, with associated increases in the invasion by weeds and by Imperata cyclindrica, which is particularly difficult to manage. By contrast, grazing did not affect the proportion of native legume in the indige­nous pastures, but the proportion of legumes in these pastures was initially low. This bears out Mannetje's viewpoint that:

• "Sown grass/ legume pastures are persistent and productive provided they consist of adapted species, are adequately fertilized and are not overgrazed" (Mannetje 1984)

All these factors are almost impossible to ensure in tropical developing countries.

 

The data in Table 9.2 and Figure 9.2 show an­imal production from pastures in three contrasting climatic environments.

 

 

 

The data indicate that:

• Although average daily gain of animals in all rainfall areas can be increased by introducing legumes and/or fertilizing the pastures, animal productivity is still well below the genetic po­tential

• Animal production per hectare can be increased substantially by increasing stocking rate accord­ing to the increased availability of forage dry matter

• The low production in Australia is largely at­tributable to seasonal differences in rainfall; ani­mals grow about three times faster on green and fertilized pasture than on the dry pasture during the rest of the year (see Figure 9.3).

 

 

Apart from the agronomic problems of retaining legumes, it is apparent that their presence in the pasture does not overcome the inherent nutritional constraints. The relatively small increases in pro­ductivity when legumes are included in a pasture, compared with the responses that can be achieved on dry pastures when the animals are supplemented with meals providing bypass protein (and minerals, partic­ularly phosphorus) (see Table 9.7), indicate that the legumes are not providing the critical bypass nutri­ents but merely fermentable N.

9.1.4       Use of fertilisers to Improve grassland

On soils of low fertility, fertilisers undoubtedly in­crease dry matter production per hectare. The most widely used fertilisers are superphosphate (S and P) and nitrogen. The increase in animal productivity from pastures fertilized with superphosphate is es­pecially noticeable in the tropics, where phosphorus­deficient soils are common. In general, phosphorus fertilisers are used at low rates to promote legume growth and thus increase pasture productivity.

 

The use of nitrogen fertilisers to maximise herbage production has generally been applied in intensively managed systems in which high value products (eg. milk) can justify the economic cost.

 

Table 9.3: Indicative amounts of feed biomass re­quired to fatten cattle to market weight (ie, 100-4 OOkg liveweight) at pasture assuming a range of growth rates and approximate conversion ratios (FeR in kg DM/kg L Wt gain).

Feed biomass LWt gain (kg/d) FeR (kg/kg) (tonnes DM)*

     .2                                            25                            7.5

     .4                                            15                            4.5

    . 6                                            12                           3.6

     .8                                              9                            2.7

   1.0                                             6                            1.8

 

9.1.5     Pasture improvement and stocking rate

In developed countries with mainly pastoral economies (eg. New Zealand and Australia), the approach to improving animal productivity has been mainly to maximise production per unit area, which has usually meant sacrificing productivity per ani­mal (Figure 9.2). The next developments must aim at increasing individual animal productivity by iden­tifying the major imbalances in nutrients absorbed by grazing ruminants and using supplements in order to correct the deficiencies.

 

The data in Table 9.3, derived from assumed rates of growth and feed conversion, indicate the differences in the quantities of feed biomass needed to raise ani­mals from weaning to market weight (ie. from 100 to 400kg) at different growth rates, and emphasise the inefficiencies of a strategy aimed at increasing pro­ductivity per unit area at the expense of individual animal productivity.

 

The foregoing discussion has highlighted the issues that should be addressed in seeking to increase ani­mal productivity on pasture, including the capacity of the forage to support efficient rumen function and to provide additional bypass nutrients and long chain fatty acids. These critical factors are not defined by chemical analysis alone.

 

9.2 Nutritive value

9.2.1     Supplementation of ruminants on green pastures

The major nutritional differences between tropical and temperate pastures are that temperate pastures are usually higher in protein and soluble sugars and lower in cell wall components and therefore are more digestible. Even on temperate pastures that are of high digestibility and contain legumes, growth rates of lambs are usually considerably lower than the ge­netic potential and sometimes less than that obtained on correctly supplemented crop residues (see Chap­ter 8). Growth rates and milk production of cattle and sheep on tropical pastures are usually lower than those of animals grazing temperate green pastures (Walker 1987).

 

Figure 9.4 illustrates the relationship between vol­untary feed intake and digestibility for a wide range of tropical and temperate pastures. At a digestibility of about 60%, which is high for tropical pastures, the forage intake of sheep is greater on tropical pasture than on temperate pasture of the same digestibility (Minson 1980).

 

In the humid tropics, in order to maintain the di­gestibility of pasture at a relatively high level, grazing pressure should be increased to a level at which the pastures are kept young and vegetative. However, this reduces the amount of feed on offer, which re­duces intake. The greatest risk is that the animal will not have enough feed for maintenance if pasture growth rate slows because of drought. In these conditions, appetite may be limited also by fatigue resulting from the work needed to gather the pasture available (McClymont 1967).

 

On the other hand, in many tropical livestock systems, animals are corralled at night and are fed cereal straws and other residues that are low in fermentable N and protein. Consuming small amounts of leafy oasture during the day would provide the nutrients needed to support the rumen ecosystem, and hence permit more efficient use of the crop residues fed at night.

 

If stocking intensity is low on tropical pastures, the proportion of mature vegetation in the pasture increases rapidly. The mature vegetation is often unpalatable and, after a certain stage of maturity, the the pasture is either not grazed or only the new shoots are grazed. Under these circumstances the cattle graze a much smaller area of land than indicated by stocking density and this area is heavily over grazed, resulting in low feed intake and low productivity. The custom of burning grasslands presumably arose from this tendency of ruminants to concentrate their grazing on small areas of land. Burning the ungrazed areas induces regrowth and encourages grazing over a widerare and also controls tick infestation to some extent (Chapter 10).

 

At high stocking rates on green pastures, production is probabably limited by availability of herbage and therefore a supplementary feeding programme should aim to supply a balanced feed. Where pasture availability is not limiting, productivity is probably constrained by low feed intake because of low digestibility and/or imbalance of absorbed nutrients in relation to the animals' requirements. Supplementary feeding therefore should aim to balance the absorbed nutrients to stimulate or maintain, rather than depress,the intake of the basal (low cost) pasture. The development of a rational supplementary feeding practice requires  a knowledge of the factors that influence the balance of nutrients from rumen fermentation according to the principles discussed in Chapters 3, 4, 5 and 7.

9.3 Feed intake at pasture

Apptetite control in grazing ruminants is complex but the most important factors are:

• The balance of protein relative to energy in the products available for absorption

• The rate of digestion of feed

• The rate of comminution of feed particles to sizes that can readily flow from the rumen.

Voluntary intake of tropical pastures by ruminants varies from 30 to 85g/kg^0.75 per day (1 to 3% of body weight) according to the pasture and time of the season. Intake on tropical pastures is generally low compared with that recorded for immature temperate pastures. For example, Minson (1980) reported an intake of 140g/kg ^0.75 per day for a temperate pasture. This suggests that there are basic differences between tropical and temperate pastures. This may be in the availability of nitrogen (both fermentable and bypass protein N) and minerals (depending on soil type), which are generally higher in temperate than tropical plants.

 

The primary factors limiting feed intake on pas­ture will determine the supplements to be used. If digestibility of the pasture is low and distension of the rumen is limiting feed intake, the supplement must have a 'low rumen load' (ie. either rapidly fer­mented or is rapidly passed to the lower tract, where it is readily digested). Soluble sugars (eg. molasses), high digestibility seed grains such as lupins and whole cottonseed, and high digestibility byproducts such as citrus pulp and sugar beet pulp might be used to increase energy intake and provide extra digestible protein and VFA energy through fermentation and growth of micro-organisms in the rumen.

 

Feeding molasses to cattle on green pastures can be highly beneficial and, provided that the pasture has a high N content, additional fermentable N is not needed. But increasing the content of soluble sugars, or other rapidly fermented carbohydrates such as grain, in a forage diet may encourage the growth of protozoa in the rumen, which may reduce the amount of microbial protein available to the animal and may also decrease fungal colonisation of plant cell walls and thus decrease the rate of digestion (see Chapters 3 and 5). This kind of supplementation can also depress cellulolysis by reducing the rumen pH or be­cause of a "soluble carbohydrate" effect (Mould et al 1983/84).

 

Chopping et al. (1970), working with dairy cows grazing irrigated and fertilized pangola grass, showed that feeding 3-4 kg of molasses daily to cows increased milk production by up to 2.5 litres per day. At higher levels of molasses feeding there was a marked decline in response, which did not occur when the animals were given cereal grain as the supplement. However, generally poorer results for such molasses supplemen­tation were reported by Preston and Willis (1974) for cattle in Cuba. The difference between the two stud­ies was probably due to the higher protein content of the irrigated pasture.

9.4  Supplementation

There are reports of responses in cattle on dry subtropical pastures to sodium chloride supplements (Murphy and Plasto 1973; see also Butterworth 1985). Under these circumstances of apparent sodium deficiency, it is claimed that supplementation with urea is ineffective unless accompanied by sodium supplementation.

 

Often supplements such as poultry litter or bone meal can provide the major minerals required al­though salt would still be needed. Molasses-based supplements (eg. molasses/urea blocks) will usually meet the animals' needs for calcium, sulphur and trace elements.

9.4.3      Fermentable nitrogen supplements

The main effect of a marked deficiency of fermentable N is to reduce feed intake (see Campling et al. 1962), due to a reduced rate of fermentation in the rumen. Supplying urea to maintain high levels of ammonia in the rumen (ie. > 200 mg ammonia N/litre) should optimise intake, increase the rate and extent of diges­tion and increase microbial protein yield relative to VFA production (see Chapters 3 and 4).

 

Urea has been widely used as a source of fer­mentable N to correct ammonia deficiency in the ru­men, and although there is, in theory, a considerable need for additional fermentable N under extensive grazing conditions, responses to urea supplementa­tion have been unpredictable (see Leng et al. 1973; Loosli and McDonald 1968). In recent times, lupins (which are 30% protein) have been used successfully in Australia to provide both fermentable N and high quality fibre to grazing sheep.

 

One of the major problems associated with supple­menting grazing cattle and sheep with urea/molasses mixtures is that nitrogen deficiency in the rumen is neither easily recognised nor predicted. The N con­tent of the pasture or faeces, or even the moisture content of the faeces, may be indicative of when to begin supplementation. Ammonia concentration in rumen fluid is probably the best guide (Stephenson et al. 1984).

 

A urea/molasses mixture can be used to provide fermentable N, but there are large differences among animals in the amount that they consume. For in-

stance, in one trial, 20 to 50% of the sheep did not consume a liquid urea/molasses mixture fed under extensive grazing conditions (Nolan et al. 1975); and 20% of cattle did not consume a urea/molasses block (R A Leng, unpublished data). In addition, animals often consume the mixture only sporadically, which may lead to large fluctuations in rumen ammonia lev­els, from above to less than the optimum for efficient rumen fermentation (see Chapters 3 and 5). On the other hand lupins appear to be readily sought after by grazing livestock.

 

Where urea has been sprayed onto low nitrogen pasture or infused continuously into the rumen of grazing animals, large increases in the intake of the basal diet have been observed. Supplying urea con­tinuously to animals on diets based on low quality pasture has increased intake of the basal feed but has seldom led to growth rates of much above mainte­nance (Leng et al. 1977). Stephenson et al. (1981) demonstrated that supplementing ewes grazing dry pasture in a tropical semi-arid environment with urea in the drinking water increased lamb survival, milk yield and lamb growth (see Table 9.4 and Table 9.5). This is somewhat surprising since in theory, a peak in rumen ammonia after drinking would have been fol­lowed by low ammonia concentration during the rest of the day. However, it may be explained if rumen bacteria are able to store a pool of nitrogenous ma­terials to draw on when rumen ammonia levels are low.

 

 

 

 

With ewes fed dry, low protein pasture hay the birth weight of the lambs was increased from 2.9 to 3.2kg by supplementation with urea either in the feed or in the drinking water (Stephenson et al. 1981) (Ta­ble 9.5). In Northern Australia recommendations to put urea in drinking water are now made on the basis of the rumen ammonia eoncentration in a sample of 6 to 10 sheep from a grazing flock. The value of this approach is illustrated in Table 9.6.

 

 

 Stephenson and Bird (1987) have recently taken their research fur­ther. \\'ith pregnant ewes under extensive grazing of dry native pastures and force-fed supplements daily from 115 days pregnant, the researchers showed that urea/ammonium sulphate supplements were effective in increasing lamb birth weight but found that ad­dition of meat meal with molasses further increased lamb birth weight (Table 9.7).

 

 

The difficulties of ensuring regular intake of urea by ruminants at pasture suggest that at times it might be advantageous to use protein meals, or even high protein grains such as lupins, to provide additional fermentable N. The fermentable N can be supplied directly by feeding a source of protein that is slowly but steadily fermented. At the same time, the amount of urea entering the rumen indirectly from blood is in­creased (the extra urea arises from the deamination of absorbed amino acids). Under these conditions protein meals have many roles, ie. to supply dietary amino acids for absorption and to provide sources of slowly available fermentable N, amino acids and pep­tides for microbial growth in the rumen.

9.4.4 Bypass nutrients supplements

Although the needs for fermentable N and bypass protein have only been quantitatively determined in recent years, it has long been recognised that feeding a protein meal to grazing animals in the dry season is highly beneficial. Marston (1932) demonstrated that supplementation with blood meal markedly in­creased wool growth of sheep on dry pastures in northern Australia, and Murray and Romyn and their co-workers in Zimbabwe (Murray €i al. 1936; Mur­ray and Romyn 1937, 1939) showed that the annual decline in condition of range cattle during the dry winter months was due to a deficiency of protein in the pasture during the earlier winter months and to a deficiency of both protein and energy during the later part of the winter. However, the latter con­clusion may perhaps be modified as they failed to provide fermentable N, which modifies the effects of the protein meal. Figure 9.1 shows a typical effect of supplementing cattle on dry winter feed with by­pass protein. Hennessy et al. (1981) and Lindsay and Loxton (1981) have since shown that giving a bypass protein meal to cattle on a diet of dry Carpet grass hay and dry Spear grass hay (45% digestible, about 0.5% N) substantially increased the feed in­take of the cattle. The results of two experiments in which urea/sulphur and bypass proteins were given to growing Hereford heifers and pregnant Brahman cross cows, respectively, are shown in Table 9.8 and Table 9.9.

 

 

Under subtropical grazing conditions, similar re­sponses have been observed in cattle given supple­ments of bypass protein in the absence of a fer­mentable N supplement (Table 9.10). Due to their greater weight gain, heifers that received supplements during the dry season reached sexual maturity one year earlier than the unsupplemented heifers. The age of heifers at first calving and their fertility in subsequent years obviously have a large effect on the animals' productivity.

 

 

 

Improvement of lifetime productivity of cows could be the most important de­terminant of whether supplements should be fed to cows grazing dry tropical or subtropical pastures.