Chapter 9 Back to contents

 

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:

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).

 

Figure 9.1 : Monthly live weight change of cattle given no supplements or supplemented with groundnut cake (Source: Murray et al. 1936)

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 prtein, 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.

 

In the heavily populated Third World countries, the land available for grazing is limited and is fre­quently of low fertility. There is considerable scope for increasing animal productivity from grass land through management. The options include:

A major problem is how to apply management strategies on communally owned land. For instance, grazing pressure is rarely controlled and fertiliser is almost never applied.

 

Agronomic aspects of pasture improvement are outside the scope of this book. The discussion is rstricted to strategies that are specifically aimed at improving the balance of nutrients for animal pro­ductivity, ego strategic supplementation, the intro­duction of legumes and use of fertiliser to increase the availability of critical nutrients in the diets of grazing or tethered ruminants on pasture-based diets.

9.1.2     Improving pastures with legumes

The role of legumes in improving pasture quality and animal production from both temperate (Ulyatt 1980) and tropical pastures (Milford 1967) is indis­putable. Animal production is nearly always greater from legume-based pastures than from pure grass pastures (Mannetje 1984; Thomson 1977; Walker 1987). A most important attribute oflegumes is that their digestibility declines more slowly with matu­rity and environmental temperature than does that of grasses (Minson 1980).

 

The use of adapted legumes to improve the quatity and quality of forage from tropical pastures has been demonstrated in many environments. In some, the introduction of legumes to native pastures has substantially increased animal production without any fertiliser input. For instance, Clatworthy and Holland (1979) observed a 53% increase in liveweight gain of cattle on Hyparrhenia pastures in Zimbabwe when legumes were introduced. Stobbs (1966, 1969) reported increases of 11 to 49% from similar pas­tures in Uganda. Modest inputs of fertiliser in ad­dition to introducing legumes have further increased liveweight gain (see Shaw and Mannetje 1970)' and, more importantly, increased the reproductive rate of cattle (see Holroyd et al. 1977). The increase in animal production is not only due to higher body­weight gains but is also due to higher carrying capac­ity, which may increase two- to six-fold. Beef pro­duction of between 260kg and 500kg liveweight per hectare per year has been obtained from sown trop­ical grass/legume pastures (Mannetje 1984). Milk production from tropical pastures and the composi­tion of the milk produced are certainly influenced by introducing legumes.

 

The increase in animal production from including legumes in tropical pastures is not attributable to a single factor. There are complex interactions between the extra dry matter available for grazing and the increased N content of the herbage selected.

 

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).

 

Table 9.1: Lambs grow faster on legume than on grass pasture

 

Grass

Mixed grass/legume

 

Legume

USA

127

172

213

New Zealand

57

 

172

New Zealand

122

 

254

New Zealand

37

128

191

Australia

82

 

159

Australia

83

 

148

Source: Archer (1980).

 

 

 

Hoever, 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 diffeent 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:

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.

 

Figure 9.2: Cattle growth on pasture is a function of pasture type, fertiliser application and legume con­tent. Productivity per unit area is maximised for the different pastures at different stocking rates: 89kg/ha for native pasture, 223kg/ha for tropical grass with legume, 682 kg/ha for tropical grass with fertiliser and on temperate pasture (clover) 1051 kg/ha (Source: Walker 1987).

 

Table 9.2: Cattle production from native (NP) and imp1>oved pasture (Imp) with and without legume and N fertiliser. The genetic potential for growth of the animals was presumed to be 0.6, 1.2 and 1.2kg/d for the three Rainfall areas A, Band C respectively

 

Grass only

Improved pasture

 

NP

Imp

+ legume

+N

Rainfall (A) 2Scm

 

 

Stocking rate, /ha

#

 

4

8

LW gain, g/d

160

 

280

360

LW gain/ha (kg/yr)

 

 

410

1050

Rainfall (B) 32cm

 

 

 

Stocking rate, /ha

 

4.2

4.2

4.2

LW gain, g/d

 

240

300

380

LW gain/ha (kg/yr)

 

367

452

595

Rainfall (C) 7.2cm

 

 

 

Stocking rate, /ha

0.3

0.7

1.1

1.0

LW gain, g/d

310

280

420

420

LW gain/ha (kg/yr)

30

76

167

167

Source: Mannetje (1982). .#Village: conditions

 

The data indicate that:

 

Figure 9.3: Seasonal variation in liveweight gain re­sponse of cattle grazing Cenchrus ciliaris (Source: Mannetje 1982).

 

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 icrease 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 anmal (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.

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

LWt gain (kg/d)

FCR (kg/kg)

Feed biomass (tonnes DM)#

0.2

25

7.5

0.4

15

4.5

0.6

12

3.6

0.8

9

2.7

1.0

6

1.8

# From 100 to 400 kg LW

 

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).

 

Figure 9.4: Relationship between voluntary intake and dl'y matter digestibllity fol' a range of tropical and temperate grasses (SoU7'ce: Minson 1980, 1982)

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:

Voluntary intake of tropical pastures by ruminants varies from 30 to 85g/kg0.75 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 supplemetation 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

9.4.1      General supplementation practices

Although productivity per animal from tropical pas­ture is usually less than that from temperate pas­tures, it is almost always uneconomic to feed supple­ments. However, as the pasture matures and turns into dry standing forage, individual animal produc­tivity declines and bodyweight losses are often excessive (see Figure 9.1). The digestibility of dry standing forage deteriorates with time and with the incidence of dews or light rain (which promote sapro­phyte growth on the pasture). Often these standing hays are less than 45% digestible and exceedingly low in soluble carbohydrates and total protein (usually around 0.5% N). They are not markedly different in composition from many cereal straws. Depending on the soil and the incidence of rain, they are often low in particular mineral elements and may contain oxalates which form highly insoluble calcium salts in the rumen.

 

The constraints to ruminant production on such feeds are obvious and a clear set of principles for sup­plementary feeding can be suggested.

 

In considering supplementary feeding practices, the first step must be to establish the order of likely con­straints. Listed below are the factors that are be­lieved to be the main limitations to the utilisation of dry feed by ruminants, which are similar to those discussed for straw-based diets. At times any one of these may be the first limiting factor to production:

Many of these factors are interrelated. For instance a lower-than-optimum fermentable N content in the diet may decrease digestibility as well as resulting in a low ratio of amino acids to energy in the absorbed nutrients. Increasing the availability of fermentable N increases the digestibility and the protein-to-en­ergy ratio in absorbed products because of increased efficiency of fermentation in the rumen, and both ef­fects lead to increased intake of pasture.

9.4.2 Mineral supplements

Mineral deficiencies are not easily predicted because of the variability in mineral content of pastures, which depends on soil, climate, previous stocking his­tory and fertiliser application. Minerals can usually be added to a diet relatively cheaply, and in practice minerals can be provided by a supplement or block lick. It is emphasised that mineral requirements are low in animals that are only maintaining weight, and mineral deficiencies will only become apparent when other limiting factors have been removed and the an­imal has the potential to grow or produce.

 

Low cost improvements of native pastures has cen­tred in Australia on augmentation with legumes and application of phosphate fertiliser. However, it has been shown that where the legumes can be sustained without fertiliser application then it is more economic to supplement the animals with phosphorus than to apply it to the soil (see Walker 1987).

 

There is a vast amount of literature on mineral de­ficiencies in grazing livestock (see Butterworth 1985). Often, recommendations for mineral supplementation are made without considering other more limiting constraints. Thus, if animals are short of feed, or there is an imbalance of essential nutrients (eg. am­monia for rumen fermentation), correcting a mineral imbalance is unlikely to affect animal performance (see Figure 8.26).

 

The high levels of oxalates in tropical pastures may reduce the availability of calcium for absorption, which at times could limit production. Horses on tropical pastures, unless supplemented, exhibit symp­toms of calcium deficiency, including the condition 'big head' (McKenzie et al 1981), but ruminants that are adapted to such diets have bacteria in the rumen that can degrade oxalate. Blaney et al (1982) demonstrated that the availability of calcium from tropical grasses, in which oxalate is present, is about 60% of that of grasses containing little oxalate.

 

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.

 

Table 9.4: The effects of supplementation of ewes grazing dry native pasture with urea on milk yield, lamb mortality and the growth of surviving lambs.

 

No supplement

Urea in drinking water (2g/litre)

Ewes

 

 

Milk yield (ml/4h)

111

123

Lambs

 

 

LWt gain (g/d)

42

76

Mortality (no.)

9

0

Source: After Stephenson et al. (1981).

 

Table 9.5: Birth weight and liveweight gain of lambs, and feed and nitrogen intake, milk' yield and liveweight loss of ewes grazing low protein, dry Flinders grass (hay) alone (A) or with access to drinking water with 2.2g/1 of urea (B) or fed Flinders grass plus urea ( C).

Attribute

Hay

(A)

Hay + urea
(B)

Hay + urea
(C)

Ewes

 

 

 

No. lambing

20

20

20

Feed intake (g/d)

900

1190

1250

N intake (g/d)

8

15

18

LWt change (kg)

-12

- 8

- 9

Ewes milked

11

 

10

Milk yield (ml/4h)#

60

ND

94

Lambs

 

 

 

Mortality (%)

40

20

20

Birth weight (kg)

2.9

3.2

3.2

LW gain (g/d)

35

81

84

Source: After Stephenson et al. (1981)
#
Mean yields measured on days 1, 11 and 21

 

 

 

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.

 

Table 9.6: Rumen fluid ammonia concentration and milk yield of unsupplemented and urea-supplemented ewes (urea in drinking water).

 

Rumen fluid ammonia
mg/100ml

Milk yield of ewe

Lactating ewe

 

 

+ urea

5.1

540

No urea

1.9

440

Non-lactating ewe

 

 

+ urea

6.6

 

No urea

5.7

 

Source: Stephenson et al 1984

 

 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).

 

Table 9.7: The effects on grazing ewes of N supple­mentation. Each ewe was drenched daily from 115 days pregnant with one of the following: water, a solution of urea plus ammonium sulphate (7 + 4)g (UI5) or a mixture of urea + meat meal + molasses (3 + 100 + 100)9 (lJ15 + MM).

 

Supplements

 

None

U/S

U/S+MM

Ewe LW change (g/d)

38

80

131

Rumen NH3 (mM/litre)

7

10

9

Lamb birth Wt. (kg)

3.5

3.7

4.0

Source: Stephenson and Bird (1987).

 

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.

 

Table 9.8: Dry matter intake and liveweight change of cattle (170kg liveweight) fed speargrass hay (predom­inantly Heteropogon contortus) supplemented with urea and/01' f01'1naldehyde-protected cottonseed meal (protected-C5M)

 

Dry matter intake (kg/d)

LWt change (kg/d)

Hay + urea

3.01

-0.32

Hay + protected CSM

3.72

+0.11

Hay + protected CSM + urea

4.43

+0.22

Source: Lindsay et al. (1981)

 

Table 9.9: Mean liveweight change and dry matter intake of pregnant cows (415kg liveweight) fed spea1'­grass (hay) supplemented with urea/sulphur (U /5) and a bypass protein meal (P P). The experiment was carried out over the last 60d of pregnancy; the hay was of low digestibility and contained 0.4 % N.

 

Hay DM

LWt

Calf birth

 

intake

change

weight

Diet

(kg/d)

(kg/d)

(kg)

Spear grass

4.2

-0.8]

22

Spear grass+U-S

6.2

-0.31

31

Spear grass+U-S+PP

8.1

+0.75

32

U-S supplied 55g Njd.

PP supplied 1kgjd of a protein pellet containing 80 % cottonseed meal, 10% fish meal and 10% meat meal (protected with formaldehyde)

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.

 

Table 9.10: The effects on the mature body size of protein supplementation (during dry season) of cattle . All calves were the same weight in 1977.

 

Mean live weight, kg

 

1978

1981

1982

Feeding system:

 

 

 

Native pasture (no supplement)

197

329

320

Native pasture plus bypass protein (during dry season)

Group 1

259

378

382

Group 2#

264

397

397

Source: Hennessy (1984).

 

 

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.  

 

Supplementation with protein concentrates in the dry season significantly increased the mature bodweight of the cows (Table 9.10), which is of consid­erable importance where the objective of the system is to produce work oxen. It appears to be likely that under many conditions draught oxen are stunted by protein malnutrition in early life. Research needs to be conducted into whether periods of protein defi­ciency in early life also reduce reproductive capacity and/or milk production.

 

During the pasture growing season, feed intake of lactating animals on tropical pasture may be lim­ited by the availability of dietary bypass protein from the pasture. Stobbs et al. (1977) showed that sup­plementing cows on a su btropical pasture (largely Rhodes grass) with 1kg/d of formaldehyde-treated casein (ie. protected from rumen degradation) in­creased milk yield (Table 9.11). The effect was at­tributed to increased pasture intake. These results indicate that the amount of essential amino acids ab­sorbed from the digestive tract of animals on such pastures is less than that needed to support high leels of milk production.

Table 9.11: Effects of supplementation with l kg/d protected (formaldehyde) or unprotected casein on yields of cows grazing Rhodes grass pasture which contained 3% N.

 

Milk

Supplement

Yield, kg/d

Fat, %

Protein, %

None

12.3

5.1

3.3

+ 1 kg casein

12.7

5.2

3.3

+ 1 kg protected casein

14.7

4.8

3.5

Source: Stobbs et al 1977

9.4.5 Other supplements

The possibility of using supplements with a low 'ru­men load' and which contain high levels of digestible carbohydrates or protein appears to be a new con­cept. Such supplements would leave the rumen quickly and have little effect on rumen distension and therefore intake of the basal feed should remain un­changed. In addition, energy and protein that avoids fermentation and is digested in the lower intestines is used more efficiently by the animal than when di­gested in the rumen.

 

Studies by Elliott et al. (1978a) showed that, in cattle on sugarcane-based diets, rice grains in a rice polishings byproduct left the rumen quickly and in­tact, resulting in increased total feed intake. Glucose availability was markedly increased by supplementa­tion with this feed (Ferreiro et al. 1979).

 

There is an urgent need to develop methods of feed processing that guarantee rapid passage of such feeds through the rumen. Feeds processed in such a way would supply essential nutrients to balance those available largely from rumen fermentation. Such sup­plements are unlikely to reduce the digestibility of cellulose in the rumen.

Lupins and cottonseed meal

In the last five years considerable interest has arisen because of the large quantities of lupins that have been successfully grown and harvested in Australia. They are finding an increasing role in supplementary feeding of cattle and sheep when pastures are dry and low in protein and digestibility. In general, it seems that lupins fed at levels compatible with their use as a supplement are providing only a fermentable N source but because they are low in starch, easily handled and easily fed they have advantages for feed­ing under extensive grazing conditions. The results of two selected feeding trials are shown in Table 9.12 which clearly indicates the different roles of these two supplements. A c.ombination of cottonseed meal and lupins is a highly efficient way of providing both fer­mentable N and bypass protein.

Table 9.12: Lupins (Lup) and cottonseed meal (CSM) are excellent N supplements for ruminants on dry pasture (5-7 % crude protein and 43- 49 % digestibility). The effects of lupins are most likely to be through providing fermentable N and carbohydrates whereas cottonseed meal appears to be largely a source of bypass protein.

Species/Supplement

Supplement intake (g/d)