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4. Animal nutrition and tropical forages


Nutritional requirements and forage quality
Digestibility and feed intake
Potential livestock production based on tropical pastures

Under traditional management systems, sheep and goats in the humid tropics are fed almost entirely on natural grazing and browse, sometimes supplemented with small quantities of household refuse. Animal feeding is limited by the quantity and quality of nutrients available from indigenous grass and shrub species throughout the year.

Nutritional requirements and forage quality

The nutritional requirements of sheep and goats in the humid tropics were recently reviewed by Oyenuga and Akinsoyinu (1976). These authors found that the daily metabolizable energy (ME) requirements for maintenance were approximately 100 kcal per kg metabolic weight2 for both species. Assuming normal growth and lactation yields, the daily requirements for ewes were an additional 194 kcal for lactation and 857 kcal for growth per kg metabolic weight, while for does the equivalent requirements were 187 and 591. The digestible crude protein (DCP) requirements for maintenance ranged from 0.74 to 1.96 g per day per kg metabolic weight, while for growth 0.26 to 2.2 g DCP were required per g liveweight gain. These energy and protein requirements for maintenance are comparable, though slightly lower, than the values calculated earlier by Devendra and Burns (1970).

2. One kg metabolic weight was calculated as equivalent to 0.734 kg liveweight.

A number of studies have been carried out on the quality and chemical composition of feedstuffs available in different tropical areas. In West Africa, these include studies by Oyenuga (1958, 1968), Mabey and Rose Innes (1964). Bille et al. (1970) and Boudet (1975b) and ongoing work at the Agricultural Research Station, Nungua (University of Ghana, 1975, 1976). Other work has been carried out by Dougall (1960) in Kenya, Devendra and Gohl (1970) and Devendra (1977a) in the West Indies, Sen and Ray (1971) in India, Castillo and Gerpacio (1976) in the Philippines and Devendra (1979) in Malaysia.

In addition, studies of voluntary feed intake have been carried out with cattle, buffaloes, sheep and goats. These have been based on indoor feeding trials, mainly because of management difficulties and the lack of reliable methods for measuring intake under grazing conditions. On the basis of such trials, the nutritive value of tropical grasses has been reviewed by Miller and Blair Rains (1963), Hardison (1966), Butterworth (1967) and Minson (1971). The nutritive value of tropical legumes has been reviewed by Stobbs (1971) and tropical hays by Marshall et al. (1961), but the effects of selective feeding on intake have not been studied systematically.

A review of this work indicates that tropical grasses are relatively low in energy and protein and high in fibre content compared with species in the temperate zone, largely as a result of their more rapid physiological growth and early maturation, as influenced by temperature and light (French, 1957; Osbourn in University of Ghana, 1976; Devendra and Gohl, 1970; Stobbs, 1971). Tropical legumes are subject to a similar process, though their loss of nutrients is less rapid and they generally retain higher levels at maturity than the grasses (Milford and Minson, 1966).

While energy consumption is the limiting factor on animal production in temperate regions (Holmes, 1951; Crampton, 1957). dietary protein is the primary deficiency among livestock in many parts of the world (Moir, 1963). In the tropics, both factors are often limiting and seriously impair animal performance, with protein deficiencies particularly important during lactation. The protein content of tropical forages has been shown to be generally low (French, 1957; Bredon and Horrell, 1961, 1962; Butterworth, 1967), falling rapidly with growth to the time of flowering. During the dry season, the crude protein content of tropical forage drops even further, reaching critical levels below 7%.

Protein and energy consumption are interlinked. When the protein content of forage is inadequate, food intake drops and the digestibility of energy is reduced (Campling et al., 1962; Blaxter and Wilson, 1963; Eliot and Topps, 1963; Smith, 1962). Milford and Minson (1968) found that the intake of sheep decreased when the crude protein content of their diet fell to 7%.

Rombaud and van Vlaenderen (1976) estimated the energy requirements of sheep at different physiological stages in terms of the nutrients required per 100 g dry matter (DM), and the digestible crude protein requirements in terms of percentage of DM, as shown in Figures 3 and 4. If these requirements are compared with the nutrients available from some of the common grasses and legumes in the humid zone, it appears that these forages are only adequate nutritionally when the plants are very young.

Shrubs and trees, however, often retain high levels of nutrients during the dry season, which increase with the first flush of new growth before the rains. At the time of year when the nutrients available from grass and legumes are inadequate, these plants become a particularly valuable source of feed, at least for goats. The value of browse plants and the extent of their intake have recently been reviewed by Devendra.

Digestibility and feed intake

Digestibility is affected by the botanical composition and stage of maturity of the forage and also by processing and chemical treatment (Miller, 1961). Improved digestibility means that a greater proportion of the feed is actually absorbed by the animal. Lower digestibility leads to higher voluntary feed intake in the temperate zone (Blaxter, 1967), but this relationship is less clear cut in the tropics due to the different lengths of time required to digest tropical feeds (Milford, 1967). For a number of tropical species, including Chloris gayana (Milford and Minson, 1968), Panicum spp. (Minson, 1971) and legume species (Stobbs, 1971), the level of voluntary feed intake has been shown to decrease with decreased digestibility, compounding, rather than compensating for, the effects of an inadequate diet.

Figure 3. Energy requirements of sheep at different physiological stages

Figure 4. Digestible crude protein requirements of sheep at different physiological stages

Source: Rombaud and van Vlaenderen (1976).

In general, it is rarely possible to achieve a digestibility level of over 70%. In practical terms, adult ruminants (except dairy goats) are unlikely to consume a dry matter equivalent of more than 3% of their body weight daily if fed mature forage of only 40% digestibility. It has been shown that the daily dry matter intake of goats increases considerably if digestibility increases up to 70%.

Feed intake in the tropics may also be restricted because of the high water content of the herbage. In the West Indies, for instance, Butterworth et al. (1961) found a dry matter content of 39.3% for pangola grass (Digitaria decumbens) during the dry season and only 23.4% during the wet season, and similar levels were observed in Thailand. As a result, the herbage contributes a large proportion of the total water consumed by the animals, but if dry matter content falls below 20% it is likely that the energy intake will be inadequate. Differences in voluntary intake may also be due to differences in chemical composition of the fodder, such as ash content.

Jollans (1960b) has suggested that one constraint on small ruminant production in the humid tropics might be that a great deal of grazing time is required to ingest the full bulk of herbage necessary to obtain an adequate level of nutrients. In order to increase intake, he carried out an investigation using supplementary light to extend the grazing day for a flock of wethers, but this produced no measurable effect on their growth rates.

Notwithstanding these observations, it is probable that some of the improved tropical grasses, such as Napier grass (Pennisetum purpureum), can produce relatively high digestible dry matter (DDM) ratios, above 65%, provided that they are grazed or cut at an early age. Different strains also vary in terms of digestibility and intake. Minson (1971), for example, reported from a study of Panicum cultivars that at a given level of digestibility the intake of var. Hamil was 27 to 50% higher than of var. Kabulabula. Devendra (1977b) found similarly for goats and sheep that at a given level of digestibility the intake of var. Coloniao was higher than of var. Serdang. Goats also showed higher levels of digestible dry matter consumption than sheep. With var. Coloniao cut at 16 to 19 days, the DDM ratio for goats was 74.6% and for sheep 68.8%. When the grass was cut at 21 to 28 days, goats showed a DDM ratio of 72.4%, compared to 68.4% for sheep. At 28 to 35 days, these ratios were 72.1% for goats and 64.1% for sheep; at 35 to 42 days, they were 64.3% for goats and 60.0% for sheep; and at 42 to 49 days, they were 61.2% for goats and 55.5% for sheep.

Potential livestock production based on tropical pastures

Glover and Dougall (1961) and Payne (1963) have suggested, perhaps optimistically, that milk production from cattle on tropical pastures can reach levels similar to those achieved in temperate regions. While this hypothesis is based on theoretical considerations, Hardison (1966) concluded from a critical examination of a great deal of published data that milk output from cattle on tropical pastures would be limited to 5 kg per day by the level of total digestible nutrients available, while the digestible crude protein available would be sufficient for maintenance and the production of 10 kg of milk daily. Dirven (1970) calculated that 9 000 kg of milk could be produced per ha of tropical pastures annually, while 1 650 kg of liveweight gain could be produced annually per ha from beef cattle in the tropics, compared with 1 000 kg/ha in the temperate zone. Butterworth (1963) concluded from a study of 29 forage species in Trinidad that adequate nutrients were available for commercial beef production. More recently, Holder (1967) demonstrated that milk production as much as 60% greater than Hardison's estimates could be achieved on Kikuyu grass (Pennisetum clandestinum) and clarence glycine (Glycine javanica) pastures in tropical Australia.

Table 5. Stocking rates and liveweight gains (LWG) in the humid zone of West Africa

Pasture species

Months and Seasons

No. of days

Animal Species

Stocking Rate

Liveweight gain

Source

actual

head/ha corrected

g/head equivalentb

kg/ha per day

kg/ha per period

Brachiaria

V - X w

180

cattle

5

50

40

2.00

360

Messager (1977)

XI - IV d

180

cattle

2

20

20

0.40

72


Digitaria

V - X w

180

sheep

24

24

38c

0.91

164

Brinkman (1975)

Cynodon-Centrosema

IV - XI w

250

cattle

3

30

36

1.08

270

Crowder and Chheda (1977)

XII - III d

110

cattle

3

30

15

0.45

50

 

Cynodon-Centrosema




3

30

21

0.63

230


I - XII w, d

300

cattle

4

40

19

0.76

274

Ruthenberg (1974)

Pennisetum purpureum-Centrosema

IV- XI w

250

cattle

3

30

32

0.96

240

Crowder and Chheda (1977)

XII- III d

110

cattle

3

30

6

0.18

20


Andropogon gayanus

VI - XII w

204

cattle

1.4

14

38

0.53

109

Crowder and Chheda (1977)

VI - XII w

204

cattle

1.6

16

76

1.22

248


a. One bovine estimated equivalent to 10 dwarf sheep or goats.
b. 300 g LWG for cattle estimated equivalent to 30 g for sheep.
c. Same LWG with or without concentrate supplementation.

Liveweight gains actually obtained using various forage species in West Africa are shown in Table 6. These production levels are still substantially lower than the potentials calculated by the authors quoted above.

Little information is available on the efficiency of energy and protein conversion among goats and sheep in tropical environments, though several estimates have been published for sheep in the temperate zone (e.g. Holmes, 1970). Devendra (1978a) has estimated the efficiency of energy and protein conversion among goats, as shown in Table 6,

Table 6. Estimated efficiency of energy and protein conversion among goats

 

Conversion Efficiencies (%)

Energy to Energya

Protein to Proteinb

Energy to Proteinc

Milk production

24.0

23.7

14.5

Meat production

 

 

 


on grass

4.7

9.1

5.1


on grass + concentrates

6.7

10.2

7.5

a. Expressed as Kcal produced per 100 kcal of metabolizable feed energy consumed.
b. Expressed as edible protein produced per 100 g of feed protein consumed.
c. Edible protein produced per Mcal metabolizable feed energy consumed.

Source: Devendra (1978a).

The estimates of milk production efficiency among goats, as reported in Table 6, are higher than those recorded for sheep by Holmes (1970). Holmes reported conversion efficiencies of 21% for energy, 23% for protein and 14.5% for energy to protein conversion. For goat meat production, the energy conversion values reported by Devendra fall within the range of 5.2 to 7.8% previously given by Tayler (1970), but are slightly higher than the range of 2.4 to 4.2% recorded by Spedding and Hoxey (1974). These estimates are generally higher than the conversion efficiencies recorded for cattle, which is consistent with other published material reviewed by Devendra (1975a).

Studies of digestive efficiency comparing goats, sheep and other ruminants have recently been reviewed by Devendra (1978a). Out of 20 studies comparing goats with other ruminants - most often sheep or cattle - 12 showed statistically significant differences indicating the higher digestive efficiency of goats in terms of dry and organic matter, crude fibre and protein digestion. Goats appear to be 3.7 to 29.1% more efficient in digesting crude fibre. Three of the studies demonstrated that this greater efficiency is associated with the intake of poor quality roughages.

Besides the nature of their diet, possible reasons for the apparently higher digestive efficiency of goats may be associated with feeding behaviour, salivation, level of feed intake, pattern of rumen fermentation, concentration of cellulolytic bacteria and rate of movement along the alimentary tract. It has been suggested that as roughages mature, goats are better able to digest crude fibre than sheep or cattle. This implies the higher availability of metabolizable energy (ME) for goats on a fibre diet, as well as possibilities for increasing their overall digestive efficiency by manipulating their selective feeding preferences. The differences in digestive efficiency among species also suggest that digestibility data should be recorded separately for goats (Devendra, 1978a).

Sheep and goats have lower protein and energy requirements than cattle, and most tropical grasses, if fed at an optimum age, should be able to meet their requirements for maintenance and meat production. Very little information is available on sheep and goat production based on tropical forages, however. On a year-round basis, daily liveweight gains vary from 0.6 to 1.2 kg per ha. During the wet season, daily gains vary from 0.9 to 2.0 kg per ha with a mean of 1.2 kg, while during the dry season they vary from 0.45 to 0.8 kg. Thus an annual liveweight gain of 250 to 300 kg per ha should be feasible. Given their somewhat higher digestive efficiency, goats should be able to produce slightly higher live-weight gains per ha than sheep.

It should be possible to increase meat production from sheep and goats in the humid tropics using a feeding system based on the intensified production of selected fodder species and the careful timing of culling or grazing. Management must also take account of the feeding preferences of the animals. Requirements for increased fibre and milk production are largely unknown, but some supplementary feeding would probably be required. Further research is needed to develop more accurate estimates of nutritional requirements and feed intakes at different times of the year.


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