Awgichew, Kassahun: Comparative performance evaluation of Horro and Menz sheep of Ethiopia under grazing and intensive feeding conditions

4

Chapter 2. LITERATURE REVIEW

2.1 Approaches to animal genetic resource evaluation

There are nearly 210 million sheep in the World (FAO, 1996). Sheep together with the other classes of livestock make a substantial contribution to the well being of multitudes of people around the World in the form of meat, milk, fibre and skin. As indicated by the FAO, (Tables 1, 2 and 3) sheep production contributes substantially to the agricultural economy of Sub-Saharan Africa. Their role is more prominent in developing countries than in developed ones. Ponzoni (1992), has reported that currently there seems to be a greater awareness of the need to identify, characterise, preserve and improve indigenous breeds which are thought to have some valuable attributes that could be used at present or some time in the future.

Man has, for a long time, been manipulating and altering the genetic composition of livestock through crossbreeding, selection and inbreeding. Recently, biotechnology is becoming more and more popular as a powerful tool for changing the genetic composition of animals (Madalena, 1993). However, genetic material cannot be synthesised and genetic improvements will still be dependent on the best possible combinations of existing DNA showing that animal genetic resources are invaluable natural resources which must be properly managed for efficient resource utilisation now and be preserved for future use.

According to Lahlou-Kassi (1987) and Peters (1989), a comparative small ruminant performance evaluation will address the following issues:

  1. Adaptation traits- these are some of the most important phenotypic traits which in one way or another might influence the adaptability of the animal to the prevailing environmental conditions (tolerance to diseases, parasites, heat, etc.)
  2. Reproductive traits (female reproduction performance such as age at puberty and first lambing, conception rate, prolificacy, male reproduction performance, etc.)
  3. Production traits (birth and weaning weight, growth rate, carcass yield and quality, fibre yield and quality, etc.) and survival rate

The usefulness of genetic diversity among livestock breeds in enabling producers to meet new goals in animal production which arise from the changes in consumer demands and also changes in economics of livestock production has been known for long (Dickerson, 1969).

In developing countries, livestock genetic resources in general have not been adequately characterised, evaluated or fully utilised through selection and in some cases local populations are threatened with extinction before their genetic value is even properly described and studied (Madalena, 1993).


5

Although the principles on which to base accurate selection decision for determining the genetic merit of animals is a well established fact, the absence or inadequacy of a well documented genetic parameter estimation of indigenous breeds makes it very difficult to develop reliable and sustainable selection indices for livestock breeding in developing countries (Timon, 1993). Similar to the other classes of livestock, the genetic diversity in sheep can be expanded by the development of synthetic breeds through crossbreeding to combine the most important traits of economical and adaptation significance (Maijala and Terrill, 1991). The role played by geographic isolation in influencing between breed differences in relation to special products, characteristics, and phenotypic appearances has also been emphasised (Maijala and Terrill, 1991). They have stated that the most important between breed variation observed was the specific adaptability of breeds to the prevailing climatic and feeding conditions within ecosystems, and these ecosystems range from sparse to ample feed and forage, desert to high humidity, from sea level to high mountains, from the Equator to the northern- and southern hemispheres.

Developing countries including those in Africa have attempted to introduce improved breeds of both sheep and goats to bring about genetic improvement without even adequately investigating the merits of local breeds. According to Ponzoni (1992), this has resulted not only in the reduction of the population of the indigenous breeds but also in endangering the existence of the local genetic material.

The choice of the right type of animal to be raised in an area where it is best adapted results in higher productivity (Madalena, 1993). Therefore, the importance of environmental components such as improved management practices and nutrition in enhancing higher productivity should not be overlooked. Despite their low productivity, indigenous breeds not only survive but also produce under harsh and mostly uncertain environmental conditions.

Appropriate genotypes must be used in environments where they could best express their inherent genetic potential (Madalena, 1993). Attempts to improve the inherent genetic capacity of any livestock population beyond the scope of the nutritional or improved health care practices under which it is maintained will be counterproductive (Timon, 1993). As indicated by Laes-Fettback and Peters (1995) and Vercoe and Frisch (1987), it is necessary to identify the merit of available genetic resources, the possible integration of the animals into various production systems and to make effective use of their potential in order to quantify the existing breed differences in growth rate, growth potential and the response of the animals to different feeding challenges. Where feed supply is a major limiting factor, it is of paramount importance to look into both biological and economical factors affecting livestock productivity (Al Jassim et al. 1996).


6

The real value of indigenous breeds is often under-estimated mostly due to their poor appearance and relatively low productivity. As stated by Hodges (1990), developing countries in most cases opt for exotic breeds to increase animal productivity through crossbreeding or if conditions allow by breed substitution without properly investigating the production potential of the indigenous breeds. Peters (1989), has reported that there is an apparent lack of information regarding identifying production problems, possible intervention and performance of animals within the existing production systems to properly utilise the available genetic diversity to enhance production. This is particularly true in developing countries where breeds or types of livestock have not yet been fully identified and characterised, despite the fact that the indigenous breeds survive and produce under unfavourable environments and limited availability of feed, above all they are also parts of the prevailing production system.

Currently there is an understanding (Seré and Steinfeld, 1996) that introducing high-yielding breeds of livestock and specialised modes of production can lead to loss in genetic diversity among indigenous animals. However, in developing countries, the less intensive production systems are the mainstay of the existing species and breeds. It is, therefore, absolutely necessary to evaluate existing livestock genetic resources from a standpoint of bio-diversity and from the standpoint of matching available genotypes with the environment under which they are maintained.

2.2 Importance of small ruminant genetic resources in sub-Saharan Africa

Livestock production in the tropics and subtropics is mostly influenced by the seasonal scarcity and low quality of feed resources. African small ruminants make a substantial contribution to the well being of the people in the region through the supply of meat, milk, fibre, pelts draught power manure and cash (de Leeuw and Rey, 1995).


7

Table 1: Sheep population and production estimates for some sub-Saharan African countries1

Region/ Countries

Sheep population (’000 head)

Mutton and Lamb production (’000 MT)

1989-91

1993

1994

1995

1989-91

1993

1994

1995

Cameroon

3407

3770F

3780F

3800F

14

16F

16F

16F

Burkina F.

5049

5520

5686

5800F

11

11F

11F

11F

Ethiopia

23320

21700F

21700F

21700F

82

77F

78F

78F

Kenya

6447

5500F

5500F

5600F

25

22F

22F

22F

Mali

6072

4926

5173F

5173F

21

24F

24F

24F

Niger

3100

3465

3678F

3789

12

12F

13F

13F

Nigeria

12477

14000F

14000F

14000F

44

51F

51F

51F

Sudan

20179

22700F

22800F

23000F

70

75F

76F

76F

Somalia

12117

1100F

13000

13500F

30

26F

33F

34F

South Africa

32060

28930

29134

28784

133

125F

119

110

Tanzania

3551

3828*

3955*

3955F

10

11F

11F

11F

Uganda

1350

1760*

1850F

1900F

7

9F

9F

9F

Zimbabwe

584

420F

450F

487

1

na

na

na

Africa

201032

202856

207279

211612

893

930

947

950

World

1172331

1102221

1089749

1067566

6942

7047

7188

7012

1 Source: FAO Yearbook. Production. Vol. 49, 1995; F = FAO estimate; * = unofficial figure; na = not available; MT = Metric Tonnes


8

Table 2: Estimate of annually slaughtered sheep and carcass yield per animal in some selected sub-Saharan African countries1

Region/ Countries

Mutton and Lamb production

Slaughtered (’000 head)

Carcass weight (kg/animal)

1989-91

1993

1994

1995

1989-91

1993

1994

1995

Cameroon

1183

1343F

1350F

1360F

12

12

12

12

Burkina F.

1205

1250F

1250F

1250F

9

9

9

9

Ethiopia

8173

7812F

7812F

7812F

10

10

10

10

Kenya

2116

1810F

1810F

1850F

12

12

12

12

Mali

1650

1900F

1950F

1950F

13

13

13

13

Niger

737

763F

782F

800F

17

16

16

16

Nigeria

4033

4600F

4600F

4600F

11

11

11

11

Sudan

4372

4700F

4750F

4800F

16

16

16

16

Somalia

2320

2000F

2500F

2600F

13

13

13

13

South Africa

10101

9754*

9565*

9400F

13

13

12

12

Tanzania

817

880F

910F

910F

12

12

12

12

Uganda

473

616*

648F

665F

14

14

14

14

Zimbabwe

42

34

35

35F

14

14

14

14

Africa

65936

68893

69998

70674

14

13

14

13

World

465307

472442

479525

477012

15

15

15

15

1 Source: FAO Yearbook. Production. Vol. 49, 1995; F = FAO estimate; * = unofficial figure

Despite their relative low productivity due to genetics or environmental constraints or both, small ruminants play an important role in the agricultural economy of sub-Saharan Africa. As indicated by Winrock International (1983), small ruminants could be an important component of the mixed crop-livestock production system of Sub-Saharan Africa and other tropical regions.


9

Local livestock populations have adapted to the various ecological niches, thus, a large variety of regional breeds or types exist. They play a very important economic role particularly in regions where subsistence farming are practised due to the limitations in input supply capability in such systems.

Livestock production is an important enterprise in Eastern Africa where about 56 % of Africa‘s livestock wealth is maintained (Winrock International, 1992).

Table 3: Annual sheep productivity dynamics (Delta %) in some East African countries*

Country/ Region

Sheep Population

Mutton and lamb production

Total annual output

Yield/animal slaughtered

(’000 head)

(Delta %)

(Delta %)

(’000 MT)

(Delta %)

(Delta %)

Kg/year

(Delta %)

(Delta %)

1987

1989-91

1995

1987

1989-91

1995

1987

1989-91

1995

Ethiopia

23400

-0.34

-7.26

82

0.00

-4.88

9.5

+5.26

+5.26

Kenya

7200

-10.46

-22.22

26

-3.85

-15.38

12.3

-2.44

-2.44

Somalia

13195

-8.17

+2.31

32

-6.25

+6.25

17.2

-24.42

-24.42

Sudan

18500

+9.08

+24.32

102

-31.37

-25.49

18.5

-13.51

-13.51

Tanzania

4500

-21.09

-12.11

12

-16.67

-8.33

13.2

-9.09

-9.09

Uganda

1883

-28.31

+0.90

11

-36.36

-18.18

17.8

-21.35

-21.35

*Adapted from FAO (1995) and FAO (1996)

According to Seré and Steinfeld (1996), per capita beef and small ruminant meat production capacity in the highland zone of sub-Saharan Africa is only about 43 % and 50 % respectively of the World average which is 39.6 kg beef and buffalo and 6.8 kg sheep and goat meat per head/year. In the Mixed Rain-fed Temperate and Tropical Highlands System for which Ethiopia is a representative country, the per capita sheep and goat meat production is only 46 % (3.6 kg/head) compared to the World average of 7.8 kg/head.

Breed differences initially arise from adaptation to environmental circumstances. Further differences are caused by random drift, migration, mutation, natural selection or targeted selection. According to Bradford and Berger (1988), natural selection favours the development of animals with a balance among reproduction, growth rate, and maintenance requirements in environments where adaptation plays a critical role. Crossing locally available and well adapted breeds with selected high performance breeds could disturb the balance, leading to a loss of adaptation. Therefore, the possibility of altering the genetic potential of animals well adapted to a particular environment is limited unless environmental constraints are minimised.


10

In most African countries, the annual variation in rainfall and feed availability coupled with seasonal fluctuations in forage availability is highly substantial. This factor is believed to represent the highest check on livestock production and reproduction in the low-input and low-output traditional livestock production systems. In the dry areas, it is often difficult and sometimes uneconomical to make attempts to improve feed resources. It is therefore clear that most livestock production systems in Africa will continue to be primarily dependent on natural forages. In such circumstances, the extent to which productivity can be improved depends on the ability to identify breeds which are best adapted to the prevailing seasonality and also have the potential of economically important characteristics. Although the available data on African sheep studied clearly show their outstanding characteristics for adaptability to harsh environments as indicated by Turner (1991), more efforts must be done to identify and characterise indigenous breeds in terms of best adaptive performance abilities.

Since there are several limiting factors, economic, social or otherwise, to alter the livestock production environment for high yielding improved or temperate breeds, the continuous improvement of the indigenous breeds for higher productivity can never be over emphasised (Setshwaelo, 1990).

The economic benefit of sheep production could be enhanced by increasing the efficiency of growth to the desired market weight. As explained by Ruvuna et al. (1992), the existence of breed differences in carcass characteristics allows the choice of breeds to match specific production objectives. This would demand a strategic identification and improvement practices focussing on existing breeds.

It has been known for long (Bradford and Berger, 1988; Dickerson, 1969) that the most effective livestock improvement can best be attained by effectively using the animals already adapted to a particular environment. As defined by Terrill and Slee (1991), adaptability is the ability to survive and be productive under whatever environment or combination of environments at which the animals are maintained. Breed comparisons of adaptability and productivity should therefore be done in comparable conditions pertinent to the prevailing production environment.

The identification of adapted breeds, which are relatively superior in important productivity indices will provide means of enhancing production at no additional input costs. However, there will always be a need to address the whole question of the relationship between the nature of the production environment and the objective of breeding programmes in the context of the level of production and adaptation. Dickerson (1973) has reported that, multiple births and long breeding seasons in meat sheep can be beneficial and could also reduce costs of breeding flocks if appropriate nutrition, housing and labour are provided, but not under stressful range conditions.


11

Local breeds or types of livestock, particularly those in sub-Saharan Africa, should be compared under extensive roughage feeding condition to see if they have real differences in their response to the seasonality of pasture availability. Results from such studies will be useful in selection programmes through which animals may be identified which are highly adapted to the harsh environmental conditions and are most efficient producers. However, production potential could only be assessed under higher level of management practices whereby external stresses such as diseases, parasites, feed limitations, etc are curtailed or minimised.

The identification of adapted breeds, which are relatively superior in important productivity indices, will provide means of enhancing production at no additional input costs. However, there will always be a need to address the whole question of the relationship between the nature of the production environment and the objective of breeding programmes in the context of the level of production and adaptation.

2.3 Birth weight, lamb weight development and average daily weight gain (ADG)

2.3.1 Birth weight

Birth weight is strongly influenced by breed (genotype), sex of lamb, birth type, age of dam, feeding conditions, season of birth and production system (Gatenby et al. 1997; Rastogi et al. 1993; Gatenby, 1986; Tuah and Baah, 1985; Dickerson et al. 1972). Birth weight of animals is one of the most important factors influencing the pre-weaning growth of the young. Martinez (1983) has reported a positive correlation between birth weight and subsequent live body weight development in sheep. In another study (Gatenby, 1986), it is stated that lambs heavier at birth grow faster than light weight lambs. Lambs which are heavier at birth are usually singles or are those produced by ewes with larger body sizes and good feeding conditions. The indication is that lambs heavier at birth have larger adult weight and a higher growth capacity. Improvement in birth weight is known to have a positive influence on other productivity parameters. The significant effect of birth weight on weaning and six month weight, growth rate and on weight at slaughter has been reported by Khan and Bhat (1981) who have worked on Muzaffarnagris sheep and their crosses with the Corriedales.

Birth weight which itself is affected by dam size, dam body condition and litter size influences the survival rate and pre-weaning growth performance of offspring's as confirmed by Laes-Fettback and Peters (1995). They have observed that kids born to relatively heavier does and those which had heavier birth weight among the multiple born kids had a better chance of survival. Other researchers (Notter et al. 1991) have also reported that birth weight of lambs is greatly influenced by production system, lamb sex, ewe effects and ewe x season interaction.


12

2.3.2 Lamb weight development and average daily weight gain (ADG)

Growth in animals is defined by an increase in body cells and by growth and differentiation of body cells (Bathaei and Leroy, 1996; Orr,1982). Growth-rate and body size along with changes in body composition are of great economic importance for efficient production of meat animals. Berg and Walters (1983) have reported that fast growing lean cattle breeds are more efficient in converting feed energy to lean tissue than those which are slow growing fatter breeds. According to Bathaei and Leroy (1996), animal growth could be expressed as the positive change in body weight per unit of time or by plotting body weight against age. In another study (Gatenby, 1986), it is reported that growth in animals is mostly measured by the increase in live weight leading to changes in body form and composition. As stated by Orr (1982), live weight increase in livestock is the gross expression of the combined changes in carcass tissues, organs, viscera and gut fill. The increase in body mass of farm animals is primarily a reflection of the growth of carcass tissues consisting of lean, bone and fat.

Growth rate of lambs, particularly during the early stages of growth, is strongly influenced by breed (genotype), milk yield of the ewe, the environment under which the animals are maintained including the availability of adequate feed supply in terms of both quantity and quality (Bathaei and Leroy, 1996; Burfening and Kress, 1993; Gatenby, 1986; Notter and Copenhaver, 1980). In another study (Laes-Fettback and Peters, 1995), it has been reported that pre-weaning growth performance is also influenced by birth weight.

As stated by Owen (1976), growth rate of lambs increases until the point of inflection which is attained when the animals are between one and five months of age. After this point is reached, the animals continue to increase in weight but at a declining growth rate as they approach maturity.

Growth performance of different sheep breeds kept in different countries and under different management conditions are compiled in Tables 4 and 5. Body weight and rate of weight gain compiled in the above tables for the various breeds of sheep indicate that performance of animals is influenced by the type of management under which they are maintained. Lower performance levels and relatively longer growing phases under field conditions and the contrary under station conditions are indicative of seasonal influences on performance levels.


13

Table 4: Growth performance of some African and other sheep breeds and crosses under field and station management conditions

Breed/ Breed Cross

Location/ Country

Management Type

Birth wt.(kg)

Body weight at (Age range in days)

Source

30-80

90-120

150-180

365

 

African breeds

 

 

 

 

 

 

 

 

Adal (Afar)

Ethiopia

Station

2.5

na

13.0

18.4

25.8

Galal, E.S.E. 1983

African Fat Tail

Rwanda

Station

2.6

6.3

11.9

17.0

31.0

Wilson and Murayi, 1988; Wilson, R.T. 1991

Blackhead Ogaden

Ethiopia

Station

2.7

na

14.2

17.7

24.8

Galal, E.S.E. 1983

Djallonke

West Africa

Station

2.0

4.7

10.0

10.9-16.6

18.6-23.2

Filius et al. 1985; Wunderlich, 1990

 

 

Field

1.2-2.5

na

4.9-9.1

7.8-13.0

14.0-22.0

Filius et al. 1985; Wunderlich, 1990

Dorper

South Africa

Station

3.5-4.5

18.2

27.9

na

31.0-45.0

Schoeman and Burger, 1992

 

Kenya*

Station

3.12

11.32

13.0

14.8

na

Bullerdieck, 1996

Horro

Ethiopia

Station

2.2-2.9

na

9.8-10.9

24.7

33.5

Yohannes Gojjam et al. 1998; Wilson, R.T. 1991

Macina

Mali

Station

2.7

5.9

10.3

14.4

24.4

Wilson, R.T. 1991

Four breeds (mean)

Mali

Field

2.8

6.0

11.8

16.1

27.2

Wilson, R.T. 1986

Menz

Ethiopia

Station

1.9-2.7

na

13.5

18.5

23.4

Mukasa-Mugerwa et al. 1994; Wilson, R.T. 1991

 

 

Field

2.4

4.4

8.3

12.5

16.4

Niftalem Dibissa 1990

Mossi

Burkina Faso

Station

4.0

6.1

10.6

13.6

21.2

Wilson, R.T. 1991

Ossimi

Egypt

Station

2.0-4.0

na

18.9-19.4

na

33.5

Lahlou-Kassi 1987

Red Masai

Kenya

Station

2.7

6.1

10.5

13.7

22.6

Wilson, R.T. 1991

Other Breeds

 

 

 

 

 

 

 

 

Barbados Blackbelly

Trinidad and Tobago

Station

2.8

na

11.2

na

na

Rastogi et al.. 1993

Blenheim grade

Trinidad and Tobago

Station

2.8

na

11.7

na

na

Rastogi et al.. 1993

Finn Sheep Crosses

USA

Station

3.9

16.7

na

42.5

na

Notter and Copenhaver, 1980

Merinolandschaf

Germany

Station

3.8-4.2

15.5

na

na

50.6

Mendel et al. 1989; Mendel 1988

Sumatra

Indonesia

Station

1.7

na

7.8

12.8

na

Gatenby et al. 1997

South Indian

Sri Lanka

Farm

1.8

na

6.6

11.8

19.3

Goonewardene et al. 1984

na= not available


14

In tropical and sub-tropical regions, where extensive grazing systems are practised, the growth rate of animals fluctuates because of the seasonality of forage availability. Forage based sheep production systems like those mostly found in the tropics and sub-tropics are usually associated with slower weight gains, but the total cost of gain may be less than those in the more intensive systems. To alleviate poor productivity performance or minimise the impact of fluctuations in seasonal forage growth patterns and feed availability, careful management practices are required. In such environments, lambs in their growing stages pass through weight gain and weight loss phases (Ehoche et al. 1992).

As expected, animals lose weight during the dry season where both the quantity and quality of forage available are limited (Velez et al. 1993). In his review, Wilson (1987) has observed that there exists very little information regarding factors affecting weight of small ruminants in sub-Saharan Africa. In his study on sheep and goats in Mali, he has indicated that these animals suffer less in seasonal feed fluctuations compared to cattle.

In another study, Vercoe and Frisch (1987) have reported that in tropical countries grazing animals have to withstand a range of environmental stresses which have supposedly multiple effects on growth. The authors suggest that in such circumstances, growth potential of animals under study could be estimated more accurately by measuring their growth rate under penned condition to minimise the environmental stresses. According to Vercoe and Frisch (1987), it is assumed that high growth potential is associated with low resistance to environmental stress and the nature of the relationship between the two factors need to be determined in order to enhance development of breeds that are tolerant or resistant to environmental stresses and which are economically productive. This could be realised through better management practices and select animals based on their weaning weight. The use of weaning weight as a selection index is reported by van Wyk et al. (1993). They have stated that the animal's weaning weight indicates its value at the desired marketing age.

Average daily weight gain and weaning weight are known to be significantly affected by the mothering ability of the dam. This is particularly important during the growth stages of lambs where there is more dependency on the milk production of the ewe rather than on forage. A similar trend was observed (Laes-Fettback and Peters, 1995) on Egyptian goats where breed and mothering ability of the doe have significantly influenced both the pre-weaning daily weight gain and the weight at 14 days of age.

A study on Caribbean sheep breeds (Rastogi et al. 1993) has indicated that average daily weight gain and weaning weight were significantly influenced by the mothering ability of the dam. Type of birth is also known to have significant influence on weaning weight and pre- weaning growth rate (Tuah and Baah, 1985).


15

The post-weaning growth rate of lambs is just as important as the pre-weaning growth performance. This should be particularly looked into if the main objective of the sheep industry is producing meat through lamb production. In general, it is considered (Gatenby, 1986) that if sheep did not reach their mature live weight, they will grow faster if provided with a better diet. In practice however, since the feed supply, particularly in the tropics is not constant throughout the year, growth rate of animals shows seasonal variation. This is more evident in the dry tropics where the growth curve for lambs is typically irregular due to losing and gaining of body weight.

In regions where nutrition is poor, a rapid growth potential of larger breeds will have no advantage since smaller breeds could grow as well or even better than lambs from large breeds (Gatenby, 1986).

Apart from breed, sex and castration are also known to affect growth-rate. Male lambs usually grow faster than females. Although castrated lambs seem to have a higher dressing % than entire lambs, castration at four weeks of age has resulted in a reduced growth-rate of lambs (Silva et al. 1980 cited by Gatenby, 1986).

Although there is very little information available comparatively evaluating average daily weight gain of lambs under station and farm rearing conditions, the summarised performance of some sheep breeds (Table 5) indicates that lambs show a better growth rate (Wunderlich, 1990; Filius et al. 1985; Wilson, 1991) under farm conditions. This could most probably due to heavier parasite burden and related health problems due to the confined husbandry in station rather than due to the inherent capability of the animals to grow fast.


16

Table 5: Average daily weight gain (ADG) of some African other sheep breeds and crosses under field and station management conditions

Breed/ Breed Cross

Location/ Country

ManagementType

Birth wt.(kg)

Rate of average daily gain (g)

Source

birth to3 months

3 to6 months

Birth to12 months

 

African breeds

 

 

 

 

 

 

 

Adal(Afar)

Ethiopia

Station

2.5

116.7

60.0

63.8

Galal, E.S.E. 1983

 

 

Field

2.4

na

na

92

Wilson, R.T. 1991

Blackhead Ogaden

Ethiopia

Station

2.7

127.8

38.9

60.5

Galal, E.S.E. 1983

Djallonke

West Africa

Station

2.0

39.0-120.0

0.6-25.5

na

Wunderlich, 1990

 

 

Field

1.2-2.5

110.3

49.0-61.9

na

Filius et al. 1985; Wunderlich, 1990

Dorper

South Africa

Station

3.5-4.5

140

60.0

100.0

Schoeman and Burger, 1992

 

Kenya*

Station

3.12

154

41

na

Bullerdieck, 1996

Horro

Ethiopia

Station

2.2-2.9

134.4

52.2

83.8

Yohannes Gojjam et al. 1998; Wilson, R.T. 1991

Macina

Mali

Station

2.7

84.4

na

59.5

Wilson, R.T. 1991

Four breeds (mean)

Mali

Field

2.8

na

na

66.9

Wilson, R.T. 1986

Menz

Ethiopia

Station

1.9-2.7

na

na

na

Mukasa-Mugerwa et al. 1994; Wilson, R.T. 1991

Mossi

Burkina Faso

Station

4.0

na

na

59.2

Wilson, R.T. 1991

Red Masai

Kenya

Station

2.7

73.0a

na

54.0

Wilson, R.T. 1991

Other Breeds

 

 

 

 

 

 

 

Barbados Blackbelly

Trinidad and Tobago

Station

2.8

152.0b

na

na

Rastogi et al.. 1993

Blenheim grade

Trinidad and Tobago

Station

2.8

156.0b

na

na

Rastogi et al.. 1993

Finn Sheep Crosses

USA

Station

3.9

276.0c

241.6

na

Notter and Copenhaver, 1980; Notter et al. 1991

Merinolandschaf

Germany

Station

3.8-4.2

258.0d

na

na

Mendel et al. 1989; Mendel 1988

Sumatra

Indonesia

Station

1.7

67.8

55.6

na

Gatenby et al. 1997

South Indian

Sri Lanka

Farm

1.8

na

na

42.7

Goonewardene et al. 1984

ADG , birth to age (days): a150 ; b 56; c43; d42; na = not available; * = Least-squares means of body weight at birth, 60, 90,and 150 days of age


17

The first stage for improved productivity of the available sheep flock should focus on improving the feeding and reproductive management practices and provide better health services. Having done that, one could also plan for a long term genetic improvement through selection within the local flock or crossbreeding or both. To bring the changes anticipated, a better knowledge and understanding of the performance of the breeds available is necessary.

In order to maximise the utilisation of available breed resources, it will be highly beneficial if the performance of animals under investigation is tested within the prevailing production system (Peters, 1989; Lahlou-Kassi, 1987). However, this may not reflect the true genetic potential of the animals being studied. As reported by Peters ( 1989), it will be essential to study animals under controlled environment to quantify their genetic performance ability. On the other hand, livestock performance under the prevailing production environment could indicate the prospects for improved productivity, management variables, production constraints and helps to identify areas for improvement. Since small ruminants have to compete with other livestock species for available feed resources, their production performance should be as highly efficient as possible (Peters, 1989).

As indicated by Notter et al. (1991), lamb growth rates could not be equated directly to the profitability of the production system. The authors have reported that those systems that promote rapid lamb growth mostly achieve higher feed efficiency on the biological scale (kg gain/kg feed) and lambs in these systems require fewer days to reach market weights. It is obvious that such production systems also require the use of more expensive feed to attain the intended higher degree of weight gain efficiency. Supplementing animals with purchased feed is for the moment beyond the reach of farmers in tropical and sub-tropical regions.

Since the main aim of sheep rearing in most production system is to produce meat, farmers will always aim to have fast growing animals that could give the maximum possible lean meat in the shortest possible time. Expressing weight of lambs at a certain age (mostly at four months) as % of adult ewe weight or weight gain per day of age might also give a good indication as to how fast lambs are growing.


18

2.4 Lamb survival rate

Reproductive wastage is one of the main constraints to lamb productivity. As shown from literature results compiled in Table 6, lamb losses before one year of age vary from 49 % to 83 %. This could be a major influencing factor of productivity of a flock as confirmed by

Mukasa-Mugerwa and Lahlu-Kassi (1995), where it was reported that lamb losses represent a major problem by nullifying all the efforts made to make the ewe flock produce and rear lambs.

Lamb mortality rate varies from one flock to another depending mostly on management level.

Lamb losses also occur during the perinatal, pre-weaning and post-weaning phases of the reproduction process (Table 6). A direct comparison of lamb survival rates to various ages summarised in Table 6 will be difficult even within a region as lambs on farm and in experimental station are reared in different management practices and weaned at different ages.

Perinatal lamb deaths, which occur around parturition time, result in significant lamb losses. The extent of perinatal mortality depends mostly on the management system, and in production systems where prolific breeds of sheep are used, management practices have evolved to minimise perinatal lamb losses to a low level. According to Gatenby (1986), perinatal lamb losses could be greatly reduced by good management. In some tropical commercial sheep flocks in Brazil and South Africa, 20 % and 10 % of the lambs are stillborn in traditionally managed sheep production systems of the tropics, lamb mortality between birth and 150 days of age is estimated to be between 10-30 % (Gatenby, 1986).

The major factors affecting lamb survival include age of lamb, litter size, birth weight, season of birth, nutrition and parity of the ewe (Gatenby et al. 1997; Armbruster et al. 1991, Notter et al. 1991).

The nutritional and physiological status of the ewe during the gestation period and at the time of lambing affect the birth weight of the offspring as well as the milk production of the ewe, both of which are known to be critically very important particularly at the early age (birth to two weeks) of the lambs. According to Fitzhugh and Bradford (1983), improvement in ewe nutrition during pregnancy has reduced lamb mortality from 23 % to 11 %. The authors have also concluded that surviving the first week after birth (perinatal stage) does not ensure a lamb‘s survival because there are also other determining factors such as poor nutrition, diseases and parasite burden before and after weaning (postnatal stage) which influence the ultimate productivity of the animal.


19

The summary in Table 6 indicates that the type and level of management in a given production system has an influence on the survival of lambs at all stages of growth particularly during the perinatal growth stage. In most cases birth weight has a quadratic relationship with mortality rate whereby mortality tends to increase at extremely low or extremely high birth weights (Mendel et al. 1989; Cooper, 1982; Notter and Copenhaver, 1980). A similar conclusion was reached by Notter et al. (1991), who reported that the relationship between perinatal survival and birth weight of lambs was curvilinear. In a study carried out on Menz sheep, Mukasa-Mugerwa et al. (1994), have recommended that lambs have to be born with birth weights of 2.0 kg or more to have a perinatal survival rate of 90 %. The importance of birth weight both on the survival and pre-weaning growth performance of young animals has been reflected in a study carried out on goat kids born from Baladi, Zaraibi and Damascus goat breeds (Laes-Fettback and Peters, 1995). In this study it has been also shown that higher litter sizes have tremendously reduced birth weight and hence, survival of the kids. In another study (Gatenby et al. 1997) have reported a higher pre-weaning mortality rate (40 %) among triplets and quadruplets.

20

Table 6: Survival rate of some African and other sheep breeds and crosses under field and station management conditions

Breed/ Breed Cross

Location/ Country

Management type

Birth wt. (kg)

Survival Rate ( %) to (Age in days)

Source

Perinatal (0-14)

Postnatal (15-150)

151-365

 

African breeds

 

 

 

 

 

 

 

Adal(Afar)

Ethiopia

Station

2.5

 

 

 

Galal, E.S.E. 1983

African Fat Tail

Rwanda

Station

2.4

 

82.5

75.6

Wilson and Murayi, 1988

Blackhead Ogaden

Ethiopia

Station

2.7

 

 

 

Galal, E.S.E. 1983

Djallonke

West Africa

Station

2.0

 

50.0-94.0

68.0

Filius et al. 1985; Wunderlich, 1990

 

 

Field

1.2-2.5

 

81.8-86.8

83.0

Filius et al. 1985; Wunderlich, 1990

Dorper

South Africa/ Kenya

Station/ Farm

3.5-4.5

92.0

87.1-94.0

 

Schoeman and Burger, 1992; Bullerdieck, P., 1996

Horro

Ethiopia

Station

2.2-2.9

88.3

67.0-71.4

 

Yohannes et al. 1995; Solomon Gizaw et al.. 1995; Wilson, R.T. 1991

Macina

Mali

Station

2.7

 

 

 

Wilson, R.T. 1991

Four breeds (mean)

Mali

Field

2.8

73.0

54.0

49.0

Wilson, R.T. 1986

Menz

Ethiopia

Station

1.9-2.7

80.2

 

 

Mukasa-Mugrewa and Lahlou-Kassi, 1995; Mukasa-Mugerwa et al. 1994; Wilson, R.T. 1991

 

 

Field

2.4

 

 

 

Niftalem Dibissa 1990

Mossi

Burkina Faso

Station

4.0

 

 

 

Wilson, R.T. 1991

Ossimi

Egypt

Station

2.0-4.0

 

86.0

 

Lahlou-Kassi 1987

Red Masai

Kenya

Station

2.7

 

 

 

Wilson, R.T. 1991

Watish

Sudan

Station

4.1

 

65.3

 

Wilson, R.T. 1991

Other Breeds

 

 

 

 

 

 

 

Barbados Blackbelly

Trinidad and Tobago

Station

2.8

97.5

83.2

 

Rastogi et al.. 1993

Blenheim grade

Trinidad and Tobago

Station

2.8

98.4

86.8

 

Rastogi et al.. 1993

Finn Sheep Crosses

USA

Station

3.9

94.0

na

 

Notter and Copenhaver, 1980

Mehraban

Iran

Farm

3.9

 

 

 

Bathaei and Leroy 1998

Merinolandschaf

Germany

Station

4.2

na

81.6; 84.2

na

Mendel et al. 1989

Suffolk Crosses

USA

Station

3.6-4.1

87.7

na

na

Notter et al. 1991

Sumatra

Indonesia

Station

1.7

88.6

75.3

56.8

Gatenby et al. 1997

South Indian

Sri Lanka

Farm

1.8

na

75.0

na

Goonewardene et al. 1984

na = not available


21

2.5 Linear body measurements

Body measurements are considered as qualitative growth indicators which reflect the conformational changes occurring during the life span of animals (El-Feel et al. 1990).

Although live body weight is an important growth and economic trait, it is not always possible to measure it due to mainly the lack of weighing scales, particularly in rural areas. However, body weight can be reasonably estimated from some linear body measurements (Mayaka et al. 1995). According to these authors, body weight of West African Dwarf (WAD) goats has been satisfactorily predicted by using heart girth as the only regressor.

Means of linear body measurements of various tropical and temperate breeds from which body weight could be reasonably estimated for the respective breeds are shown in Table 7.

Table 7: Means of body weight (kg) and linear body measurements (cm) of various tropical and temperate sheep breeds

Breed/ Breed Type

Location/ Country

Age (months)

Body wt. (kg)

Linear body measurements (cm)

Source

HG

WH

BL

TL

WAD x N

Ghana

8

11.9

52.1

53.4

52.8

na

Arthur and Ahunu, 1989

Sahel x WAD x N

Ghana

8

12.1

51.2

54.6

53.2

na

Arthur and Ahunu, 1989

West African Rams

Venezuela

mature

57.0

92.4

69.9

68.9

na

Stagnaro, C. G., 1983

Sudan Desert Rams

Sudan

mature

60.0

na

80.0

na

na

Wilson, R.T.,1991

Adal (Afar) Rams

Ethiopia

mature

38.0

na

66.0

na

na

Galal, E. S. E., 1983

Red Masai Rams

Kenya/ Tanzania

mature

41.0

na

70.0

na

na

Wilson, R. T., 1991

South Indian

Sri Lanka

mature

24.5

39.3

52.4

83.2

na

Goonewardene et al. 1984

WAD = West African Dwarf; N = Nungua Blackhead; na = not available; HG = heart girth, WH = wither height, BL = body length, TL = tail length


22

Linear body measurements have been also used to give information on differences in body proportion as well as short- term fluctuation of body proportion mainly due to weight loss and gut fill (Arthur and Ahunu, 1989). Indices using body measurements have been used to estimate shape which is usually difficult to quantify due to its subjectivity in comparison with size.

Body dimension measurements will be particularly useful when it is not possible to take direct measurements of the main meat production traits such as body weight and carcass traits (El-Feel et al. 1990). They have observed that weaning system has influenced some linear body measurements of cow calves and buffalo calves up to two years of age. This same study has also shown that birth season had an influence on body measurements of buffalo calves at six months of age. In a study on calves from dairy cows in Turkey, Tuzemen et al. (1993) have indicated that body weight could be predicted accurately from body measurements. The authors have also observed a positive and significant relationship between body weight and height at withers and heart girth on both types of calves. Through principal component analysis of body measurements (Arthur and Ahunu, 1989), it could be possible to identify a relatively small number of factors that can be used to describe relationship among sets of several interrelated variables. Factors developed through such techniques could also help to contrast animals of different shapes and sizes (Brown et al. 1973).


23

2.6 Fattening performance of Tropical sheep breeds

2.6.1 Carcass characteristics and composition

Before we attempt to optimise approaches to lean lamb production, it is essential to understand environmental and genetic factors influencing the lean : fat ratio. Characterisation of breeds for carcass composition is one such method through which potential genetic resources for lean lamb production could be identified. This will lead us to a better understanding of management alternatives required for different genotypes. The existence of genetic variation among breeds in growth and carcass characteristics have been described by Dickerson et al. (1972) and by Crouse et al. (1981).

Table 8: Carcass tissue proportions for various Temperate and Tropical sheep breeds and crosses

Breed/Breed cross

Location/ Country

Age (months)

Slaughter wt./ EBW1 (kg)

Carcass composition
(%)

Source

Lean

Fat

Bone

Rest

Awassi

Saudi Arabia

9

24.5

55.3

19.5

25.2

na

Gaili and El-Naiem 1992

Najdi

Saudi Arabia

9

32.0

54.3

20.3

25.4

na

Ibid

Baluchi

Iran

6.5

28.0

75.9

6.7

17.1

na

Farid 1991

Border Leicester

U.K.

na

35-40

56.1

25.4

na

na

Kempster et al. 1986

Crossbreds

Germany

na

>30

58.2

23.6

na

na

Streitz et al. 1994

Hampshire Down

U.K.

na

35-40

54.6

27.7

na

na

Kempster et al. 1986

Ile de France

U.K.

5

35

55.8

26.3

16.4

na

Wolf et al. 1980

Karakul

Iran

6.5

33.5

77.4

4.9

17.5

na

Farid 1991

Merino

Portugal

na

20-40

55.7

24.1

15.7

na

Teixeria and Delfa 1994

Oxford Down

U.K.

5

35

56.3

24.6

17.5

na

Wolf et al. 1980

Sudan Desert

Sudan

na

41.1

57.3

19.0

21.0

na

Khalafalla and El Khidir 1985

Suffolk

Portugal

na

20-40

55.5

23.8

16.0

na

Teixeria and Delfa 1994

Texel

U.K.

5

35

60.5

21.5

16.5

na

Wolf et al. 1980

na = not available; 1 EBW = Empty Body Weight


24

The proportion of muscle, fat and bone in carcasses change as animals grow. Relative to empty body weight, bone tissue matures early followed by muscle (lean) and the fat tissue maturing late (Afonso and Thompson,1996, Taylor et al. 1989, Orr, 1982). The review by Anous (1991) shows that the ratio between the weight of muscle (lean) and bone tissues is the most critical determinant of the value of carcasses. Tissue proportions of various sheep breeds and their crosses are compiled in Table 8. Most reports state a lean content of between 54 % and 58 %. Higher values are reported for Texel (60 %) and for Baluchi and Karakul (75 - 77 %). Colomer-Rocher et al. (1992), have reported that the mean muscle content of male New Zealand Saanen goats to be about 60 % and stated that this was higher than that normally found in sheep. Ruvuna et al. (1992) have reported a lean: fat: bone ratio of 73:9:18 for 14 ½ month old goats. A similar carcass composition is also reported by Farid (1991) for carcasses from Baluchi sheep (Table 8).

In meat production enterprises, lean is the most important economic component of the carcass. Producing and marketing of lean lamb to meet the consumer demand for less fat has become a challenge for livestock industry particularly in developed countries. As stated by Farid (1991), the relative merit of different sheep breeds for meat production is determined by a high proportion of lean, and a low proportion of fat and bone in the carcass.

While the tendency in developed countries is to produce meat animals with a decrease in fatness at the appropriate market weight and an increase in growth-rate and mature size, animals with higher degree of fatness regardless of size and weight fetch a higher premium in most tropical countries (Thatcher and Gaunt, 1992, Terrill and Maijala, 1991; Lee, 1986). This is particularly true in Ethiopia where during festivities, lambs and Wethers with higher fat cover are also priced high.

The commercial value of sheep carcasses is influenced by the proportion of muscle, fat and bone in a whole sale cut (Taylor et al. 1989). This is strongly governed by consumer preferences in the developing countries while the proportion of lean in carcasses play a lesser role in determining market price in less developed countries. The characteristics of a superior carcass are: high proportion of muscle (lean), low proportion of bone and an optimal level of fat cover. According to the authors, the proportions are in turn influenced by breed, sex and could also be mostly influenced by the stage of maturity or mature size of the animal. Due to a strong breed influence on body composition (Taylor et al. 1989), there could be better opportunities to select among breeds for differences in this trait even at a similar maturity level in body weights.


25

As reported by Berg and Walters (1983), the proportion of muscle (lean) in a carcass varies indirectly with fat proportion whereby a higher fat proportion is associated with a lower proportion of muscle and vice versa (Table 8). The change in bone proportion is minimal. It is suggested (Berg and Walters, 1983) that in meat producing animals, the proportion of muscle to live weight could be a valuable index of yield since genetic differences appear to be of major importance.

Other factors influencing lean meat % are carcass weight, body conformation (muscling), dissection method, lean mass measuring method and procedure (Walstra and de Greef, 1995).

Tissue growth patterns and the resulting changes in chemical composition of the body are very much influenced by many interrelated environmental as well as genetic factors (Orr, 1982). According to the above author, animals of the same species mostly vary in their mature body size and weight which is also reflected in the differences of their carcass composition.

Fat deposition is believed to start out relatively slowly and to increase geometrically as the animal enters a fattening phase (Berg and Walters, 1983). The authors have also reported that there exist genetical differences in fat deposition exists among breeds due to different growth capacity and maturity. Plane of nutrition is also another major factor influencing the fat deposition pattern of animals whereby high plane of nutrition promotes earlier fattening while a low plane results in a delayed or slower fattening process.

Carcass composition could be used as a parameter in breed characterisation to identify potential genetic resources for lean lamb production and also to identify management alternatives for different breeds (Snowder et al. 1994). The authors have observed the existence of genetic variation among the American sheep breeds studied in growth rate and carcass characteristics. Some of the breeds had higher percentage of kidney, pelvic and subcutaneous fat while others like the Suffolk had higher growth rate and 22 % less kidney and pelvic fat. Snowder et al. (1994) have concluded that when slaughter weight is held constant, carcass characteristic differences of breeds contribute to the variation in quality of lamb meat. This suggests that a relatively late maturing sheep could produce heavier carcasses of higher lean percentage.

Breed effects are known to influence not only carcass composition and quality but also carcass conformation. Breed differences in carcass merits could influence the choice and development of breeds for specific production objectives. This could be realised through strategic identification and utilisation of existing breeds.

The other most important factors that are known to influence carcass composition are sex and feed. In a study undertaken by Cantón et al. (1992) it was observed that nutritional level is related to carcass yield, carcass quality, and fat tissue development and composition.


26

In a study on some Egyptian sheep breeds, El Karim and Owen (1987), have observed no significant breed type or sex differences in the proportion of lean, bone and fat in dissected sides of carcasses. However, they have observed that the fat content was more variable than either lean or bone percentage. According to this study, fat depth over the rib eye muscle was significantly influenced by breed and sex. This is in agreement with the findings of Snowder et al. (1994) where it was concluded that such differences in carcass characteristics are expected particularly if breeds differ in their physiological maturity.

Iason and Mantecon (1993), have reported that food restriction followed by compensatory growth delays growth and maturation of animals thereby affecting carcass composition. However, since body fat is mobilised to provide nutrients for body maintenance during periods of limitations in food intake, the performance of animals during and after a period of food restriction is likely to be affected (Afonso and Thompson, 1996).

A study on South African Merino sheep (Cronjé and Weites, 1990), has shown that carcass composition, expressed as a proportion of carcass weight, was found to be highly influenced by maize supplementation. They observed that the proportion of fat was doubled with a 200g per day allowance compared to the control.

In another study (Villete and Theriez, 1981), it is indicated that birth weight has an indirect effect on carcass composition by influencing age at slaughter. According to these authors, a study on some French sheep has shown that for every 1 kg increase in birth weight, there was a decrease by 13 days of slaughter age. It is possible to manipulate growth paths of lambs maintained on relatively poor quality pasture to produce carcasses of better quantity and quality (Thatcher and Gaunt, 1992).

Fat is deposited only if surplus nutrients are available. According to Gatenby (1986), the higher the level of nutrition or the lower the growth capacity, the more fat is deposited in lambs at any given age and body weight.

In carcass merit evaluation, dressing percentage is an important trait. However, according to the review by Ruvuna et al. (1992), dressing percent is known to be affected by breed, age, castration and it is also highly affected by feeding and degree of fattening.

They have also reported that proportion of lean and fat in carcasses increased with age while the proportion of bone decreased. Gruszecki et al. (1994) have reported that the carcass composition of Polish Lowland sheep and its crosses, whose slaughter weights range from 38-40 kg, to be in the range of 61-63 % lean, 17-20 % fat and 19-22 % bone. In a similar investigation on mutton-type lambs of diverse genetic background (Streitz et al. 1994), it was observed that lean and fat content of carcasses were 62.4 % and 16.8 % respectively for those lambs which had below 30 kg live weight and 58.2 % and 23.6 % respectively for those above 30 kg live weight.


27

In their observation on some seven British sheep breeds, Taylor et al. (1989), have concluded that as breed size increased, proportion of carcass muscle and bone has decreased. It was also observed that the breeds have not only differed in the proportion of carcass muscle, fat and bone but also in their distribution. As animals get older and heavier, their body composition also change. It has been reported (Gatenby, 1986; Berg and Walters, 1983) that the viscera, skin, head and feet of lambs grow relatively slower than the carcass tissues. The author has also stated that the ratio of fat to muscle and that of muscle to bone increase with age. However, according to Taylor et al. (1989) there might be no significant breed differences in the way the muscle, fat and bone distribution changed as animals grew older. Teixeira and Delfa (1994) have reported a carcass composition of 55.5 % and 55.7 % muscle, 23.8 % and 24.1 % fat, and 16.0 % and 15.7 % bone for Suffolk and Merino sheep carcasses respectively. A similar result (58.3 % lean, 24.3 % fat and 17.4 % bone), obtained through logarithmic regression, was reported by Taylor et al. (1989) for 7 British sheep breeds. In a study on some one year old Egyptian sheep breeds El Karim and Owen (1987) have reported a dressing percentage of 43.55-45.35 and a carcass composition of 58.13-59.24 % lean, 14.20-15.80 % fat and 18.92-19.95 % bone. The slaughter weight of these animals ranged between 25.67 and 33.80 kg.

Gaili (1979) and Gatenby (1986) have reported that tropical sheep tend to deposit more intramuscular and internal fat and less subcutaneous fat compared to temperate mutton breeds. There is evidence that tropical and temperate breeds do not differ in their carcass composition (Amegee, 1981 cited by Gatenby, 1986). However, according to the author, tropical and temperate breeds do differ in size and distribution of fat deposited in the body.

The development of fat depots over the growing-finishing period in cattle, pigs and sheep is consistent in indicating that subcutaneous fat grows faster than intermascular fat (Berg and Walters, 1983). On the other hand, the growth of kidney knob and channel fat in relation to the other fat depots is more variable. Kempster (1981) has reported that breeds of sheep also differ in fat partitioning whereby breeds which have been improved for fat lamb production tended to have a higher proportion of subcutaneous fat than those where adaptability and maternal performance have been important.


28

In wool sheep, body composition is known to be influenced by breed, sex and nutritional level (Cantón et al. 1992). Their review also shows that nutrition level is related to carcass yield, quality, fat deposition and composition while breed influences strongly carcass conformation, composition and quality. According to the review by Cantón et al. (1992), although nutritional influence on body composition is similar for wool and tropical hair sheep, the later have a different body conformation and composition compared to wool breeds. This is mainly attributed to their smaller mature body weight. In a study by Friggens et al. (1994) and Iason et al. (1992), the degree of maturity was observed to be an important predictor of carcass composition for some European breeds. This has been also conformed by Frutos et al. (1997) where it was reported that live bodyweight was the most effective means to predict body composition. Since large breeds of sheep reach their mature live weight at relatively older ages than smaller ones, differences in carcass composition will be observed between the two groups unless they are compared at a similar proportion of mature live weight (Iason et al. 1992). This is also supported by another study (McCutcheon et al. 1993) where it was observed that carcasses of similar weight had the same proportion of water, protein and fat.

Frutos et al. (1997) have also reported that individual breeds have a distinctly different fat distribution within the body. According to the review, the ability of sheep to retain and mobilise body reserves is of considerable importance in determining the productivity or even the survival of sheep, particularly in arid and semiarid environments where limitations in feed supply are more pronounced.

2.6.2 Feed intake and fat deposition in small ruminants

As reported by Gatenby (1986), the supply of animal feed in the tropics, except that in the humid zones, is not constant both in quantity and quality leading to seasonal variation in growth-rate of the animals.

In intensive livestock production systems where milk and meat are the main production objectives, feed costs account as a major component of the expenses.

Efficiency of feed utilisation is an important trait in meat production enterprises (Terrill and Maijala, 1991), and should be included in selection programmes for genetic improvement of animal performance (Parker et al. 1991). However, it has always been difficult to improve feed resources particularly in dry areas.

In sub-Saharan Africa, the main source of livestock feed is grazing on natural pasture which mostly suffers from seasonal variations both in quality and quantity. This is considered to be the most important constraint to livestock production and reproduction in traditional systems where the low economic input to the system is evident.


29

The existence of feed intake differences between genotypes has been reported in several studies (van Arendonk et al. 1991; Barlow et al. 1988; Wagner et al. 1986).

The relationship of intake to productivity is very complex and depends on several factors including nutrition and genetics. Genetic groups are known to have differences in feed intake (van Arendonk et al. 1991; Barlow et al. 1988) but very few studies show the amount of genetic variation between breeds in this trait. In his review, Black (1990) has reported that feed intake is closely correlated with both the amount of pasture available per animal per day and the digestibility of the forage selected. Another review by Said and Tolera (1993) shows that plant cell-wall is the major restrictive determinant of feed intake. However, the authors also indicated that the actual feed intake of an animal depends on its genotype and physiological state, the quality and quantity of the feed available during grazing. In an earlier study, Arnold and Birrel (1977) have reported that herbage intake of grazing sheep is influenced by age, size, weight and physiological state of the animal, climatic conditions and the availability and quality of the feed.

A feeding experiment on British Angora goats (Shahjalal et al. 1992) has indicated that growth and lean tissue deposition are influenced by both protein and energy intakes. Carstens et al. (1991) have reported that increase in dietary energy intake can enhance the proportion of body fat deposition in animals. In a similar investigation, Coleman et al. (1995) have observed that at the end of the growing phase steers fed grain had higher body weight and higher percentage fat in the empty body and carcass compared to those fed forages. By manipulating dry matter intake of animals, particularly in the finishing period, it may be possible to increase the proportion of lean in carcasses. In a study on bulls and steers, Steen and Kilpatrik (1995) have concluded that for animals maintained on high-forage diets and slaughtered at moderate levels of fatness, reducing dry matter intake during the finishing period by 20 % has reduced the carcass fat estimate and increased lean and bone contents. Kabbali et al. (1992) have concluded that weight loss of lambs during feed shortages results in the loss of weight in internal organs and such lambs recover the lost weight during realimentation through compensatory growth resulting in better feed efficiency and leaner carcasses.

In their study on restricted feeding and realimentation using lambs from Timahdit, D´man and Ile de France x D´mann sheep, Kabbali et al. (1992) have observed that feed efficiency was better in lambs that have gone through a phase of compensatory growth. A similar result was reported by Zervas et al. (1997) where it was stated that daily dry matter intake by lambs was influenced partly by the nature, energy and nutrient density of the available feed.


30

As stated by Kabbali et al. (1992), body composition is dynamic and changes always depending on environmental factors. This study shows that body composition changes occur in animals undergoing compensatory growth. In another study by Graham et al. (1991), it is reported that body protein gain was dependent on both voluntary feed consumption (VFC) and weight, whereas fat gain and wool growth were influenced by VFC alone. According to a review by Dulphy and Demarquilly (1994), voluntary feed intake of ruminants is determined by the ingestibility of the feed and the intake capacity of the animal. Daily feed intake is greatly influenced by the digestion rate of the digestible material and the rate of passage of indigestible material ( Khandaker,et al. 1998; Dulphy and Demarquilly, 1994).

Level of nutrition is known to influence body or carcass composition significantly (Black, 1983; Butler-Hogg, 1984; Taylor and Murray, 1991). Supporting such theory, Aziz et al. (1992), have observed that the body fat of sheep serves as an immediate source of energy during under nutrition. This is particularly important during the dry seasons when both the quantity and quality of feed are in short supply. In an earlier study (Little and Sandland 1975), it was observed that during weight loss periods, the first body tissue to be mobilised is the subcutaneous fat. If the duration of the weight loss phase is prolonged, a greater proportion of muscle fat will be mobilised (Aziz et al. 1992).

Body weight is the main determinant of body composition of animals of the same breed and sex group regardless of age or nutritional level (Turgeon Jr. et al. 1986). They have observed that more fat and less protein has been deposited per unit of weight gain as growth rate increased giving an indication that body composition of weight gain is influenced more by growth rate. Body composition is also likely to be influenced by growth capacity. According to Nicholas et al. (1992), small-framed lambs were significantly fatter than the medium and large -framed Rambouillet lambs. The authors have recommended that there should be different management approaches for the various groups in order to fulfil the yield grade required by the market and obtain a premium value for the produce.

The review done by Cantón et al. (1992) shows that body composition is affected by sex, nutritional level and breed. They have also observed that tropical hair sheep have a different body conformation and body composition compared to wool breeds mainly due to their smaller mature body weight. The summary also shows that sex and nutrition have similar effects on body composition of both hair and wool sheep breeds. In another study (Marais et al. 1991) on compensatory growth, it was observed that body composition remained uniform at a specific body mass and was independent of feeding level. However, this study has involved only one breed type and a similar sex and age group. In another study (Villete and Theriez, 1981), it has been reported that an increase in birth weight has resulted in a decrease in perigastric and perirenal fat.


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The idea to modify composition of weight gain through different levels of nutrition has been supported by Turgeon et al. (1986) who have concluded that slow growth rate restricts body fat deposition allowing a higher proportion of the gain to be deposited as protein. However the review made by Kabbali et al. (1992) indicates that there is no uniformity in the results of nutritional manipulation and compensatory growth. Some studies have shown increases in fat content, others have reported increases in protein and water and still others have reported no body composition changes in animals which have gone through a period of feed restriction followed by realimentation. The contradictory results are thought to be mainly due to the diversity of the factors involved in compensatory growth. This could indicate the true spectrum of the results which could be expected from studies on small sample size with highly variable body composition and under various conditions of nutritional restriction and realimentation. The changes in the rates of protein and fat deposition which occur as a result of changes in environmental temperature are naturally reflected in the body composition of the animal (Fuller, 1969). He has also indicated that protein deposition is proportionately less affected by environmental stress than fat deposition. According to Graham et al. (1991), it will be beneficial to the producer if it could be established where in the weight gain phase fat deposition starts. Once this is determined, it may be possible to manipulate the changes in weight gain to produce lambs of desired carcass composition. Prasad and Sinha (1992) have reported that subcutaneous fat increases with maturity of the animal. They also observed that the intermuscular fat was always more than subcutaneous fat at all maturity levels. According to their review, body weight at which lambs begin the fattening phase is related to the mature size.

The relative abundance of feed in tropical regions during wet seasons, is in most cases adequate to enable animals to deposit body fat. According to Ørskov (1998), ruminants are capable of adapting to seasonally fluctuating pasture forage quality and (or) availability by conserving energy from the lush period in the form of body fat. He has also indicated that fat deposition generated by high intakes of quality pasture could be an efficient way of conserving forage. The stored fat could then be mobilised to sustain growth, lactation or maintenance requirements.

Carcasses from lambs that have gone through weight loss/ weight gain (compensatory growth) phase were found to be leaner compared to those which went through a normal growth phase (Kabbali et al. 1992).


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2.6.3 Feed influence on development of non-carcass tissues

In a study conducted on Dorset x (Border Leicester x Merino), Lee (1986) found no significant effect of nutrition on the weight of Omental, kidney plus channel or mesenteric fat depots. This is contrary to other findings (Taylor and Murray, 1991, Black, 1983; Butler-Hogg, 1984), where nutrition is known to have influenced fat deposition in all parts of the body and at all stages of weight gain.

In another study, Burrin et al. (1990) have reported that there exists a relationship between nutrition level and visceral organ size and metabolic activity whereby the level of feed intake changes the relative proportion of visceral organs to body mass (Table 9). On the other hand as suggested by Mehrez et al. (1977), feed intake could be reduced if the rumen ammonia concentration is limiting the rate of fermentation in the rumen.

In a study on some British sheep breeds, Wood and MacFie (1980) have observed that breed differences in partition and distribution of carcass fat deposition within and between breeds are smaller. On the otherhand, the authors also reported larger variation in the site of body fat deposition within an animal.

Fat content of carcasses could be also determined through chemical analysis. One such method is Ether extract. According to Snowder et al. (1994), a higher total chemical fat indicates a higher percentage of intermuscular fat. Farid (1991) has reported ether extract values of 26 %, 30 % and 32 % for untrimmed meat from Karakul, Mehraban and Baluchi sheep respectively. Wishmeyer et al. (1996), have reported ether extract values of 11.82 % - 30.52 % for whole body composition of Rambouillet Wether lambs. They have concluded that body weight is a better predictor of body chemical composition.


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Table 9: Fresh weight of sheep non-carcass organs by breed or breed crosses and body weight

Breed/ Breed Crosses

Location/ Country

Slaughter wt. (kg)

Fresh non-carcass organ weight (g)

Source

Liver

Heart

Kidneys

Lungs + Trachea

Spleen

Skin

Head

Feet

 

Horro

Ethiopia

24.9

376.8

112.4

68.6

481.8

73.7

2200.0

1900.0

570.0

Ewnetu et al. 1998

Menz

Ethiopia

24.7

324.5

99.4

64.3

476.6

69.7

2700.0

535.1

585.4

Ibid

Blackbelly

Mexico

38.2

64.2

161.0

104.3

406.5

56.9

na

na

na

Cantón et al. 1992

Crossbred

USA

31.0

495.0

na

84.0

na

na

na

na

na

Burrin et al. 1990

D´man

Morocco

30.0

651.0

190.0

98.0

410.0

61.0

na

na

na

Kabbali et al. 1992

Ile de Fr.x D´man

Morocco

30.0

607.0

189.0

90.0

432.0

62.0

na

na

na

Ibid

Timahdit

Morocco

30.0

564.0

180.0

92.0

400.0

56.0

na

na

na

Ibid

Karakul

Canada

20.7

na

128.0

169.0

na

na

na

na

na

Kennedy et al. 1995

Merino

Australia

33.0

537.0

141.0

102.0

383.0

53.8

2316.0

874.0

1740.0

Aziz et al. 1993

Romney

New Zealand

42.5

669.1

219.2

126.2

463.2

50.7

na

na

na

McCutcheon et al. 1993

Suff. x Ramb.

USA

38.0

572.0

175.0

na

na

na

na

na

na

Turgeon, Jr. Et al. 1986

Suff.x Ramb. x Fin.

USA

34.0

887.0

171.0

149.0

na

65.0

na

na

na

Ferrell et al. 1986

Ile de Fr. = Ile de France, Suff. = Suffolk, Ramb. = Rambouillet, Fin. = Finnish Landrace, na = not available


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