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Phenotypic and genetic parameters of growth traits of Blended goats at Malya, Tanzania

S.M. Das,1 J.E.O. Rege2 and Mesfin Shibre2

1 Livestock Production Research Institute
ZRTC (C), P. O. Box 292, Mpwapwa, Tanzania
2 International Livestock Centre for Africa (ILCA)
P.O. Box 5689, Addis Ababa, Ethiopia

Abstract
Introduction
Materials and methods
Statistical analysis
Results and discussion
Conclusion
References

Abstract

Twenty-five year (1965-89) data on the Blended goats from Malya Research Centre totalling 4799 records were analysed for live weights and growth traits from birth to 72 weeks of age. The Blended goat developed in Malya, Tanzania, is a stabilised three-way cross of the Kamorai (55%), Boer (30%) and indigenous (15%) goats. Birth weight and weaning weight (16 weeks) averaged 2.47±0.02 kg and 11.14±0.15 kg, respectively. Live weights at 24, 48 and 72 weeks of age averaged 12.47±0.26 kg, 16.42±0.32 kg and 25.800.43 kg, respectively. Average daily gain from birth to weaning, and to 24,48 and 72 weeks of age averaged 80.0±1.0 g, 59±2 g, 41±1 g and 46±1 g, respectively. Year of birth, sex of kid and type of birth significantly (P<0.001) affected birth weight, weaning weight and live weight at 24, 48 and 72 weeks of age. Season of birth significantly (P<0.01) affected birth weight and live weight at 48 and 72 weeks of age. Age of dam significantly (P<0.01) affected birth weight and weight at 48 weeks of age. Heritability estimates for birth weight, weaning weight and weight at 24 weeks were 0.0151±0.036, 0.099±0.39 and 0.0148±0.053, respectively. Repeatability estimates for birth weight, weaning weight and weights at 24, 48 and 72 weeks of age were 0.202±0.18, 0.1870.22, 0.533±0.022, 0.0160.035 and 0.174±0.38, respectively.

Introduction

Genetic improvement is an integral part of many goat development programmes in the tropics, where breeding policies mostly aim to upgrade local goats by crossbreeding with, either temperate or tropical exotic breeds (Lavaraj and Gore 1987; Schmidt 1991). Selective breeding, utilising the variability within a population to upgrade that population, is less often a part of such programmes. This is due to easy availability of apparently superior stock from elsewhere, and due to difficulties in running field-based performance recording. Where local goat breeds are involved the flocks are mainly kept in institutional farms, in which low numbers of animals and management constraints limit selection intensity and genetic progress (Devendra and Burns 1983). The potential for genetic improvement is largely dependent on the heritability of the trait and its genetic relationship with other traits of economic importance upon which some selection pressure may be applied. Information on heritabilities is essential for planning efficient breeding programmes, and for predicting response to selection.

In Tanzania, Blended goats are kept mainly for meat production. Thus, traits affecting economic viability include those associated with growth. Body weight and rate of gain are among the most economically important and easily measured traits of meat animals. Although weight is an important objective in selection, knowledge of the phenotypic and genetic parameters of the growth trait upon which to base selection is of utmost importance. Genetic and phenotypic parameter estimates are scarce in goats reared under Tanzanian conditions and where such information is available, analytical methods used tend to be inadequate. The purpose of this study was to estimate phenotypic and genetic parameters of growth traits in a flock of Blended goats at various stages of growth from birth to 72 weeks of age.

Materials and methods

Blended goats are three-way crosses (55% Kamorai, 30% Boer and 15% indigenous), developed at Malya, Tanzania, which were stabilised in late 1960s (Des 1989). Phenotypically, they have large pendulous ears and resemble Anglo-Nubians. The records of Blended goats used in this study were collected from Livestock Research Centre, Malya, which lies at approximately 4°S latitude and 37°S longitude, and at an altitude of about 1050 metres above sea level. The centre covers an area of 1800 ha of which 1400 ha have natural vegetation of savannah grassland. The natural vegetation at Malya comprises Combretum wood species and grass species of Bothrichloa, Hyparrhenia, Themeda and Cynodon types; a few paddocks are sown to Chloris gayana. At the centre, goats were grazed from 0800 to 1700 hours and were housed indoors at night.

Breeding of goats was done in the wet and dry seasons. The wet breeding season extended from March to May, while the dry breeding season extended from September to November. Lambs were ear-tagged soon after birth and left to suckle their dams during grazing until weaning at 16 weeks of age. Kids were separated by sex at weaning into different weaner flocks. Records taken on lambs included birth and weaning weights, and weights at 24, 48 and 72 weeks of age.

Statistical analysis

The 4799 records used in the analyses were collected over 25 years (1965-89) at Livestock Research Centre, Malya. The records included weight records from birth to 72 weeks of age. The analyses were carried out by fixed effects models using the General Linear Model (GLM) procedure of SAS (1987). The fixed effects included in the model were year and season of birth, sex of kid, type of birth and age of dam. The residual mean square was used as the error term to test the significance of all differences evaluated among classes. Where analysis of variance depicted significant differences for variables, linear contrasts of least squares means were estimated to test pair-wise differences within factors. Heritability and repeatability estimates were computed by the method of paternal half-sib analysis using VARCOMP procedures of SAS (1987).

Results and discussion

Live weight

Overall least squares means of birth and weaning weights were 2.47±0.02 kg and 11.14±0.15 kg, respectively, (Table 1). Overall live weights at the age of 24,48 and 72 weeks were 12.48±0.26 kg, 16.42±0.35 kg and 25.80±0.44 kg, respectively, (Table 2). Year of birth, sex, type of birth and age of dam significantly affected birth and weaning weight. Season of birth significantly affected birth weight but was not significant for weaning weight. Weights at birth and weaning tended to increase from 1965 to 1975 and thereafter tended to decline. However, weaning weight showed a small recovery in 1987. Live weights at all ages in the present study are higher than those reported earlier by Das (1989) on Blended goats at the same location when data from four years were analysed.

Year of birth, sex, and type of birth significantly affected the post-weaning weights at 24, 48 and 72 weeks of age. Season of birth was significant for live weights at 48 and 72 weeks but not for live weight at 24 weeks of age. Age of dam had significant effects on live weight at 24 and 48 weeks of age, but had no effect on live weight at 72 weeks of age. Post weaning weights tended to increase from 1965 to 1976 and thereafter declined, in some years, to below the live weight at 1965.

The high variation in both pre-weaning and post weaning weights due to year of birth can be explained by variations in amount of annual rainfall which in turn influenced pasture production and availability of feed. Kids, prior to weaning, depend mainly on dam's milk as food, the production of which is directly related to the availability of feeds to does (Pears 1982; Mukundan and Bhat 1983; Groot et al 1993). Seasonal influence on birth weight operates through its effect on the dam's uterine environment mostly in late gestation (Eltawil et al 1970). Season of birth plays an important role in growth performance indirectly through its influence on the dam's nutrition and hence amount of milk available to the unweaned lamb. In the post-weaning period its influence is related to its effect on the quality and quantity of pasture available to the weaned kids.

Birth weight and live weight at all ages were significantly affected by type of birth in the present study. Generally birth weight decreased with increase in litter size. Robinson et al (1977) reported that for lambs in utero, as the number of foetuses increases, the number of caruncles attached to each foetus decreases, thus reducing the feed supply to the foetus and hence reduction in the birth weight of the lambs.

In this study, males were significantly heavier and grew faster from weaning onward, implying that sex effects are more pronounced with age after puberty. These have been attributed to hormonal differences between sexes and their resultant effects on growth (Bell et al 1970).

Weaning weight would reflect mothering ability of dam as well as the inherent growth potential. Thereafter growth potential would predominate. Age of dam was found to have a non-significant effect on live weight at 72 weeks of age only, showing that the mothering ability of does had carried over effect up to 48 weeks of age. Wilson (1987) found that the effect of age of dam was significant on birth weight and growth rate at pre-weaning and that young ewes tend to produce smaller progenies at birth. It is generally known that mothering ability, especially milk production, increases with parity. Older ewes are larger in body and tend to be better milkers (Stobart et al 1986). The effect of parity of dam on kids is thus imparted as maternal influence whose direct influence is limited to the nursing period.

Growth - average daily gain

Overall average daily gain from birth to weaning, and to 24, 48 and 72 weeks of age were 77±1.3 g, 59±1.5 g, 41±1.0 g and 46±1.0 g, respectively (Table 3). Year of birth, sex, and type of birth significantly affected average daily gain from birth to 72 weeks of age. Season of birth had no significant effect on average daily gain except at 48 weeks of age. Age of dam had significant effect on average daily gain up to 48 weeks of age. Growth of kids tended to increase from year 1965 to 1976 and thereafter declined.

It is generally recognised that the early post-natal phase of growth in goats is a critical stage because this is the stage when there is little maternal protection and the kid is exposed to environmental stress which limits rate of growth. The rate of growth of a kid after weaning, however, is partly determined by the genetic potential of the kid and the level of environmental influence, especially during the immediate postweaning stage. Pre-weaning growth rate was lower than the 94.3 g per day reported by Das and Sendalo (1990) for Blended kids at Malya. Blended goat kids in the present study had higher average daily gain compared to that of Black Bengal goats reported by Singh et al (1983) and Black Bengal x Beetal crossbred goats reported by Kanaujia et al (1986). The mean overall post-weaning growth rate was higher than that reported by Reynolds (1989) for West African Dwarf goats in Nigeria.

In the present study single born and male kids grew faster than twin born and female kids. It would seem that birth type effects are commonly observed at pasture (Beischer et al 1992) and it is so mainly because of competition for the limited supply of doe milk. This is supported by Norton and Banda (1993) who found no differences in growth between single and twin born kids when subjected to artificial rearing of the kids.

Table 1. Least squares means of birth and weaning weight of blended goat kids at LRC, Malya.


Factor

Birth weight (kg)

Weaning weight (kg)

N

Mean

SE

N

Mean

SE

Overall

3691

2.47

0.02

2745

11.14

0.15

Year of birth

(0.0001)

(0.0001)

1965

36

2.48

0.08

17

10.41

0.61

1966

80

2.36

0.05

54

10.58

0.35

1967

47

2.39

0.07

29

9.60

0.49

1968

50

2.62

0.07

29

11.61

0.48

1969

135

2.56

0.05

97

12.28

0.31

1970

105

2.51

0.05

69

11.18

0.34

1971

225

2.42

0.04

173

10.81

0.23

1972

92

3.01

0.06

69

13.00

0.35

1973

161

2.58

0.04

82

11.25

0.30

1974

211

2.57

0.04

139

12.06

0.28

1975

161

2.59

0.05

106

14.38

0.30

1976

241

2.80

0.04

167

13.60

0.26

1977

185

2.63

0.04

156

11.03

0.27

1978

308

2.76

0.04

165

11.37

0.27

1979

202

2.57

0.04

174

10.13

0.25

1980

188

2.44

0.04

180

10.13

0.26

1981

132

2.35

0.05

103

9.46

0.30

1982

196

2.38

0.04

130

9.99

0.27

1983

176

2.27

0.04

163

10.22

0.26

1984

192

2.34

0.04

133

9.82

0.21

1985

122

2.21

0.05

113

10.00

0.30

1986

99

2.13

0.06

92

11.11

0.32

1987

111

2.32

0.05

106

13.70

0.30

1988

134

2.19

0.05

113

9.71

0.30

1989

102

2.36

0.05

86

11.09

0.32

Season of birth

(0.0001)

(0.5438)

Dry

607

2.39

0.03

450

11.07

0.23

Wet

3084

2.55

0.02

2295

11.25

0.15

Sex

(0.0001)

(0.0001)

Female

1814

2.37

0.02

1344

10.58

0.16

Male

1877

2.57

0.02

1401

11.70

0.16

Type of birth

(0.0001)

(0.0001)

Single

2070

2.66

0.02

1567

12.44

0.16

Twin

1621

2.29

0.02

1178

0.16

0.16

Age of dam (years)

(0.0001)

(0.0001)

<2

17

2.51

0.12

9

12.90

0.80

2+-3

328

2.35

0.03

246

10.16

0.18

3+-4

1063

2.38

0.02

751

0.12

0.12

4+-5

752

2.49

0.02

573

0.14

0.14

5+-6

618

2.53

0.02

475

11.23

0.15

>6

913

2.57

0.02

691

11.15

0.13

SE denotes standard error.

Table 2. Least squares mean weight (kg) of Blended goat kids at 24, 48 and 72 weeks of age.,LRC, Malya.

Table 3. Least squares means for average daily gains (g) from birth to weaning and advanced ages of blended goat kid at LRS, Malya.

Genetic parameters

The heritability and repeatability estimates for live weight and growth rates are presented in Table 4. Heritability estimates obtained in this study were moderate to low for live weights and average daily gain, ranging from 0.099±0.039 for live weight at weaning to 0.153±0.053 for average daily gain from birth to 24 weeks of age. Repeatability estimates ranged from 0.004±0.035 for average daily gain from birth to 48 weeks of age to 0.534±0.022 for average daily gain from birth to 24 weeks of age.

The heritability estimates in the present study are within the range reported for meat goats by Singh et al (1993), whose heritability estimates for body weights ranged from 0.144±0.149 for 12 months weight to 0.360±0.110 for 6 months weight. Heritability for 3 and 9 months weight was calculated as 0.330±0.110 and 0.295±0.107, respectively. Roy et al (1989) working on Jamunapari kids raised under semi intensive conditions reported that heritability of body weights at 3 and 12 months were 0.432±0.152 to 0.127±0.116, respectively.

Table 4. Heritability and repeatability estimates of live weight and growth traits of Blended goat kids at LRC, Malya.

Trait

Heritability estimates

Repeatability estimates

Birth weight

0.151±0.036

0.202±0.018

Weaning weight

0.099±0.039

0.187±0.022

Weight at 24 weeks

0.148±0.053

0.533±0.022

Weight at 48 weeks

-

0.016±0.035

Weight at 72 weeks

-

0. 188±0.038

ADG1 up to weaning

0.100±0.039

0.171±0.022

ADG up to 24 weeks

0.153±0.053

0.534±0.022

ADG up to 48 weeks

-

0.004±0.035

ADG up to 72 weeks

-

0. 174±0.038

1 ADG = average daily gain.

It is clear from the present and other results that post-weaning growth generally has higher heritability estimates than pre-weaning growth. This would indicate that environmental factors, in relation to additive genetic factors, had more influence on early kid gains than on gains later in the kid's life. This may be attributed to the high maternal influence associated with kid growth performance early in life. High maternal influence has a tendency to increase the component of variance environmental to the lamb thereby lowering heritability estimates (Thrift et al 1973). One could concentrate on traits with high heritability as long as there exists a high positive correlation with other traits of economic value.

The moderately higher heritability estimates for 24 weeks live weight and gain in this study indicate that to select kids for their own genetic merit for weights and gains, it would be best to use body weight at 6 months of age as the selection criterion rather than weaning weight as is often practiced. The 6 months live weight gain should be superior to weaning weight and pre-weaning growth rate since it is much less influenced by maternal effects which tend to obscure the direct additive genetic effect for growth. Selection directed towards weights at later ages would maximise response in birth weight and possible increased frequency of dystocia (Thrift et al 1967; Olson et al 1976; Martin et al 1980). However, selection for weights at later ages would be expected to lead to increased yearling weights which is desirable for meat animals, but may be associated with increased maintenance costs for breeding animals.

Conclusion

The present results on live weights and growth rates of Blended goats indicate that in order to improve breeding value, selection must be based on genotypic rather than environmental superiority. Thus variation due to definable environmental effects must be removed by use of suitable adjustment factors. It is necessary that all known sources of variation influencing the traits of importance be included in the model of analysis, otherwise the results of the study may not be reliable. This can, however, be done only in on-station recorded flocks.

The trait of interest ought to be investigated in the environment under which it is to be typically expressed, since this environment may be the one which is necessary for revealing certain desirable or undesirable genes or, in contrast, the sought after genetic differences may be of little importance or indistinguishable in this environment. Statistical techniques presently utilised for the estimation of breeding values in selection programmes rely on estimates of genetic variation within and between traits of economic importance. The overall impact of any selection programme will depend on the direct and correlated responses that result from selection on the selection criterion. These responses can be predicted a priori by using estimates of genetic and phenotypic relationships between all traits of economic importance.

It is therefore, important that a breeding and selection strategy be developed, preferably involving farmers, such as "Open Nucleus Scheme" for further improvement and enhancing dissemination of Blended goats in Tanzania.

Acknowledgements

The authors are grateful to the Commissioner for Research and Training, Ministry of Agriculture, Tanzania, for permission to present the paper. The authors are thankful to the Zonal Director, ZRTC (C), Mpwapwa and the staff at the institute for contribution in data collection and classification and also thankful to Animal Breeding and Production Systems, ILCA, Ethiopia, for analysis of data.

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