Effects of genetic and non-genetic factors on body weight, pre-weaning growth, birth type and pre-weaning survivability of lambs in a sheep nucleus station

B A Hagan, S Salifu¹, C Asumah, E D Yeboah and K Boa-Amponsem¹

Department of Animal Production and Health, University of Energy and Natural Resources, P O Box 214, Sunyani, Ghana
bernard.hagan@uenr.edu.gh
¹ CSIR - Animal Research Institute, P O Box AH 20, Achimota, Accra, Ghana

Abstract

The effects of genetic and environmental factors on birth weight (BWT), weaning weight at 12 weeks (WWT), pre-weaning average daily gain (PADG), body weight at 6 months (BW6), 12 months (BW12), birth type and lambs’ pre-weaning survivability were studied in Djallonké (DD) and Djallonké-Sahelian (DS) crossbred lambs at a sheep nucleus breeding centre in Ghana. Data on body weight, growth and pre-weaning survivability collected between 2014 and 2016 were analysed using GLM and logistic regression procedures of SAS statistical software. The overall mean BWT, WWT, PADG, BW6, BW12 and litter size were 2.48 kg, 8.28 kg, 68.3 g/day, 13.0 kg, 17.4 kg and 1.15, respectively. Breed group influenced BWT and WWT of lambs. Parity, year of lambing, season of lambing and birth type influenced (p<0.05) BWT, WWT and PADG but not BW12. Year of lambing, season of lambing and birth type had effect (p<0.05) on BW6. Sex of lamb did not influence (p>0.05) any of the traits studied. Lambs born in the wet season were heavier at birth (2.34 vs 2.09 kg) and weaning (7.71 vs 6.34 kg) and had higher PADG (64.1 vs 51.1 g/day) than lambs born in the dry season. Singleton lambs had heavier BWT (2.51 vs 1.92 kg), WWT (8.26 vs 5.78 kg) and PADG (68.5 vs 46.6 g/day) than lambs born as twins. Year of lambing and birth weight were the only factors that influenced (p<0.05) the chances of lambs’ pre-weaning survivability. This could be altered by improved management to increase the chances of lamb survival. Both genetic and environmental factors had substantial influence on the body weights, growth and pre-weaning survivability of lambs and thus these factors could be manipulated to enhance the traits studied in lambs.

Keywords: Djallonké, Ghana, Sahelian, sheep

Introduction

Small ruminants play important roles in the socio-economic livelihood of households in Ghana. There is no age, social and gender limitations to the ownership of sheep and goats (Baah et al 2012) in northern Ghana unlike cattle, which are owned solely by men. Among the benefits of raising sheep and goats are source of income from the sale of animals, provision of manure for improvement of soil fertility, source of savings and store of wealth, insurance against crop failure and for performing social and cultural functions (Weyori et al 2018).

The West African Dwarf sheep, popularly known as the Djallonké sheep, is the predominant sheep breed in Ghana (Agbolosu et al 2005; Baffour-Awuah et al 2007). Despite the advantage of being hardy, tolerant to tick and trypanotolerance (Koney 2004; Bosso et al 2006), the Djallonké sheep has generally low productivity in terms of growth rate and matured body weight (Gbangboche et al 2006a) and lower reproductive rate (Annor et al 2007; Salifu 2014). The 2-year matured body weight of Djallonké sheep is between 18 and 35 kg (Alhassan et al 1995; Koney 2004). The sale of sheep during the lean season to raise money for the purchase of food, mostly cereals, for feeding entire households in northern Ghana is common. However, the incomes these households obtain from the sale of their animals are usually low due to the small size and relatively low matured body weights of the breeds of sheep households raise. Hagan and Boa-Amponsem (2015) reported mean matured 2-years body weights of 17.20, 23.75 and 20.25 kg for Djallonké sheep in Sagnarigu, Talensi and Wa East districts, respectively, in Northern, Upper East and West regions of Ghana. Households in these parts of the country yearn for improved breeds of sheep with relatively bigger matured body size and well adapted to the harsh tropical environment (Aboe et al 2013).

The Sahelian sheep breed, which is popular in northern neighbours of Ghana (Burkina Faso, Mali and Niger) are known to have higher growth rate, higher matured body weight and higher productivity than Djallonké sheep (Yaro et al 2012), although the Sahelian breed is susceptible to diseases and have low survivability. The lack of well organised and serious crossbreeding programmes between the Djallonké and Sahelian breeds has partly contributed to the low productivity of the sheep stock in Ghana. This is despite the fact that the national breeding objective for sheep is to increase growth rate and matured body weight (Ahunu et al 1995). The importation of live sheep into the country increased significantly between 2008 and 2018 (Veterinary Services Directorate of MoFA 2018) contributing to loss of foreign exchange.

A sheep breeding programme was designed by Animal Research Institute of the Council for Scientific and Industrial Research (CSIR-ARI) to improve the body weight and growth rate of the local Djallonké sheep in northern Ghana. This was aimed at partly achieving food security and reduce poverty in northern parts of Ghana. Different cross breeding strategies were used to obtain two genotypes of sheep in addition to the Djallonké sheep at the sheep nucleus breeding station of CSIR-ARI.

The objectives of this study were to determine the influence of genetic and non-genetic factors on body weights at birth, weaning, 6-month and 12-month, pre-weaning growth rate, birth type and pre-weaning survivability of lambs at the CSIR-ARI Small Ruminant nucleus breeding station.

Materials and methods

Study area

The study was undertaken at the Small Ruminant breeding station of CSIR-ARI at Nyankpala in the northern region of Ghana. Nyankpala is located on latitude 9° 24' N and longitude 0° 59' W in the Guinea Savanna agro-ecological zone with a mean annual rainfall of 1204 mm (MoFA 2011). Grasses interspersed with shrubs characterise the vegetation of the area. It has an erratic and unimodal rainfall pattern from May to October (Konlan et al 2014) and a mean temperature of 28.3°C. There are cold harmattan winds from November to February and a warm dry period in March and April. The dry season is prolonged from November to April (Ansah et al 2012).

Project concept

The project involved a hierarchical structure of a nucleus centre, multiplier and beneficiary farmers (Figure 1). The nucleus centre was established at Nyankpala station of CSIR-ARI where sheep foundation stocks were assembled in 2013 for different matings. Multiplier farmers are elite small ruminant farmers who have at least 40 sheep or goats and proper housing structures to house the animals. They undertake improved management practices such as supplementary feeding, routine control of ecto- and endo-parasites, provision of mineral licks and identification of animals using ear tags. Selected progeny (young rams), which were obtained from the matings at the nucleus centre were distributed to elite farmers at the multiplier level for subsequent matings with gimmers and ewes of the elite farmers. Progeny produced at this level were given to beneficiary farmers at the commercial level of the hierarchy.

Figure 1. Hierarchical structure of the Small Ruminant Breeding Programme

Three years’ data were obtained on the foundation stocks and progeny from different matings at the nucleus centre. The data obtained was subjected to preliminary analysis to determine the performance of the progeny being produced and the influence of both genetic and environmental factors on the expression of some economically important traits.

Management at the sheep nucleus station

Animals were housed in cement block walled and concrete floor pens with aluminium roofing sheet. They were grazed on developed paddocks with pastures comprising Stylosanthesis sp, Cajanus sp and Panicum maximum between 8 and 15 hours daily. Sheep were also offered supplementary feeds of crop residues (Cajanus pod residues, Cajanus leaves, groundnut haulms and urea treated rice straw) and formulated concentrate of corn chaff, maize, soya meal, wheat bran, dicalcium, mineral and vitamin premix and salt before and after grazing. Fresh water was provided freely to all animals. Flock was routinely treated against ecto-parasites by dipping using acaricide and drenching with Albendazole against endo-parasites once a month. Animals were also vaccinated against Peste des petits ruminants once a year. Young rams were grazed in a different paddock from other animals.

All animals were identified by an ear tag with unique identification number. Different genotypes had tags with different colour.

Matings

Five Sahelian breeding rams aged 2 years old served as sires and were used for mating gimmers and ewes. The sires were zero-grazed and pen-matings were done throughout. The sires were pure Sahelian breeds. These sires were thus designated as SS genotype. Each sire was used for mating an average of 20 ewes during each mating season. Ewes were either pure Djallonké (0% exotic gene), designated DD or Djallonke-Sahelian crossbred, (50% exotic genes) designated DS. Ewes were allowed to spend some grazing time in the paddocks during the breeding season and were taken to their assigned rams (sires) when they returned from grazing. Matings between the sires and the two ewe groups generated two types of lamb genotypes or breed groups. Mating between Sahelian sires and Djallonké dams produced lamb breed group with average of 50% Sahelian and 50% Djallonké (DS50) genes; and mating between Sahelian sires and Djallonké-Sahelian crossbred dams produced lambs with average of 75% Sahelian and 25% Djallonké (DS75) genes. These breed groups were named based on the average Sahelian genes lambs carry. Some of the Djallonké breed foundation stocks purchased were mated to three Djallonké rams already at the station prior to the arrival of the five Sahelian rams. Progeny from this crossing had 0% Sahelian genes and were designated as DD00. After three years of matings, three lamb breed groups were generated (DD00, DS50 and DS75) at the nucleus station.

Trait measurements

Records on birth weight (BWT), weaning weight (WWT), pre-weaning average daily gain (PADG), body weights at 6 (BW6) and 12 months (BW12) of age were taken on 366 lambs at the nucleus centre from 2014 to 2016.

Birth weight was taken within 24 hours of birth using a digital weighing scale. Weaning weight was the weight of a lamb at 12 weeks of age when lamb was separated from its dam. Pre-weaning average daily gain was calculated for each lamb, which survived till weaning using the formula below:

Body weights at 6 and 12 months of age were the body weights of lambs that survived until 6 and 12 months of age, respectively measured with digital weighing scale.

Birth type or litter size is the number of lambs born per dam at each parturition and pre-weaning survivability is the proportion of lambs that survived till weaning at 12 weeks.

Statistical analyses

Data on BWT, WWT, PADG, birth type, BW6 and BW12 were analysed using the General Linear Model procedure of SAS (2013) and mean differences for a given trait by factor were carried out using the Tukey’s test. Results were considered statistically significant when p<0.05. The statistical model used for analysing the traits studied is presented below.

Y_ijklmnpqr = m + G_i + P_j + T_k + S_l + X_m + L_n + e_ijklmnpqr

Where:

Y_ijklmnpqr = the observation of birth weight, weaning weight, birth type (litter size), pre-weaning average daily gain, body weights at 6 and 12 months;

m = overall mean;

G_i = fixed effect of the i^th lamb breed group (DD00, DS50 or DS75);

P_j = fixed effect of the j^th parity of dam (Parities 1, 2, 3 or 4);

T_k = fixed effect of the k^th year of lambing (2014, 2015 or 2016);

S_l = fixed effect of the l^th season of lambing (Dry or Wet Seasons);

X_m = fixed effect of the m^th sex of lamb (Female or Male);

L_n = fixed effect of the n^th birth type (Singleton or Twin) (fitted for only BWT, WWT, PADG, BW6 and BW12);

e_ijklmnpqr = random residual associated with each observation ~N(0, σ² _e), where σ²_e is the residual variance.

The two-way interactions of the factors were fitted for all the traits but none of them was significant at p<0.05; therefore, the interaction terms were dropped from the model.

Logistic regression was used to predict the probability of pre-weaning survivability of lambs using lamb breed group, parity, lambing year, lambing season, sex and birth type as categorical predictor variables and birth weight as continuous variable. Pearson correlations among BWT, WWT, BW6, BW12 and PADG were also determined.

Results

The effects of both lamb breed group and environmental factors on BWT, WWT and PADG are presented in Table 1. The mean BWT, WWT and PADG of lambs were 2.48 kg, 8.28 kg and 68.3 g/day, respectively. Both genetic and environmental factors studied had effect (p<0.05) on BWT and WWT of lambs, with the exception of sex. Birth weight of lambs increased (p<0.05) with increase in level of Sahelian genes in lambs. The mean birth weight of DS75 lambs was 60% and 18% heavier than those of breed groups DD00 and DS50, respectively. In addition, birth weights generally increased with increase in dam parity. There was no difference in birth weights between lambs born in 2015 and 2016. The mean birth weight of singleton lambs (2.51 ± 0.05 kg) was higher than twins (1.92 ± 0.08 kg).

The mean WWT of group DS75 was higher (p<0.05) than DD00 but not DS50. Weaning weight of parity 1 lambs (7.36 ± 0.24 kg) was higher (p<0.05) than lambs from parity 2 but not lambs from parities 3 and 4. Weaning weights generally decreased as lambing years advance (2014 – 2016). The mean WWT of lambs that were born in the wet season (7.71 ± 0.27 kg) was higher than lambs born in the dry season (6.34 ± 0.25 kg). Lambs born as single (8.26 ± 0.21 kg) were heavier than twins (5.78 ± 0.34 kg) at weaning.

Lamb breed group and sex did not influence (p>0.05) PADG of lambs. However, parity, year of lambing, season of birth and birth type influenced (p<0.05) PADG of lambs.

The overall mean birth type (litter size), BW6 and BW12 were 1.15, 13.0 kg and 17.4 kg, respectively. All the factors studied did not influence birth type except season of lambing. The mean litter size of lambs born in the wet season (1.21 ± 0.04) was higher (p<0.05) than lambs born in the dry season (1.12 ± 0.04). Year of lambing, season of lambing and birth type influenced (p<0.05) BW6. Lamb breed group and environmental factors did not have any effects (p>0.05) on BW12 of lambs.

Table 3 presents the results of the logistic regression in prediction of the odd ratios of pre-weaning survivability of lambs. With the exception of lambing year and birth weight, all other predictors of pre-weaning survivability were statistically not important (p>0.05). For lambs born in 2015, the odds of surviving to weaning were four times less (OR=0.25) than the livelihood of survival for a lamb born in 2016 (odds ratio = 1). In addition, lambs born in dry season have about 50-50% chance of surviving till weaning as with lambs born in the wet season. Females, on the other hand, have 1.54 times more chance of surviving till weaning compared to male lambs although not statistically significant (p>0.05). For any 1 kg increase in birth weight, the odds of pre-weaning survivability significantly increase by a factor of 1.641.

Table 4 presents the phenotypic correlations among BWT, WWT, BW6, BW12 and PADG. Medium to large positive correlations were observed among the traits studied with the highest correlation observed between WWT and PADG (0.959).

Discussion

Birth weight is an important predictor of later health outcomes in farm animals (Gardner et al 2007) as low birth weight is associated with neonatal mortality (Alexander 1974; Gardner et al 2007). The overall mean BWT of 2.48 kg was higher than the mean birth weights of between 1.70 and 2.07 kg reported by other authors in Djallonké sheep populations in Ghana (Tuah and Baah 1985; Agbolosu et al 2005; Baffour-Awuah et al 2007; Salifu 2014; Ampong et al 2019). The variation in BWT could be attributed partly to management and breed differences in the sheep populations. Breed type is reported to be an important source of variation in lamb birth weight (Hafez et al 2000; Gardner et al 2007). Whilst the sheep population in this study involved Djallonké, Sahelian and their crosses, those of the other authors were solely Djallonké breed.

The overall mean WWT of 8.28 kg was similar to 8.23 kg reported by Agbolosu et al (2005) and Tuah and Baah (1985) but 33% lower than the finding of Ampong et al (2019) in a University Research farm in Ghana. The variation in WWT could be attributed to the differences in the ages at weaning. Whereas lambs were weaned at 12 weeks in this study, lambs at the University research station were weaned at 4 months of age. In Benin, Gbangboche et al (2006a) also reported a higher weaning weight of 10.6 kg at 90 days. The higher WWT in their study compared to this study could be attributed to the higher nutritional status of lambs. Sheep were grazed all year round on cultivated and natural pasture in addition to daily provision of supplementary feed to sheep. These would enhance the production of milk by dams hence better nutrition for their lambs, leading to higher growth rate of lambs.

The overall PADG of 68.3 g/day was similar to reports of Agbolosu et al (2005) and Senou et al (2009) who reported pre-weaning daily gains of 70.5 g/day and 64.9 g/day in Sheep Breeding Stations in Ghana and Benin, respectively. However, Ampong et al (2019) and Gemiyo et al (2014) reported higher PADG of 76.9 g/day and 89.2 g/day, respectively. The difference could be attributed to differences in management of the flock such as provision of better nutrition.

Birth weight

The effect (p<0.05) of lamb breed group on BWT is indicative of the strong genetic influence on birth weight (Gardner et al 2007). Jainudeen and Hafez (2000) indicated that differences between breeds and strains result from genetically determined differences in the rate of cell division. The BWT of groups DS50 and DS75 with average of 50% and 75% Sahelian genes were 36% and 60% respectively heavier than the pure Djallonké lambs (DD00). This is indicative that higher levels of Sahelian genes in lambs correlates positively with lambs’ birth weight. Momani et al (2010) also reported significant effect of lamb genotype on the birth and other body weights of lambs.

The effects (p<0.05) of non-genetic factors such as parity, year of lambing, season of birth and birth type on birth weight corroborate with reports of several authors (Babar et al 2004; Yilmaz et al 2007; Senou et al 2009; Gemiyo et al 2014). Birth weight largely increased with dam parity similar to other reports (Poivey et al 1982; Ebangi et al 1996; Senou et al 2009) with parity 4 lambs being higher than other parities. Heavier BWT in later parities is attributed to heavier dam weight, larger size and physiological imprint in the uterus during the first pregnancy which supports relatively greater foetal growth in subsequent pregnancies (Gardner et al 2007). The differences in BWT due to year of lambing is mainly due to the differences in management and availability of feed at the station. During the early years of the breeding program, the availability of unlimited resources facilitated the procurement of sufficient crop residues and supplementary feed resources for the animals at the station, in addition to the pasture for grazing. This could have affected the availability of nutrients to dams enhancing their nutritional status and thus foetal development. Obese et al. (2013) indicated that birth weight is a function of net supply of nutrients to foetus. Ampong et al (2019) however did not detect significant effect of year of birth on birth weight at the University Research Farm in Ghana due probably to homogeneity of management across years.

The higher BWT of lambs born in the wet season as against lambs born in the dry season could be attributed to the availability of good quality forages to pregnant ewes leading to improvement in their nutritional status before lambing. However, in the southern part of Ghana, Ampong et al (2019) did not record any significant effect on season of birth on lamb birth weight.

Lambs from single birth were 31% heavier than those from twin birth. This agrees with the works of other authors (Baffour-Awuah et al 2007; Gemiyo et al 2014; Ampong et al 2019). The finite capacity of the maternal uterus space to hold foetus, coupled with the competition between foetuses for available space and nutrients in the uterus contribute to the lighter weights of multiple births (Robinson et al 1977; Gardner et al 2007). Sex did not influence (p>0.05) BWT in this study contrary to other reports (Momani et al 2010; Gemiyo et al 2014; Ampong et al 2019).

Weaning weight and Pre-weaning average daily gain

Although lamb breed groups DS50 and DS75 had similar WWT, both were different (p<0.05) from DD00. This is indicative that genetic effect was not eroded even at weaning stage. Benyi et al (2006) and Momani et al (2010) have also reported significant effect of lamb genotype on lamb weaning weight in Ghana and Jordan, respectively. The effects of some non-genetic factors on lamb WWT in the present study agree with results of several studies in Ghana (Agbolosu et al 2005; Benyi et al 2006; Baffour-Awuah et al 2007; Ampong et al 2019), Benin (Gbangboche et al 2006a; Senou et al 2009) and Ethiopia (Gemiyo et al 2014). The variations in WWT due to lambing year and season of birth could be attributed to temporal variations in feed availability and quality, disease incidences and other management practices at the centre. The higher WWT of singletons relative to lambs from multiple birth is due to the absence of competition for dam’s milk. Other authors, however, did not observe effect (p>0.05) of lambing year (Benyi et al 2006; Momani et al 2010; Ampong et al 2019), parity (Senou et al 2009; Gemiyo et al 2014) and birth type (Ampong et al 2019) on WWT of lambs. The non-significant effect (p>0.05) of sex on WWT in this study agrees favourably with the work of Tuah and Baah (1985).

The non-effects of the genetic factors studied on PADG corroborate the findings of Benyi et al (2006) but is contrary to the results of Momani et al (2010). The effects of parity, lambing year, season of birth and birth type on PADG has been corroborated by several authors (Tuah and Baah 1985; Agbolosu et al 2005; Gbangboche et al 2006b; Benyi et al 2006; Senou et al 2009; Ampong et al 2019). The wet season recorded higher PADG due to the availability of abundant quality feed in the grazing pastures. Similarly, temporal variations in management over the years might have partly contributed to the differences in PADG recorded over the lambing years. Sex of lamb was the only non-genetic factor that did not influence (p>0.05) PADG. This finding is in agreement with the results of other studies (Benyi et al 2006; Gemiyo et al 2014; Ampong et al 2019). The availability of adequate quantity and quality of feed for dams at the station ensured the production and provision of enough milk to feed the lambs until they were weaned (Senou et al 2009). Other studies, however, reported contrary results in Djallonké lambs (Gbangboche et al 2006b; Senou et al 2009).

Litter size

The mean birth type or litter size in this study is similar to the 1.14 reported for tropical sheep (Ibrahim 1998). Other authors have reported higher litter size (1.30 – 1.40) for Djallonké sheep in West Africa (Fall et al 1982; Tuah and Baah 1985; Gbangboche et al 2006a). Besides season of lambing, all the factors studied did not have effect (p>0.05) on birth type. This suggests the positive influence of dam nutrition during pregnancy on birth type. Ibrahim (1998) reported that litter size of sheep can be increased by 10 to 40% using better nutrition or hormonal treatment. Although the genetic factors did not influence birth type, the high CV of birth type suggests that adequate environmental variation exists within the trait and can thus respond positively to better management.

Body weights at 6 and 12 months of age

The non-effect (p>0.05) of the genetic factor on BW6 and BW12 is inconsistent with the study of Kumar et al (2008) who reported effect of genotype on body weights from birth to 12 months. The non-effect of lamb breed group could be attributed to the improved nutrition and care provided for the lambs post-weaning. This might have masked the genetic differences of body weight at 6 and 12 months. The non-effects (p>0.05) of all the non-genetic factors studied on BW12 suggest that environmental factors had little effect on the lamb body weight at 12 months. Agbolosu et al (2005), however, reported significant effects of sex, season of birth and year of lambing on 12 months’ body weight of Djallonké lambs in the middle belt of Ghana. The significant variations in BW6 of lambs due to year of lambing, season of lambing and birth type are generally as a result of variations in management over the years, availability of forage for lambs and competition between lambs for dam’s milk.

Pre-weaning survivability of lambs

The odd ratios of pre-weaning survivability for all the predictor variables studied were not significant (p>0.05) except for lambing year and birth weight. This finding agrees with Hagan and Dwumah (2015) who also reported no effects of many environmental factors with the exception of year of birth on pre-weaning survivability of lambs in a research station flock in southern part of Ghana. The odd ratio of a lamb born in 2015 surviving to weaning as against a lamb born in 2016 surviving to weaning was 0.25. This indicates that the odds of pre-weaning survivability of lambs born in 2015 is minor (OR= 0.25) compared to lambs born in 2016 (OR=1). Year of birth has also been reported to have effect on kid pre-weaning survivability (Hagan et al 2014). The differences in pre-weaning survivability of lambs due to year of lambing in this study could be attributed to differences in management, diseases and climatic conditions over the years (Al-Najjar et al 2010). Favourable management and climatic conditions would probably have offered the lambs born in 2016 a better chance at survival to weaning than lambs born in the other years. The odds of female lambs surviving to weaning was 1.54 times as those of male lambs and this is in agreement with the results of Turkson and Sualisu (2005) who suggested that male lambs have a lower survival rate than female lambs. The odds of pre-weaning survivability in single born lambs is not different (p>0.05) from those of twins and this agrees with Turkson and Sualisu (2005) whose reported no difference between the odds of death in twin lambs as against single born lambs. Our finding could be explained by the good care given to dams and their lambs during the pre-weaning stage, which ensures all lambs have equal chance at survival till weaning.

A one unit increase in birth weight of lambs increases the odds of pre-weaning survivability by 1.641 times. This suggests that lambs with higher birth weight have greater chance at surviving to weaning than lambs with lower birth weight. This finding corroborates the findings of Turkson and Sualisu (2005) in Sahelian sheep in northern part of Ghana kept under on-station conditions.

Correlations among traits

The medium to high phenotypic correlations between BWT and WWT, and WWT and PADG are similar to the report of Jawasreh et al (2018) in Awassi sheep in Jordan. The high phenotypic correlations among the traits studied could be attributed to the common environmental effects that the traits shared (Falconer and Mackay 1996). The very high phenotypic correlations (0.715 – 0.959) between WWT and later lamb life body weights (BW6 and BW12) and PADG suggest that weaning weight of lambs could be used to predict later life body weights of lambs in a population provided animals share a common environment.

Conclusions

• Both genetic and many of the environmental factors studied influenced body weight of lambs at birth and weaning at the nucleus breeding station in Nyankpala. Breed group of lambs influenced only BWT and WWT whilst all the genetic and environmental factors did not influence BW12.
  
  
• Sex did not influence any of the traits studied whilst year of lambing and birth weight were the only significant environmental determinants of lamb pre-weaning survivability in the nucleus population.
  
  
• There is the need to improve management and health conditions at the nucleus breeding station to further enhance the preweaning performance traits and survivability of lambs at the station.

Acknowledgement

The authors are grateful to the technical staffs of the Nyankpala station of CSIR-Animal Research Institute for helping in data collection and management of the sheep flock. We also acknowledge the efforts of the veterinary technician, Mr. Michael Baba Agombire, for the treatment of the flock.

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