Application of Physical Body Traits in the Assessment of Breed and Performance of WAD Sheep in a Humid Tropical Environment

U K Oke and E O Ogbonnaya

Department of Animal Breeding and Physiology,
College of Animal Science and Animal Production,
Michael Okpara University of Agriculture, Umudike, Umuahia, Abia State
dumukal@yahoo.com

Abstract

The study attempted application of physical body traits in the assessment of breed and performance in WAD sheep in the humid tropical environment. Data were collected from 1080 sheep comprising of male and female animals stratified into three age groups, namely 0-12 months (kids), 13-24 months (growers) and 25 months and more (mature) which were sampled from three agricultural zones. The effect of sex, age and location on physical body traits were analyzed in a completely randomized design.

The live weight and all physical body traits were significantly (P<0.05) influenced by location except height at withers (HW). The mean value of these traits identified sheep population in Umuahia zone as  longest in body (49.8±0.89cm) and tail (16.7±0.33cm); Aba zone as heaviest 25.7±0.43kg), largest in heart girth (50.5±1.16cm) and longest in earlobes (9.49±0.21cm) and Ohafia zone as leggy (28.7±0.58cm). This demonstrates a rich diversity of the indigenous sheep population in the region. From description and geographical locations, three natural types were recognized – medium type, large type and small type. Pearson correlation modules showed that most of the phenotypic correlation between body measurement were positive and significant (P<0.05) for all the age groups. Heart girth  was the most related to live weight with correlation  coefficient of   0.98  in kids, 0.72 in growers and 0.90 in adults. Other physical body parameters which are directly related to the size of the animal displayed moderate to very high positive correlation with one another thus any one of these traits could serve as selection criteria. Stepwise linear multiple regression analysis evinced that addition of body length to heart girth in the prediction model increased the precision of body weight prediction. From the foregoing, genetic characterization, which is underway, will hopefully help quantify the relationship among sheep breed, existing diversity and evolutionary history.

Keywords: linear body characteristics, native sheep, productivity, type

Introduction

Characterization is a vital tool in animal selection and breeding. However, dearth of information on the genetic potential and diversity of sheep in Nigeria discourages attempt towards their exploitation and conservation for increased productivity (Buvanendran et al 1980).  The close adaptation of African sheep to their habitat would suggest that until more is known of the characteristics much emphasis should be placed on within-breed selection for genetic improvement.

Unfortunately, records of performance which form the basis of this technique have not yet been developed in most African countries. Although there is an acute awareness of this deficiency, most countries have embarked on development of sheep performance recording scheme at local and national level and are collaborating at international levels (FAO/ILCA/UNEP 1980; James et al 2007). A pre-requisite to the discovery of either useful ecotype or strain is that the breeds will have to be studied and traits which make them unique, characterized phenotypically.

The West African dwarf sheep are indigenous to Nigeria, and have long been adapted to extreme harsh environmental conditions of nutrition, climate and diseases and might be more productive in their own environment than exotic breeds. They can also be valuable experimental animals in fundamental research and potential store of unique genes which may be useful to the industrialized nations. Nigeria has basically four definitive breeds of sheep – the West African dwarf (WAD), the Uda (UD), Balami (BAL) and the Yankasa (YANK) (Olunfunmilayo 2000). They are all adapted to different ecological niches except for Yankasa which is widely spread across the country.        

The WAD sheep is a small breed but not dwarf in the genetic sense. True dwarf are physically weak and poor, whereas the sheep of West Africa have notable physical and sexual vigour and robustness that enables them to withstand the stress of the climate, disease and irregular feeding (Charray and Levif 1992).      

Ibe (1989) stated that body weight and linear measurement of meat animal have been found useful to quantify body size and shape. Searle et al (1989) reported that body length in sheep is almost independent of the environment and therefore literally indicate inherent size. In their study, Jeffrey and Berg (1972) observed that while heart and Parch girth at any age reflects body condition and animals’ skeletal size. Most of the linear measurements reflect primarily the length of the long bones of the animal and when taken sequentially over a period of time, they generally indicate the way in which the animal’s body is changing shape.       

Dorji et al (2006) claims that characterization of indigenous sheep genetic resources and management systems are essential for planning national domestic diversity conservation plans. Furthermore, characterization of sheep in the natural habitat will provide crucial information for a comprehensive breeding policy for the species and permit proper classification of sheep in the humid tropics into varieties and strains. The objective of the study was therefore to characterize sheep in Nigeria based on measurable phenotypic traits. 
 

Materials and methods

The study covered the three agricultural zones of Abia State, Nigeria. Abia State lies between longitude 04^o 45’ and 06^o 17’ North and latitude 07^o 00’ and 08^o 10’ East. It is located in the tropical rainforest zone of Nigeria, with an area of 6420km². The predominant livestock management system found in the area is extensive system.

The data used for the study were obtained from 1080 sheep stratified into kids (0-12 months old), growers (13-24 months old), and mature animals (25 months and above). From each of the zones, three Local Government Areas were purposively selected based on priority in sheep farming. Thus, from Umuahia zone (Umuahia North, Umuahia south, Ikwuano), Ohafia zone (Ohafia, Bende, Isiukwuato), Aba zone (Isialangwa North, Isialangwa south, Ukwa east) Local government Areas were selected.         

Twenty farmers were selected from each of the local Government areas and six animals consequently sampled per farm. Thus, 120 animals were sampled for data collection from each local government area which summed up to a total of 1080 animals from the entire 9 local government areas selected. Data were collected in the morning before the animals were led out for grazing. The age of each animal was judged both by dentition according to Charray and Levif (1992) and enquiries from the sheep farmers. Pregnant ewes were excluded from the study.      

Data collection and statistical analysis      

Data collected were on linear body traits.  Each animal were posed on a level platform with its four feet squarely placed on a level platform so that it appears to be standing normally without strain and with the aid of a flexible tape and measuring stick, measurement were taken. All reference points for the linear measurement were observed according to Searle et al (1989). The parameters were measured as follows:

•  Body length was the distance from the head of humerus to the distal end of the pubic bone

• Heart girth, measured as body circumference just behind the foreleg

• Height at wither, distance between the most cranial palpable spinosus and the ground

• Neck girth, as taken as the distance round the mid-neck region

• Foreleg length, the distance from the proximal extremity of the olecranon process to the mid-lateral point of the coronet

• Tail length, measured from the base of the tail to the end of the cocygeal vertebrae

• Live weight, measurement with the aid of 500kg-capacity carrying weighing scale.

 The general linear model (GLM) procedure in SPSS (V. 13.0) appropriate for CRD was used to test the effect of sex, age, location on linear body parameters. The effects of these factors were represented in the following general model:

Y_ijk = µ + S_i + A_j + C_k + e_ijk

Where

Y_ijk = the observation of the i^th animal within the j^th age of i^th

µ = Overall mean

S_i = Effect of the i^th sex

A_j = Effect of j^th age

C_k = Effect of the k^th location/zone

E_ijk = Error term, iind (0, σ²)

Significant means were separated using Duncan’s Multiple Range Test (DMRT). The Pearson correlation analysis was used to determine the nature of the association between growth traits. Linear stepwise multiple regressions were then fitted to determine the relative importance of live animal measurement in a model designed to estimate body weight. The general regression model used is given as

where 

Y = Liveweight

a = Intercept

b = Regression coefficient

X_i = Linear body traits                       

e = Standard error.
 

Results and discussion     

The structure of indigenous WAD sheep population sampled from the three Agricultural zones of Abia State is presented in Table 1.

 Generally, there were more female than males. These findings were similar to those of Katongole et al (1996), who observed 69% females and 31% males from their study. According to them, such flock structures arise because farmers know that maintaining a constant flock size depend on keeping the more reproductively active females longer than males. The 13-24 months age group dominated the sampled population. This is explained by the findings of Nsoso et al (2006) that the traditional meat market favours animals of older age groups (greater than 12 months). Older age groups have been selected for various traits and culls are sold for meat production or to other farmers who also continue to do some selection to suit their farming needs.           

Mean values of the live weight and linear body measurement of the sheep population are presented in Table 2.

 Generally, all linear body traits significantly (P<0.05) increased with age, the highest being in the older animals (25 months and above) and the least in the kids, with the growers being intermediate. Similar trends were also observed irrespective of location or zone and sex. These observations corroborate the findings of Owen et al (1977) Katongole et al (1996) and Nsoso et al (2006) who reported similar trends in sheep breeds. Such trends represent growth and development of the animals where weight and other measurement increase with age.           

Table 3 presents the mean values of the linear body parameters for male and female WAD sheep studied. 

Some of the linear body traits were significantly (P<0.05) affected by sex difference only at 0-12 months of age. There were heart girth, height at withers and neck girth, for which the males had the highest mean value, 41.8±1.43cm, 39.3±1.41cm and 18.0±0.55cm respectively. Moreover, the average value of live weight and other body measurement in male kids were higher than those of the female of similar age group. This trend was repeated for growers except for live weight, heart girth and body length. Fitzhugh and Bradford (1983) reported higher value in male than in female sheep.           

The mean values of the live weight and linear body measurement at various age groups are shown in Table 4.

The linear body traits significantly (P<0.05) increased with age, the highest being in the older animals (25 months and above) and the least in the kids, with the growers being intermediate. Similar trends were observed irrespective of location or zone and sex. These observations corroborate the findings of Owen et al (1997), Katongole et al (1996), Nsoso et al (2006) who reported similar trends in sheep breeds. Furthermore, Riva et al (2003) observed that body measurement varied with increasing age, the smallest were 12-18 months old ewes, whereas little variation was observed after 18 months of age.  Such trend represents growth and development of farm animals where weight and other measurements increase with age.  

Correlation analysis of the measured parameters showed strong associations (Table 5). Most variable pairs were significantly (P<0.05) and positively correlated. The coefficient of the correlation between the linear body measurements and body weight were between 0.33-0.98 in kids, 0.12-0.72 in growers, and -0.20-0.98 in mature animals. This is in line with Salako (2006) who reported a value of 0.41-0.99 in WAD sheep. The heart girth was the most related (P<0.05) to live weight and the correlation between these two traits were 0.98 (kids), 0.72 (growers) and 0.98 (mature animals) respectively. Other variables which are directly related to the size of animals displayed moderate to very high, positive significant (P<0.05) correlations. This agree with Tiamiyu et al (2000) who observed that interrelationships among linear body measurements in sheep were high and positively (P<0.05) correlated with one another. The magnitude of the coefficient reflects active growth at this age group in the species. Many workers have used correlation among body parameters to assess the parameters and positively to implicate the genetic control of certain parts on the same genomic region. Such knowledge may be crucial to the constructive manipulation of measurable parts in an attempt to alter conformation (Brown et al 1974). Since all the body measurements, except tail length in kids and growers, has high correlation with body weight, anyone of the traits may be used as selection criteria (Hamayun et al 2006) as  it suggests high predictability among them (Hall 1991).

 The relationships among live weight and heart girth and other traits were further studied using a stepwise multiple regression analysis by adding other body measurements, one at a time, to chest girth (Table 6). The essence was to determine how other body measurement influence the precision of live weight predictions compared to using chest girth alone (Vargas et al 2000). The result of this work shows that heart girth was the singular predictor of body weight in both sexes at 0-12 months of age. The R² values showed that heart girth contributed to 98% and 95% in the variation of body weight of male and female respectively. In the female, growers, body length assumed the best predictor of body weight with R² value of 57.2%, which increased to 65.1% on addition of heart girth. This scenario could be explained by the findings of Mukhergee et al (1986) and Singh et al (1987) who reported that at later stages (19-24 months, and onwards), the body length assumes more importance as an indicator of live weight in beetal goats. In the male sheep of same age range, heart girth solely explained 73.9% variation in body weight. This finding supports the report of Dlamini (1988) that heart girth is a more reliable predictor of live weight in male than in female indigenous Swazi goats. The overall result of the multiple regression analysis indicated that addition of other measurements to heart girth resulted in significant improvement in accuracy of prediction. However, under field condition, live weight estimation using heart girth alone would be preferred to combination with others   because of difficulty of proper animal restriction during measurement.

Conclusion

• There is rich diversity among the indigenous sheep population of Abia State, Nigeria, in terms of linear body traits and live weights.

• From description and geographical locations, three natural types were recognized – Ohafia medium type, Aba large type and Umuahia small type. Existence of these diverse local types in terms of body size and conformation gives good opportunity to develop the indigenous sheep industry through selective breeding and thus may also ensure long-term conservation and sustainable use of native sheep genetic resources in the humid tropics.
   

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Received 14 June 2010; Accepted 23 October 2010; Published 1 February 2011

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