Application of Categorical Traits in the Assessment of Breed and Performance of 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, Abia State, Nigeria
dumukal@yahoo.com

Abstract

The study attempted characterization of West African dwarf sheep in the humid tropical environment using categorical  traits namely coat colour, colour type, coat texture, presence of horn, horn shape and orientation, ear type, orientation and tail type. Data were collected from 1080 sheep, comprising male and female animals stratified into three age groups, namely 0-12 months (kids), 13-24 months (growers) and 25 months and above (matured), which were sampled from nine local government areas, three from each of the three agricultural zones of the region. The effects of sex, age, location and categorical traits on linear body traits were analyzed using the General Linear Model in SPSS appropriate for a completely randomized design, while some categorical traits were scored using percentages.

Analysis of the categorical traits revealed that all the sheep populations had predominantly (greater than 60%) straight, coarse and black coat; thin short tail; horizontal horns and polled females, and reduced ear type. Coat colour did not significantly (P>0.05) influenced live weight and heart girth, however, black sheep had the highest values for these traits (25.6±0.85 kg and 53.6±6.3 cm) respectively. Similarly, coat type had no significant effect on body length; but sheep of straight coat type had longer bodies (45.7±0.64 cm) than the curled type (37.3±0.13 cm) These traits may not be related to these characters, however, they may signify breed background and could be used to sort a breed type of interest and then work within that type to be used for selection .Selection within the local types offers some advantages in specific cases.  Improvement programmes within native types are important when they are used as pure breed or crossbreed in crossbreeding systems. Thus, there is need for more details and extensive documentation on performance of African sheep.  

Keywords: native sheep type, productivity, qualitative characters

Introduction

Phenotypically, the African sheep exhibit great variation in conformation, coat colour, size, height, length and size of tail, presence or absence of horns and their shape, and behaviour patterns, among others (Ngere 2002). A pre-requisite to the discovery of either useful ecotypes or strains is that the breeds will have to be studied, and traits which make them unique, characterized phenotypically.

The reason why sheep farming is important for the African humid tropics is thus not the size of the sheep population but rather the fact that sheep are kept by many small peasant farmers who often have a diet that is deficient in animal protein (FAO 1989). Moreover, the research work carried out in last 10-15 years now allows coherent sheep improvements programmes to be implemented (CBABG 1971). Categorical traits could be used to formulate breed standards for this species, which are non-existent at present. Such breed standards are used as strict benchmarks for controlling entry and continued membership of a breed society. However, most of the categorical  traits, namely coat colour and type, presence/absence of horns, shape of horn, horn orientation and ear type may appear to be of little importance at present but most of them are very important especially if they could be found to be highly genetically related with traits of economic importance.

Furthermore, it is worth noting that colour is one of the few physical characteristics that farmers use to define a breed (Willis 1991). Thus, the various colours of indigenous breeds of sheep indicate that there have been little selection efforts for certain colours, which is the way selection is done in other breeds to result in a particular colour or few closely related colours (Katongole et al 1996) so that they would be known for a certain colour, which can be used in their marketing. The objective of the study was therefore, to characterise sheep in Nigeria based on categorical traits and to determine the importance of coat traits on linear body traits of WAD sheep in the humid tropics.

Materials and methods

The study covered the three Agricultural zones of Abia State, south east, Nigeria. Abia State lies between longitude 04⁰45^I and 06⁰17^I North and latitude 07⁰00^I and 08^o10^I  East. It is located in the tropical rainforest zone of Nigeria, with an area of 6420km²; about 2.6% of the population of Nigeria; and an average population density of 364 per square kilometer (km²). The prominent livestock management system found in the area is the extensive system.

The data used for this study were obtained from 1080 sheep, stratified into kids (0-12 months old), growers (13-24 months old) and matured animals (25 months old and more), which were sampled from the three agricultural zones of Abia State between May and August, 2008. Data were collected in the morning before the animals were let 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.

The following categorical traits were studied:

• Coat colour: This was scored either as black, white, black/white, brown;

• Coat type: Scored either as curled hair or straight hair;

• Coat texture: Scored either as coarse or fine;

• Incidence of horn: Scored either as present or absent;

• Shape of horn: Scored either as spiral or curved;

• Ear orientation: Scored either as horizontal or lateral;

• Ear type: Scored either as normal or reduced;

• Tail type: Scored either as short thin or long thin.

Experimental design and data analyses 

The general linear model (GLM) procedure in SPSS (v. 13.0) appropriate for a completely randomized design (CRD) was used to test the effects of sex, age, location and coat traits on growth parameters. The effects of these factors were represented in the following general model:
 

Y_ijklm  = μ + S_i + A_j + C_k + C_l + C_m + L_n +  е_ijklmno   

Where Y_ijklm = the observation of the i^th animal within the j^th age of i^th sex category possessing k^th coat trait and in l^th location

S_i = effect of the i^th sex (i = male, female)

A_j = effect of the j^th age (j = kids, growers, mature)

C_k = effect of the k^th coat texture (k = coarse, fine)

C_l = effect coat colour (white, black, black/white, brown)   

C_m = effect coat type (straight, curled)

L_n = effect of the l^h location or zone (l = Umuahia, Ohafia, Aba)

μ = Overall mean.                               

е_ijklmno = Error term iind(0,δ²).

Significant means were separated using Duncan multiple range test (DMRT).

Results and discussion

The result revealed that coat colour did not influence heart girth and live weight (Table 1). However, sheep in black coats had the highest values for live weight (25.6±0.35 kg), followed by sheep possessing black/white coat (23.4±0.38 kg), while those of white coats and brown coats had the least values (21.6±0.75 kg and 21.7±0.69 kg) respectively. This trend was repeated for heart girth. These findings contradict the  work of  Gratten et al (2008), who revealed that dark coat colour in Soay sheep is associated with large size which is heritable and positively correlated with fitness.  The fact is that coat colour has an important role to play in the adaptability of animals to different ecological zones (Banergi, 1984). Buttressing this, Katongole et al (1996) reported that white coat colour and/or white in combination with other colours especially black or brown, appears to be an adaptive characteristic of Tswana goats to withstand pronounced seasonal fluctuations in the intensity and duration of light, heat and cold experienced in the region. This argument has also been supported by the assertion of Hemmer (1990) that animal domestication can be aided by the use of certain coat colours/patterns which are related to behaviour through the common metabolic pathway of pigments and catecholamine neurotransmitters. The prevalence of black sheep in Abia State, southeast Nigeria could be explained by the usual selection against white coloured sheep which are often preferred for sacrifices.

The predominant coat type was straight hairs, with incidence of curly hairs being low and of similar magnitude (approximately 20%) in combined and separate sexes of sheep within the zones and the entire region studied (Table 2). The predominance of hair in the sheep is consistent with the report of Devendra and McLeroy (1988) that sub-tropical and tropical sheep are predominantly haired. The coat texture, on the other hand, were predominantly coarse hairs, followed by fine hairs and the least (or negligible) being mixture of coarse and fine hairs in combined and separate sexes.

The commonest incidence of colour was black, followed closely by mixed combinations of black/white then brown, while white was the least incident in both sexes within the zones and the entire region. According to Katongole et al (1996), such diverse colours are an indication that there have been little selection efforts in this breed. According to Willis (1991), colour is one of the few physical characteristics that farmers use to define a breed, thus there is need to select the WAD sheep so that they are known for certain colours, for instance black and black/white, which can be useful in marketing.The most prevalent tail type in the WAD sheep was short thin (more than 70%) in both sexes and within the entire region. This corroborates the claims of Williamson and Payne (1978) who observed a slender and short tail without any fat deposits in WAD sheep.
 

Horns were predominantly a feature of male sheep, with no incidence in females in the entire region. This is supported by the findings of Charray and Levif (1992) who asserted that only the rams have horns. Incidence of polled rams has also been reported by some authors. The shape of horns varied, with the commonest type being spiral (more than 60%) in males and entire region and then the curved (less than 40%). 

There was a greater incidence of reduced ear type (about 60%) in both sexes within the zones and the entire region studied, compared with the incidence of normal or long ear type (about 30%). The dominant ear orientation was horizontal (above 60%), with the lateral type lower. These corroborate Charray and Levif (1992), who reported that the ears of WAD sheep are short and usually in horizontal position.

Table 3 shows the summary of WAD sheep type and there characteristic features.

From the foregoing, it appears that the sheep populations have different incidences of these traits . Although, most of the categorical traits may appear to be of little economic importance at present, most of them are very important especially if they could be found to be highly genetically related with traits of economic importance. Figueiredo (1986) reported heritability and repeatability of 47% and 87% respectively for colour in some sheep breeds suggesting that they are heritable and thus could be selected for improvement.

References

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Charray J M and Levif J  1992 Manual of sheep production in the humid tropics of Africa. CTA; CAB International Wilingford Oxon, UK pg 1.13.

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Received 26 June 2010; Accepted 23 October 2010; Published 1 February 2011

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