LITERATURE REVIEW

2.1 Origin of Indigenous Chickens

According to modern ornithology, there are four species of the jungle fowl and the red jungle fowl (Gallus gallus) is found to be the major contributor or an ancestor to the domestic fowl (Crawford, 1990). It is believed that the other three wild species (G. sonnerati, G. lafayettei and G. varius) interbred with Gallus gallus and those domestic fowls are carriers of the inheritance from these three species. These fowls give rise to variety of domestic hens of all kinds. These ―fancy‖ breed are of important value. It is very important that these breeds are maintained in the future as ―gene bank‖ because they may comprise major genes that could be exploited cost-effectively (Smith, 1990). Indigenous chickens of today resulted from many cross-breeding with exotic breeds and random breeding within flocks of indigenous fowl. As a consequence, it is not possible to homogenize the characteristics and performance of indigenous chickens (FAO, 1998). Mutant genes such as the frizzled feathers, naked neck, pea, rose and walnut combs are widespread within local birds (Anonymous, 2007). The Naked neck mutation originated in Transylvania, Romania and spread across Europe many centuries ago, while the frizzle feathered chicken was first described by Western explorers in Fiji during the seventeenth century (FAO, 2000).

2.2 Overview of Major Genes in Poultry

2.2.1 The naked neck (Na) gene

Naked-neck chickens look like a cross between a turkey and a chicken with their completely featherless necks and faces. They are often referred to as turkens, Transylvania Naked-necks, bare necks, Hackle-less and Rubber necks and are characterized by the naked-neck trait, caused by a single autosomal incompletely dominant gene (Nthimo, 2004). The naked-neck gene (Na) is incompletely dominant and the heterozygote (Nana) can be identified by a tuft of feathers on the ventral side of the neck above the crop. The homozygous dominant chickens (NaNa) however, either lack this tuft or it is reduced to just a few pinfeathers or small feathers (Crawford, 1976). Scott and Crawford (1977) demonstrated that the presence or absence of the tuft could be used to identify the two genotypes accurately at hatching. The resulting bare skin becomes reddish, particularly in males as they approach sexual maturity (Somes, 1990). The naked-neck chicken is thought to have originated from Transylvania, Romania and was spread all over the world by a Dutch East Indian Company in the course of trading around the 17th century (Ramsey et al., 2000). The Na gene is associated with significantly less plumage cover than chickens not carrying the gene (Nthimo, 2004). They are very colorful – white, red, brown and black feather combinations are found. The autosomal incompletely dominant naked-neck (Na) gene is not only responsible for defeathering the neck region, but it also restricts the feathered area around the body by 20 to 30% in heterozygous (Nana) and up to 40% in homozygous (NaNa) genotypes because of the incomplete dominance of the Na gene (Islam and Nishibori, 2009). The Na gene received greater attention in the recent past in broiler production because of its association with heat tolerance (Merat 18 tropical climate (Horst, 1987). In broiler chickens, the Na gene results in a relatively higher growth rate and meat yield than the normal birds at normal temperature and the effect is more pronounced at high temperature (Cahaner et al., 1993). Higher meat yields were reported for Na genotypes (Younis and Cahaner, 1999; Galal and Fathi, 2001; Patra et al., 2002; Fathi et al., 2008). In expression of sex differences, Na females have 4.8% greater naked area compared to Na males (Howlider et al., 1995). Bordas et al. (1978) reported that the Na birds tend to have more feathers cover as compared to their Na counterparts (41 to 27%) and (33 to 22%) for males and females respectively. Normally, the apterium carry scattered down and semi-plume feathers, but the apterium of the naked-neck birds contain no feathers. The feather tracts themselves are also either absent or reduced in area so that birds have greatly reduced feather cover (Nthimo, 2004). Feather pterylae are absent from the head and neck except around the comb, the anterior spinal tract and two small patches on each side around the comb. Islam et al. (2004) suggested that the Na gene and its effects on heat dissipation positively affect appetite and this happens for two opposing reasons; in cool climates, because of higher energy demands, and in hot climates because of an increase in the upper limits of the critical body temperature. Under such conditions, feed intake increases, resulting in improved body weight, egg sizes and livability. The introduction of the Na gene in chicken breeds seems to improve the resistance of the birds to heat stress (Islam and Nishibori, 2009). The incorporation of Na gene in commercial breeds might contribute to the production of birds with a high genetic potential and better performance at high temperatures. The relationship between the presence of the Na gene and the resistance of the naked-neck 19 20

2.2.3 Normal feather chicken

Birds have evolved many unique and interesting features, allowing them to adapt and radiate into various ecological niches. They display a great degree of diversity in feathers and other body parts. Domesticated birds exhibit an even greater diversity in phenotypes than their wild ancestors, thus providing an excellent opportunity to explore the genetic basis underlying variation in morphology, physiology, and behaviour (Ng et al., 2012). The complex organization of feathers allows for a variety of potential morphological changes to occur. Modifications of the feather include deterrence of feather development, changes in feather structure, inhibition of feather molting, and alterations of feather growth rates. The Nigeria native chickens have very colourful plumage; solid white or black, brown, red and grey or combination of these. They possess different feather patterns, mostly, barred, penciled, laced and partridge with skin and small almond shaped ear lobes that are white in colour. The colour of their egg shell is mainly white or tinted.

2.3 Characteristics of Nigerian Indigenous Chickens

The Nigerian indigenous chicken can be classified as a light breed, with small oblong head and vigorous outlook (Ibe, 1993). Hill and Modebe, (1996) reported that the neck of the indigenous chicken varied in length ranging from 5.6cm to 14cm with a mean of about 10.17cm. They further described the indigenous chicken as having pin lash-white skin, usually covered at a fairly long and densely covering fluff. Eshiette and Okere (1990) reported that the Nigerian chicken has multicoulored feather with several variation. Peters (2000) reported the superiority of frizzled feathered birds over naked-neck birds with regards to body weight and chest girth. However, the naked-neck broiler had higher growth rate and meat yield compared to normal birds under high temperature, (Younis and Cahaner, 1999). He also reported that the frizzle feathered gene, increases heat conductivity of the feather coverage by affecting feather structure, and its effect on layers performance was reportedly significant.

2.4 Adaptation of the Chickens in the Tropical Environments

The strong nature of the local chicken as manifested in their resistance to certain diseases and their ability to thrive well under harsh condition had helped in their adaptation in the tropical environment which is characterized by heat stress (Horst, 1989). Peters (2000) reported that the naked-neck chicken has the highest egg weight followed by frizzle feathered and lastly normal feathered chickens. Variation in egg weight/size, egg length and breadth is said to be influenced by the possession of major genes, dam’s genotype and environmental factors. The possession of major genes influences the utilization of available food reserve for egg production as reported by Peters (2000). Peters (2000) attributed the superior performance of the naked-neck and the frizzle feathered birds when compared to the normal feathered birds in terms of feed efficiency to the thermoregulatory roles of the genes they possess.

2.5 Local Poultry as Sources of Pertinent Genetic Materials

Modern breeding strategies for profitable poultry centre on dedicated production lines derived by vigorous selection from a few breeds and a very large population with a great genetic uniformity of traits under selection (Ac Amovic et al., 2005). There are fancy breeds throughout the world that are characterized by medium or low performance and are often maintained in small populations (Horst, 1989). The genetic erosion of these local breeds may lead to the loss of valuable genetic variability in specific characteristics that are momentarily unimportant in commercial breeding strategies (Ladokun et al., 2008). It can be assumed that local breeds contain the genes and alleles pertinent to their adaptation to particular environments and local breeding goals. Local breeds are needed to maintain genetic resources permitting adaptation to unforeseen breeding requirements in the future and a source of rich material (Notter, 1999).

2.6 Crossbreeding

Crossbreeding is one of the tools for exploiting genetic variation (Adebambo, 2005). Crossbreeding, the mating of two individuals with different breed make-up is one type of a larger class of mating systems called outbreeding. Out breeding has the opposite effect of inbreeding and hence results in increased heterozygosity in a population and decreased homozygosity. A crossbreed or crossbred animal usually refers to an animal with purebred parents of two different breeds, varieties, or populations. The intention is often to create offspring that share the traits of both parent lineages, and producing an animal with hybrid vigor. Crossbreeding is beneficial for two primary reasons. First, a well designed crossbreeding system allows the producer to combine the desirable characteristics of the breeds involved in the cross while masking some of the disadvantages of the breeds. This is frequently referred to as breed complementarity. The second benefit arises from heterosis, which is often referred to as hybrid vigor. In addition to these primary benefits, Adebambo (2005) reported that crossbreeding enables a producer to change a population rapidly with the introduction of new breeds. Adebambo (2005) further reported that crossbreeding is to produce progeny which are more disease resistant, healthier and hardier.

2.6.1 Breed complementarity

Breed complementarity, a major advantage of crossbreeding, is often very important to the success of crossbreeding programmes (Adebambo, 2005). It refers to the production of a more desirable offspring by crossing breeds that are genetically different from each other, but have complementary attributes. Breed complementarities is the result of mixing and matching the mean breeding values of different biological types of breeds.

2.6.2 Heterosis

The term heterosis, also known as hybrid vigor or outbreeding enhancement, describes the increased strength of different characteristics in hybrids; the possibility to obtain a genetically superior individual by combining the virtues of its parents (Shawn, 2012). It is a measure of the superior performance of the crossbred relative to the average of the purebreds involved in the cross. The probable cause of most heterosis is combination of genes from different breeds, concealing the effects of inferior genes. Heterosis may result in the crossbred being better than either paternal breed or simply better than the average of the two. Heterosis is measured by crossing populations to produce an F1 generation, which is compared to the parental populations (Shawn, 2012). Theoretically, the magnitude of heterosis is inversely related to the degree of genetic resemblance between parental populations (Wilham and Pollak, 1985) and is expected to be proportional to the degree of heterozygosity of the crosses (Sheridan, 1981). Thus, heterosis is a result of non-additive genetic effects and may be viewed as overall fitness as well as an expression of a specific trait. It is usually greater for reproductive traits than for growth traits (Fairfull, 1990). Heterosis is influenced by maternal and dietary effects (Liu et al., 1995) and may vary with regard to complex traits (Gram and Pirchner, 2001). In addition, Lamont and Deeb (2001) reported that heterosis for body weight was age dependant. Generating hybrid vigor is one of the most impo Nagpure et al. (1991) demonstrated the effects of crossbreeding using the Japanese Shamo Game, a popular meat bird in Hawaii. This local breed was crossed with Rhode Island Reds, Barred Plymouth Rocks and White Leghorns. The four pure breeds were compared with crossbreds resulting from matings of the Shamo Game males to females of each of the other three mentioned breeds. There was definite improvement in hatchability of all crossbreds over the purebreds except in the case of the Rhode Island Reds. The crossbreds in each group had a lower chick mortality rate than the purebreds of the corresponding group. The crossbreds grew more rapidly than any purebred except the Shamo Game and ate less through the first eight weeks than did any of the purebreds. Kitalyi (1998) outlined the importance of crossbreeding by crossing two strains of chickens. The offspring of the strain cross laid about eggs per year more than offspring of the pure strains. In addition the hybrids laid larger eggs, matured earlier, and had a larger body size than the pure strain hens. There are three types of heterosis, which include Individual heterosis, maternal heterosis and paternal heterosis. Individual heterosis is the advantage of the crossbred individual relative to the average of the purebred individuals; maternal heterosis is the advantage of the crossbred mother over the average of purebred mothers; while paternal heterosis is the advantage of the crossbred sire over the average of purebred sires. Paternal heterosis generally has an effect only on conception rate and aspects of male reproduction. The male parent does not have any direct environmental effect on the survival of the offspring, so the benefits are more limited than those for maternal heterosis. However, the benefit in added conception rate can be substantial, particularly if young males are being used (Anonymous, 2007).

2.7 Effect of the Naked Neck Gene on Performance of Birds

The naked-neck genes conferred better feed conversion, growth rate, feed efficiency and dressing percentage than the normal feathered chicken. The feather structure and feather distribution genes are well adaptive to the harsh tropical environment; survive on low energy feed, highly resistant to diseases and superior to their exotic counterparts. Crossbreeding with the exotic breeds improved body weight greatly at 12 weeks of age. Merat (1986) and Horst and Rauen (1986) studied the effect of temperature variation on the egg production performance of two genotypes (naked-neck and normally feathered birds). Their studies showed that there was a different response of the naked-neck and normally feathered genotypes to high environmental temperature.

2.7.1 Carcass characteristics

The reduction of plumage (20 - 40%) gives 1.5 - 3.0% more carcass yields to the nakedneck genotypes than their normal feathered counterparts regardless of the temperature. Due to the higher proportion of muscle in the pectoral region of naked neck birds, there is 1.8-7.1 percent more meat in them than normal feathered birds when their carcasses are dressed (Merat, 1986). Fathi et al., (2008) reported that the naked-neck genotypes (NaNa or Nana) exhibited higher relative weight of dressed carcass, drumstick and breast muscles compared to normally feathered individuals (nana) and that the proportion of abdominal fat was decreased in both naked-neck genotypes compared with normally feathered ones. Intramuscular and subcutaneous fat in naked-neck birds is low due to the utilization of a larger fraction of energy for thermoregulation (Merat, 1990). N’Dri et al. (2005) observed that slow growing homozygous and heterozygous naked-neck birds under fluctuating temperature, tended to reach the weight of 2 kg 3.3 days sooner than normally feathered birds and that carcass yield of Na birds was higher than that of 27 normally feathered birds (81.6 % vs. 80.0 %). Singh et al. (1996) reported that heterozygous naked-neck broilers gained about 3% more weight than their normally feathered counterparts under commercial conditions during the spring and summer months, and that this advantage was almost tripled at high ambient temperature of about 32°C.

2.7.2 Body weight and growth rate

At 20°C, adult body weight was lower in naked-neck hens, especially the homozygote, than in hens with complete plumage cover, but the trend reversed when the temperature increased above 30°C (Cahaner et al., 1993). The reduction of feather coverage provides relative heat tolerance and therefore, in high ambient temperature, heterozygous nakedneck chickens are superior to their normally feathered counterparts (Cahaner et al., 1993). The naked-neck gene has been associated with increased laying rate, egg size and egg mass in hot environments (Garces et al., 2001; Younis and Galal, 2006). AbdelRahman (2000) researched into the effect of the naked-neck gene on the egg production performance of Sharkasi chickens under subtropical conditions and reported that the naked-neck birds showed significant increases in egg production, 90-day egg number and egg mass by 9.0, 17.80 and 13.30% for Na/na and 3.70, 7.30 and 7.30% for Na/Na respectively compared with the na/na genotype. Garces et al. (2001) and Younis and Galal, (2006) observed that the naked-neck birds also reached sexual maturity significantly earlier than the normally feathered birds by about 5 days. The naked-neck birds were also heavier at 24, 40 and 72 weeks than normally feathered birds (P<0.05 at 40 and 72 weeks of age). The average mortality rate during the laying season was less in naked-neck birds than normally feathered (na/na) ones; however, the differences were not significant. Garces et al. (2001) and Younis and Galal, (2006) stated that the Na gene also reduced feed intake by 12.40 and 13.60% in Na/na and Na/Na genotypes, respectively. The naked-neck birds had a significantly better feed conversion than na/na genotypes. The Na gene led to a significant reduction in egg yolk and shell percentages. Eggs produced from naked-neck birds had a lower breaking strength and egg shell thickness compared with the na/na genotypes. Other effects of this gene on productivity noted by other researchers include reduced effect of high ambient temperature on fertility, (Ladjali et al., 1995), less body weight loss under heat stress and superior levels of heat shock protein, Hsp 70 (Hernandes et al., 2002). Similarly, Fraga et al. (1999) observed the lowest incidence of diseases such as cloaca cysts, ascites, prolapse, Mareks disease, Coccidiosis, Osteodystrophy and Salmonellosis in the naked-neck birds studied. According to Yushimura et al. (1997) among the indigenous chickens, the naked-neck is found superior in terms of egg production, egg size and body weight in a hot and humid environment. Other positive effects associated with this gene on broiler stocks are increased body weight and meat yield, higher body weights, lower fat content and better feed efficiency (Merat, 1986). A study by Njenga (2005) on productivity and socio-cultural aspects of local poultry phenotypes in coastal Kenya showed that the naked-neck phenotypes had significantly higher body weights compared to the normally feathered counterparts. Egg weights ranged from 38±2.9 g to 45±4.5 g, with the naked-neck phenotypes having the highest. The overall mean eggshell thickness for the birds was 0.31mm. The naked-neck had the highest average daily gain among the other four phenotypes. The author concluded that the naked-neck phenotype is superior in productivity when compared to the other phenotypes. Barua et al. (1998) showed that among the indigenous chickens of Bangladesh, the naked-neck fowl performed better in terms of egg and m eat production, and were more resistant to diseases than their fully feathered counterparts. They observed that the crosses between the indigenous naked-neck fowl and the exotic standard breeds performed better than similar crosses using fully feathered indigenous fowl.

2.8 Effects of the Frizzle Gene on Performance of Birds

There is not as much information on the effects of the frizzle gene on productivity as there are in the naked-neck gene. Nevertheless, there is evidence to indicate that the gene may be useful in stocks that have to perform under hot humid conditions (Gowe and Fairfull, 1995). Gowe and Fairfull, (1995) stated that the gene was capable of reducing the insulating properties of the feather cover thereby making it easier for the bird to radiate heat more efficiently from their body. Merat (1990) showed that the frizzling gene resulted in an increase in egg number and mass, alongside reducing mortality under hot and humid conditions. Work by Haaren-Kiso et al. (1988) on F/f and f/f progenies compared under two temperatures, (18-20°C) and (32°C), revealed that the birds carrying the F gene laid more eggs over a 364 day laying period in the hot (32°C) environment. On the other hand, the F gene birds laid only 3 eggs less on average in the cooler (18°C) environment. There was also an increase in egg weight, feed efficiency and viability under the hot environment for the frizzled birds. According to Horst (1988) the F gene is associated with increases in egg number, egg mass and reduction in mortality when the birds are raised under hot and humid conditions. Haunshi et al. (2002) worked on the effect of the naked-neck and frizzle genes on immune-competence in chickens and reported that there were significantly 30 higher haemolytic complement levels in serum observed for the frizzle feathered birds than their normally feathered sibs. Younis and Cahaner (1999) suggested that when reared at high ambient temperature (32°C), birds with frizzle genes perform better in terms of weight gain from 4-7 weeks than their counterparts which are normally feathered. The results indicated that the reduction in feather coverage by the frizzle gene provided relatively better heat tolerance, and therefore, under hot climates the F/f broilers were superior to their normally feathered counterparts. They concluded that frizzled broilers should be preferred in hot climates. Nwachukwu et al. (2006) also observed that the birds with the frizzle gene outperformed their sibs which were either naked-neck or normal feathered in body weights and most of the egg traits evaluated, thus indicating that the frizzle gene may be advantageous in poultry production in the humid tropics.

2.9 Interaction between the Naked-neck (NA) and Frizzle (F) Genes

According to Gowe and Fairfull (1995) some major genes like naked-neck and frizzling are used to improve heat tolerance and are often incorporated in breeding programs with local chickens to increase poultry production. Studies by Younis and Cahaner (1999) have shown that combining the naked-neck allele with another heat tolerant gene like frizzling resulted in a favourable additive effect on various productive parameters. Mathur and Horst (1992) reported that the three genes Na, F and dw interact so that the combined effects of two genes are lower than the sum of their individual gene effects. Mukherjee (1992) observed a positive additive effect on performance when Dahlem Red naked-neck strains were crossed with Dahlem White frizzle strains. Mahrous et al. (2008) observed that the naked-frizzle genotypes had significantly better egg performance, egg quality traits and attained sexual maturity earlier than the normally feathered females.

2.10 Growth Performance of the Indigenous Chickens

Although the Nigerian indigenous chickens possess small body size and grow slowly, it has been concluded that they reach point of inflection earlier than the exotic (Nwosu et al., 1980). Body size of an individual chicken is also determined by its rate of growth (Ibe, 1993). Olawunmi et al. (2008) found that the Fulani ecotype chicken was bigger in size than the Yoruba ecotype chicken 1.76+0.4 and 0.79+0.21 kg respectively. Indigenous male chicken was also bigger in size than their female counterparts 1.5+0.06 kg versus 1.29+0.04 kg respectively (Ajayi et al., 2008). It has also been reported that the frizzle feathered and the naked neck genes conferred better feed conversion on these genotypes when compared to their normal feathered counterpart (Horst, 1997; Gunn, 2008).

2.10.1 Effects of day-old chick weight on subsequent growth of chickens

Due to the lovely appearance, wonderful taste and other unique characteristics, indigenous chickens are preferred to fast growing broilers in Nigeria (Chin, 2003). Genetic selection for high hatching egg number will results in smaller egg weight and thus decrease day-old chick weight (Yannakopoulos and Tserveni-Gousi, 1987). However, the effect of day-old weight on growth rate of indigenous chickens varied among different studies. Some researchers have found that day-old chick weight affect performance of broiler chicks to market age (Mendes et al., 2007), while other studies have shown that, for broiler, the advantage of initially higher chick weight diminishes rapidly during early growth period and has little influence on economic traits at market time (Pinchasov, 1991). 2.10.2 Body weight and body linear measurement Series of techniques are available to gain information about an animals’ body mass and body composition. Some of these techniques use simple, inexpensive equipments and others require sophisticated equipment (Lerner, 1973). The animals’ mass is usually directly determined by weighing. However, under some circumstances a scale may not be available. Alternative way is to measure body parts and relate them to body weight (Lerner, 1973). Shank length has been commonly related to body weight in poultry. This is simply because different coefficients and exponents are needed for different breeds of chickens. Body Weight Adeyinka et al. (2006) obtained body weight of naked neck broiler chickens at day old, 2, 4, 6 and 8 weeks as 37.22±0.32g, 210.46±1.97g, 744.33±4.31g, 1,351±7.91g and 2428.1±14.61g respectively. Okon et al. (1996) observed that live body measurement at 28 days (4 weeks) were 6.21 and 6.09 times their 7 days (1 week) old values for the Lohman Brown and Anak strains respectively and added that the chicks doubles its weight 3 to 5 times before 6 weeks of age. Mature body weight of indigenous chickens in Africa under scavenging system ranged from 1170g – 1500g, 1354g – 1652g for chickens under intensive management, while it was between 1140g - 2000g and 1125g – 2000g respectively in their Asian counterparts (Horst, 1989; Huque, 1999 as cited in Sonaiya and Swan, 2004). Breast Girth Continuous increase in breast girth from 3 – 12 weeks in broiler chicken has been reported (Okon et al., 1996). They recorded a range of 15.59±0.08 to 37.08±0.24 from 3- 33 weeks of age. Essien and Adeyemi (1999) had also observed that breast circumference increased with age from 1 to 7 weeks in Lohman Brown and Anak broiler strains. Okpeku et al. (2003) obtained a slight difference in breast circumference between male and female local chickens in Edo state. 39.03±4.89cm was obtained for male while 38.01±4.50cm was obtained for female. Shank Length Essien and Adeyemi (1999) showed that shank length increases with age and was significantly different among broiler strains. According to Kabir (2006), Rhode Island chickens recorded the mean shank length values of 9.74+0.19 and 12.69+0.06 at 20 and 40 weeks of age respectively. Okoro and Ogundu (2006) observed that shank length varied between sexes in turkey and they recorded higher shank length for males than females. This was also observed in local chicken in Edo State as reported by Okpeku et al. (2003) with males recording up to 9.52±0.32cm and females 8.99±0.60cm. Thigh Length Essien and Adeyemi (1999) reported that thigh length increased with age and differed between strains of broilers. However, Okoro and Ogundu (2006) observed that thigh length differed between sexes in turkey and male had higher thigh length than females. The authors also noticed that breed had significant effects on thigh length.