Review: Effects of different growth rates in broiler breeder and layer hens on some productive traits

Review: Effects of different growth rates in broiler breeder and layer hens on some productive traits M. Buzala1 and B. Janicki Department of Animal Biochemistry and Biotechnology, UTP University of Science and Technology, Mazowiecka 28, 85-084 Bydgoszcz, Poland

Key words: feed intake, hormone, muscle, adipose tissue, genetic selection 2016 Poultry Science 0:1–9 http://dx.doi.org/10.3382/ps/pew173

INTRODUCTION

ity by increasing egg production in layers and reducing the number of rearing days necessary to achieve market body weight in broilers (Hafez and Hauck, 2005). In 1985, a 1.40-kg broiler required 3.22 kg of feed at 35 d of age; 25 years later, a 2.44-kg broiler was produced on 3.66 kg of feed (Siegel, 2014). Between hatching and slaughter, broiler breeders increase their weight by up to 60 times, and on d 42 of age their body weight is 5 times as high as in layer hens during the same rearing period (Zhao et al., 2004; Druyan, 2010). In response to genetic selection, the body weight of broiler breeders increased by over 400% and during the same time feed conversion improved by 50%. Selective breeding also increased the yield of pectoralis minor (by 30% in males and by 37% in females) and pectoralis major muscles (by 79% and 85%, respectively) (Zuidhof et al., 2014). The negative consequences of intensive genetic selection in broiler breeder and layer hens include abnormalities in cardiopulmonary performance (Olkowski, 2007), skeletal system (Julian, 2005), immune system (Leshchinsky and Klasing, 2001; Koenen et al., 2002; Parmentier et al., 2010), as well as deterioration of meat quality (Bailey et al., 2015) and fatty tissue buildup (Resnyk et al., 2013; Dong et al., 2015). Therefore, the aim of the study was to discuss the consequences of divergent growth rates resulting from long-term genetic selection on the productive traits of broiler breeder and layer hens.

Genetic selection has been carried out for several dozen years mainly to improve reproductive traits in layer hens and production traits in broiler chickens (Emmerson, 1997; Buzala et al., 2015; Nangsuay et al., 2015). It caused considerable differences in the rate of growth and development and thus in poultry metabolism, both during embryogenesis and after hatching (Bednarczyk et al., 1985; 2000b; Rosinski and Bednarczyk, 1997; Bednarczyk and Rosinski, 1999; Sawicka et al., 2015). These differences are due to both genetic (Sato et al., 2006a,b; 2007; Rubin et al., 2010; Alexander et al., 2015) and epigenetic modifications (Ho et al., 2011; Al Musawi et al., 2012). The basic molecular mechanisms that have led to these differences in broiler breeder and layer hens are not completely understood despite visible changes in systemic metabolism of the birds (Julian, 2005; Buzala et al., 2015). These differences are seen in yolk sac, hormone and lipid metabolism, gas exchange and thermogenesis, and contribute to differences during hatching or body weight of the chicks (Buzala et al., 2015). After hatching, differences are seen in broiler breeder and layer hens mainly in feed consumption, growth rate, efficiency of nutrient utilization, and muscle fat content and development (Havenstein et al., 2003a,b; Reyer et al., 2015). This increases the production profitabil-

FEED CONSUMPTION AND GROWTH RATE

 C 2016 Poultry Science Association Inc. Received January 24, 2016. Accepted April 5, 2016. 1 Corresponding author: [email protected]

The intensive genetic selection of poultry has produced highly specialized lines/strains of birds selected 1

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broilers and layers. The paper discusses the consequences of different growth rates resulting from longterm genetic selection on feed intake, efficiency of nutrient utilization, and development of muscles and adipose tissue, with particular consideration of the hormonal mechanisms of appetite control in broilers and layers. The information presented in this review paper shows that it would be worth comparing these issues in a meta-analysis.

ABSTRACT Genetic selection that has been carried out for several dozen years has led to significant progress in poultry production by improving productive traits and increasing the profitability of broiler breeder and layer hen production. After hatching, broilers and layers differ mainly in feed intake, growth rate, efficiency of nutrient utilization, and development of muscles and adipose tissue. A key role can be played by hormonal mechanisms of appetite control in

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EFFICIENCY OF NUTRIENT UTILIZATION Genetic selection has also resulted in less efficient nutrient utilization in broilers than in layers (Sakomura, 2004). Research showed that regardless of the type of feed, broiler chickens metabolize about 2.5% (1 to 7%)

less energy than layer hens, which may be associated with differences in the shape and function of the digestive tract. The average apparent metabolizable energy (AMEn) of wheat and barley is similar in both bird types, and that of corn, soybean and wheat bran is higher in layer hens compared to broilers. In broiler breeders, excessive consumption of metabolizable energy may cause fat deposition, and its deficiency in peak-of-lay hens may reduce egg production (Pishnamazi et al., 2005; Swennen et al., 2007). A larger and more muscular gizzard as well as longer intestines in relation to body weight in layer chicks compared to broiler chicks may increase food breakdown and the absorptive capacity of the intestine (Shires et al., 1987). Layer hens have longer digesta retention times in the crop and gizzard compared to broilers, which may increase nutrient digestibility (Shires et al., 1987; Pishnamazi et al., 2005). From hatching to 14 d of age, the proportion of intestine and liver is greater in broilers, but pancreas growth is greater in layer hens (Nir et al., 1993). Parker et al. (2015) concluded that long-term selection for body weight is associated with differences in pancreas development. Pancreases from chickens selected for high body weight were found to have larger pancreatic islets, less pancreatic islets mass, and more pancreatic inflammation compared to chickens selected for low body weight (Sumners et al., 2014). Broiler chickens have a greater absolute intestinal weight compared to unselected birds (Jackson and Diamond, 1996; Schmidt et al., 2009). At 56 d of age, gut weight as a percentage of live body weight in broiler chickens increased by 73% in response to genetic selection (Zuidhof et al., 2014). It was also found that genetic selection has contributed to differences in the expression of nutrient transporters in the small intestine of chickens selected for high or low body weight, which may influence the growth and development of these birds (Mott et al., 2008). In addition, broiler- and layer-type chickens differ in the structure and function of lymphoid organs, such as the spleen (Koenen et al., 2002). A higher spleen weight was found in broilers, but relative spleen weight was higher in layers (Parmentier et al., 2010). Other studies found that in contrast to layer selection, broiler selection reduces relative heart and spleen weight, and increases liver weight (Sandercock et al., 2009; Schmidt et al., 2009). The activity of digestive pancreatic enzymes is similar in both types of hens, but the activity of enzymes in small intestine is lower in broiler chickens. The secretion of digestive enzymes in newly-hatched broilers could be a limiting factor in digestion and, consequently, in food intake and growth (Nir et al., 1993). In newly-hatched broilers, the activity of disaccharide hydrolases (maltase and saccharase) is 2- to 5-fold higher than in layer hens. From 7 d of age, their activity is significantly higher in layers than in broilers. Until 14 d of age, plasma amylase activity gradually increases and is higher in broilers than in layers. No differences were noted in the activity of alkaline phosphatase and lactate dehydrogenase

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to maximize reproductive traits or to improve growth rate and feed conversion (Bednarczyk et al., 2000a; Nowaczewski et al., 2010; Wolc et al., 2010). As a result, daily feed intake and feed consumption rate is 2- to 3-fold higher in broilers than in layers starting from 2 d of age (Masic et al., 1974; Nir et al., 1993; Mahagna and Nir, 1996; Hocking et al., 1997; Saneyasu et al., 2011), but feed consumption in relation to body weight gain is lower compared to layer chicks (Yuan et al., 2009). During the first 3 wk of life, the feed conversion ratio is better in broilers (Mahagna and Nir, 1996). Birds spend the same proportion of time ingesting the feed, but the share of time spent on the eating is smaller in layers than in broilers in the morning, but similar in the afternoon. Feed consumption rate is higher in the morning than in the afternoon in both strains (Hocking et al., 1997). In turn, other studies revealed that broilers spend less than half the time eating compared to layers, but eat more meals which are larger and shorter than in layers (Masic et al., 1974). What is more, broilers spend more time drinking compared to layers (Hocking et al., 1997). It was also found that broilers have more taste buds and are more sensitive to the bitter taste compared to layers (Kudo et al., 2010). Birds selected for egg production stop eating when their metabolic needs are met, whereas broiler breeders selected for meat production do not stop until their gut is completely full (Hafez and Hauck, 2005). This confirms that the satiety mechanism dominates the hunger mechanism in broilers, and satiety and hunger mechanisms are equally involved in regulating feeding behavior in layer chickens (Bokkers and Koene, 2003). In addition, contrafreeloading is the observed behavior when an animal is offered a choice between food provided or food that requires effort, and chooses the food that requires effort. It was found that broiler chickens show significantly less contrafreeloading, and are more inactive and perform less active behaviors than layer hens. Contrafreeloading in these birds may indicate adaptive reallocation of energy resources in response to selection for increased production traits (Lindqvist et al., 2006). As a consequence, 42-day-old broilers weigh 4 to 5 times as much as layer hens (Zhao et al., 2004; Hassanpour et al., 2010; Ho et al., 2011). The difference in body weight between these two types of chickens is due to the considerable increase in growth rate in broilers during the first 2 wk of age (Druyan, 2010). Also between 4 and 20 wk after hatching, growth rate is higher in broilers than in layers (Cooke et al., 2003). The body weight of 42-day-old broilers decreased by 51% when fed with layer diets, whereas that of layer hens increased by 35% when fed broiler food (Zhao et al., 2004).

PRODUCTIVE TRAITS IN BROILER BREEDER AND LAYER HENS

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Table 1. Appetite-regulating peptides. Orexigenic Noradrenaline Growth hormone-releasing hormone (GHRH) Melanin-concentrating hormone (MCH) Neuropeptide Y, S, and AF Actin-Related Protein (ARP) Ghrelin Orexin A and B Galanin

Anorexigenic Serotonin, Dopamine Leptin Corticoliberin (CRH) Alpha-melanocyte-stimulating hormone (alpha-MSH) Glucagon-like peptide (GLP1) Insulin Tumor Necrosis Factor-α (TNF-alpha) Amylin

(after Strzalka et al., 2010, as modified by the present authors).

HORMONAL APPETITE CONTROL Neurohormonal appetite regulation involves many digestive tract, adipose tissue, and endocrine gland hormones, many central, especially hypothalamic peptides, and numerous neurotransmitters. They can be categorized into orexigenic (appetite stimulating) and anorexigenic (appetite suppressing) neuropeptides (Table 1) (Keer-Keer et al., 1996; Yuan et al., 2009; Strzalka et al., 2010). Differences were found between highand low-weight-selected chickens in feed intake due to differences in gene expression profiles of different neuropeptides in the hypothalamus (Newmyer et al., 2013; Yi et al., 2015). Although most of the orexigenic neuropeptides in the hypothalamus did not differ between low- and high-weight-selected chicks, low-weightselected chicks expressed higher levels of anorexigenic neuropeptides, which may directly contribute to the reduced appetite in low-weight-selected chicks (Yi et al., 2015). Agouti-related proteins stimulate feed intake in layer-type chickens but not in broiler chickens, which suggests that they are not responsible for increased feed intake in broilers (Tachibana et al., 2001a). Administration of neuropeptide Y (NPY) considerably increases feed intake in broiler and layer chickens, thus playing a key role in stimulating food intake in the chicks. The hypothalamic content of this neuropeptide is lower in day-old layer than broiler chicks (Zhou et al., 2006). At 7 d of age, broiler and layer chicks show a similar expression of hypothalamic NPY (Yuan et al., 2009). mRNA level of hypothalamic NPY, which has an effect on feed intake, is lower in broiler than in layer chicks. Furthermore, mRNA level of NPY receptors (Y1, Y5) is also much lower in broiler chickens compared to layer hens (Saneyasu et al., 2011). Zhang et al. (2013) reported that after intraperitoneal injection of insulin,

NPY and receptor subtypes 1 and 5 mRNA were significantly greater in the hypothalamus of low-weightselected compared with high-weight-selected chickens. Yuan et al. (2009) found no significant difference between the 2 bird types in the hypothalamic expression of neuropeptide Y, agouti-related protein, proopiomelanocortin, orexin, leptin receptor, acetyl-CoA carboxylase, and fatty acid synthase. In turn, injection of the appetite-suppressing polypeptide insulin into brain ventricles significantly inhibits feed intake in layer chicks, but not in broiler chickens. mRNA expression level of hypothalamic insulin receptor is significantly lower in broiler chickens compared to layer hens fed on an ad libitum basis. Feed deprivation significantly decreases the level of insulin receptor mRNA in layer hens but not in broilers. In addition, plasma insulin concentration shows a significant negative correlation with hypothalamic insulin receptor expression in both types of birds fed ad libitum. These findings indicate that insulin resistance occurs in the central nervous system of broiler chickens probably because of persistent hyperinsulinemia, which reduces insulin receptor expression in the central nervous system compared to layer chicks (Shiraishi et al., 2011). Smith et al. (2011) showed that the decrease of the anorexigenic effect of insulin in highweight-selected chicks might be one of the causes of the difference in body weight between high-weight-selected and low-weight-selected lines. It should be noted that the effect of insulin-like growth factor (IGF) (Duclos et al., 1999; Wu et al., 2011) and anorexigenic effect of amylin (Cline et al., 2010), corticotrophin releasing factor (CRF) (Cline et al., 2009), leptin (Kuo et al., 2005; Ninov et al., 2008), neuropeptide S (NPS) (Cline et al., 2008a) or neuropeptide AF (NPAF) (Newmyer et al., 2010), as well as orexigenic effect of galanin (Hagen et al., 2013), ghrelin (Kaiya et al., 2009; Xu et al., 2011), obestatin (Xu et al., 2011; Song et al., 2012) and gonadotropin-inhibitory hormone (GnIH) (McConn et al., 2016), on feed intake in layer and broiler chickens has not been adequately studied. Other studies demonstrated that intracerebroventricular injection of muscimol (agonist of γ -aminobutyric acid, GABAA ), baclofen (agonist of GABAB ) or nipecotic acid (GABA reuptake inhibitor) stimulates feed intake in layer chicks but not in broiler chickens. The presence of differences in the central GABAergic system between chick types confirms that this system has an important

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between meat-type and layer-type hens (Mahagna and Nir, 1996). In addition, the activity of lipoprotein lipase increases much more rapidly in broilers than in layers (Griffin et al., 1991). Broiler chickens also had greater creatine kinase activity than layer hens, which is indicative of greater muscle pathology (Sandercock et al., 2009). In addition, circulating uric acid, glucose, triglyceride, and free fatty acid levels were significantly higher in layer compared with broiler chickens (Swennen et al., 2007).

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to layer chicks. This expression is associated with a significantly lower corticotropin-releasing hormone mRNA and higher accumulation of CRH peptide in hypothalamus, suggesting an augmented GR-mediated regulation of CRH transcription and release in broiler chicks. The expression of the genes related to energy homeostasis and obesity, such as GR, CRH and FTO (fat mass and obesity associated gene), rather than orexigenic (appetite-stimulating) neuropeptides, is impacted by genetic selection and these genes contribute to body weight regulation in the chicken (Yuan et al., 2009). Furthermore, exendin (5–39) enhanced food intake of layer-type chicks under ad libitum feeding. However, this effect was not confirmed for broiler chicks. Endogenous glucagons-like peptide-1 (GLP-1) may play an important role in the regulation of feeding in layer-type chicks but not in broiler chicks, at least early during rearing (Tachibana et al., 2001b).

ADIPOSE TISSUE DEVELOPMENT Reducing accretion of adipose tissue is a new challenge for poultry producers because this allows more nutrients to be directed toward muscle growth, thus increasing the profitability of production (Chen et al., 2014). During embryogenesis and in the first d after hatching, adipose tissue growth does not differ between layer hens and broiler chickens, which suggests that these differences occur later in the rearing period. Fat pads in layer hens and broiler chickens can be seen starting at embryonic d 9. Between embryonic d 20 and hatching, fat pad weights in layer hens and broiler chickens decline, which is due to increased energy expenditure during hatching. Interestingly, neck and leg fat pad weights do not greatly differ between layer and broiler embryos, and broilers have lighter neck fat pad weights at embryonic d 20 and on the day of hatching. When the fat pad weights were compared with total embryonic weight or body weight, layer embryos usually had a higher fat percentage within the body (Chen et al., 2014). Differences in the rate of abdominal fat deposition begin to appear from 4 wk of age (Leclercq et al., 1980). In 5-week-old broiler chickens, the rate of intramuscular fat deposition is 4 times as high as in layer hens (Griffin et al., 1991). It was found that putative adipocyte precursor cells isolated from adipose tissue of broiler chickens proliferate more quickly in vitro than in layer hens, which confirms the differences in the rate of adipose tissue accumulation in these birds (Donnelly et al., 1993). High-weight-selected chickens had significantly reduced efficiency and rates of fatty acid oxidation in abdominal fat compared to low-weight-selected chickens. In addition, high-weight-selected chickens had significantly larger and fewer adipocytes per unit area than those selected for low weight (Zhang et al., 2014). The fat mass and obesity associated gene (FTO) probably plays a major role in liver function and energy metabolism of hens, and its expression is significantly greater in the hypothalamus of broilers compared to

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role in regulating feed intake (Bungo et al., 2003). It was also found that dopamine and dihydroxyphenylacetic acid levels in the brain of one-day-old broiler chicks are significantly higher than in layers (Saito et al., 2004). In addition, the concentration of a neurotransmitter such as serotonin and its metabolite, 5-hydroxyindoleacetic acid increases with the age of layer and broiler chickens (Yamasaki et al., 2003). In chickens selected for low and high body weight, changes in serotonergic and dopaminergic signaling pathways in response to insulin injection suggest a role in whole-body energy homeostasis (Zhang et al., 2015). On d 18 of embryonic development, the concentration of norepinephrine and adrenaline is almost the same in broiler and layer chicken embryos. Thereafter, the content of these two neurotransmitters rapidly increases after hatching, with layer embryos having a significantly higher adrenaline concentration compared to broiler embryos. The concentration of dihydroxyphenylacetic acid and 4-hydroxy-3methoxyphenylacetic acid on d 18 of embryogenesis is higher in broiler than layer embryos, but these differences decrease post-hatching. The changes in these neurotransmitters after hatching may partly explain the difference in feed intake and performance between layer and broiler chickens (Yamasaki et al., 2003). According to Honda et al. (2012), intracerebroventricular injection of α-melanotropin (α-MSH) considerably decreased feed intake in layer and broiler chicks, but other authors (Tachibana et al., 2001a) revealed different effects of this hormone. In broiler chickens selected for low body weight, all doses of α-MSH caused a potent decrease in feed intake, while in broiler chickens selected for high body weight, only the highest dose was effective at reducing feed intake. α-MSH did not influence water intake in either line, and these differences may be due to differential hypothalamic signaling (Cline et al., 2008b). In turn, β -MSH significantly inhibits feed intake in layer chicks but not in broiler chickens, whereas α-MSH has no effect on feed intake in both bird types. Probably, absence of the anorexigenic effect of β -MSH in broiler chickens may be associated with increased feed consumption. What is more, the lack of a significant difference in the hypothalamic proopiomelanocortin (POMC) mRNA between these bird types suggests that the voracious appetite of broiler chickens is not caused by reduced hypothalamic POMC mRNA (Honda et al., 2012). Both postprandially and after insulin injection, POMC mRNA was greater in the hypothalamus of chickens selected for high body weight compared to chickens selected for low body weight, while in fasted birds the expression decreased in both lines, which may contribute to differences in the feed intake and growth rate of these birds (Rice et al., 2014; Yi et al., 2015). Appetitesuppressing effects of corticotropin-releasing hormone (CRH) are stronger in layer hens than in broiler chickens (Tachibana et al., 2006). In a study by Yuan et al. (2009), broiler chicks expressed significantly higher glucocorticoid receptor mRNA in hypothalamus compared

PRODUCTIVE TRAITS IN BROILER BREEDER AND LAYER HENS

layers (Yuan et al., 2009; Tiwari et al., 2012). In addition, the level of FTO mRNA is higher in the liver and skeletal muscle of 8-week-old compared to 4-week-old broiler chickens (Tiwari et al., 2012). The highest FTO mRNA expression was found in the liver of broilers, and in the hypothalamus and cerebellum of layer hens. Significantly higher expression of this gene in the liver occurs in one-week-old broiler chicks compared to layer chicks, whereas significantly higher expression of FTO mRNA in abdominal fat and cerebellum was observed in layer hens. Adult laying hens compared to week-old chicks have a higher FTO mRNA expression in the liver and abdominal fat, whereas adult broiler chickens show greater expression in the hypothalamus and cerebellum. Therefore, FTO mRNA of hens has a greater expression in the tissues which play an important role in energy homeostasis and lipid metabolism (Wang et al., 2012).

One of the most important traits improved as a result of genetic selection in poultry is body weight, which translates into muscle development and thus meat quality (Maiorano et al., 2009, 2011; Bednarczyk et al., 2010; Traveniello et al., 2014). Broiler chickens have more muscle fibers of greater size and their breast muscles grow 8 times as fast as in layer hens (Aberle and Stewart, 1983; Oshima et al., 2007; Zheng et al., 2009; Hassanpour et al., 2010). The size of muscle fibers and their growth rate is 2- to 3-fold greater in broilers than in layers (Zheng et al., 2009). Compared to layer hens, carcass yield of broiler chickens is higher by 100 g/kg (about 16%) of body weight (Sandercock et al., 2009). Breast muscles in both bird types grow faster than leg muscles during the first 2 wk of life (Saunderson and Leslie, 1988). Rate of protein synthesis in breast muscle of 2-week-old broiler chickens is significantly greater than in layer hens (Jones et al., 1986). This is supported by other studies, which showed that muscle cells of broiler chickens are able to accumulate more muscle proteins due to the much slower rate of protein degradation (Orcutt and Young, 1982). The more rapid growth of muscle is paralleled by a lower rate of protein degradation, although at the age of less than 2 wk, differences in protein synthesis rate may also contribute to the muscle growth (Saunderson and Leslie, 1988). Broiler chickens selected for rapid growth are slower to degrade muscle proteins compared to layer hens selected for egg production. The rate of protein degradation in skeletal muscles of young layer hens is between around 1 and 9 times greater compared to broiler chickens (Muramatsu et al., 1987; Harper et al., 1999). Furthermore, it was found that during embryogenesis, carnosine concentration in breast muscles is higher in broilers than in layers (Sato et al., 2009). In layer chicks, excretion of NT methyl histidine derived from degradation of myofibrillar proteins is higher compared to broiler chickens in relation to body weight and

muscling. In broiler and layer chicks, cathepsin D and H activity in breast and leg muscles decreases significantly with age (muscle size) of the chicks. It was noted that lysosomal proteases are not responsible for the differences in muscle protein degradation between layerand meat-type chicks from 1 to 29 d of age (Saunderson and Leslie, 1989). Unlike layer chickens, broiler chickens show low m-calpain activity and high calpastatin (calpain inhibitor) activity, which suggests that m-calpain and calpastatin activity in skeletal muscles differs between chicken types, which have different rates of muscle growth (Johari et al., 1993). During the growth of chickens, the rate of protein degradation in breast muscles is not controlled by ubiquitin mRNA expression or ubiquitin conjugation. The expression of some 26S proteasome regulatory subunits in breast muscle of broiler chickens may be related to the growth of chickens (Harper et al., 1999). The latest research (Saneyasu et al., 2015) indicates that the differences in the expression of genes associated with the proteasome-ubiquitin system in skeletal muscle proteolysis in layer hens and broiler chickens when no feed is available are one of the reasons for the high growth rate of broiler chickens. Thus, the regulatory mechanism of the proteolytic system in skeletal muscles may differ between broiler chickens and layer hens. In addition, the expression of parvalbumins, a group of calcium-binding proteins that play a role in muscle contraction is higher in leg muscle than in breast muscle of both layer hens and broiler breeders. The leg muscle of layer hens shows higher expression of proteins associated with muscle development, growth, oxidative stress and locomotion of hens. These results may suggest that leg muscles of layer hens compared to meat-type chickens are more susceptible to oxidative stress (Jung et al., 2007). Furthermore, the differences in muscle development between layer hens and broiler chickens are reflected in the quality of the meat. Broiler chickens were found to be more often affected by meat quality aberrations compared to layer hens (Oda et al., 2009). Under optimum conditions for growth, the body weight of layer hens is significantly lower than in broiler chickens as a result of their inner genetic differences. The different expression of genes in broiler breeders and layer hens is indicative of considerable discrepancies in the growth rate of skeletal muscles during development and the difference in body weight gain between broiler chickens and layer hens is most pronounced within 2 to 6 wk of hatching (Zheng et al., 2009). In chickens selected for low body weight there is enhanced expression of genes necessary for proliferation of progenitor muscle cells and muscle cell differentiation at day of hatch compared with chickens selected for high body weight, but by d 28 the expression of these genes is reversed (Yin et al., 2014). Several important metabolic pathways were also found, which showed significantly different gene expression between broiler chickens and layer hens. Thus, the differential gene expression profile may be positively or negatively correlated to the different rate of

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MUSCLE DEVELOPMENT

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growth in broiler chickens and layer hens (Zheng et al., 2009).

SUMMARY The different growth rates due to intensive genetic selection of layer hens and broiler breeders result in large differences in the mechanisms of feed intake and utilization of nutrients, which influence muscle and adipose tissue development. A great role in these differences is played by hormonal mechanisms of appetite control. The efficiency of intensive genetic selection in layer hens and broiler chickens can be seen in the higher efficiency of productive traits, and thus in the greater profitability of poultry production. However, it is worth comparing these issues in a meta-analysis.

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