Introduction
The selection of broiler chickens in the second half of the 20th century has been primarily aimed at obtaining high meat yields at minimum production costs, primarily feed costs. This has led to the development of hybrid broiler strains whose productive traits and goals include attaining maximum biological potential, i.e. fast growth rate, high yields of primal carcass cuts and favourable feed conversion efficiency, ensuring maximum cost-effectiveness and profitability from this type of production.
However, over the last couple of decades, poultry meat production has diversified and become focused more specifically on some other aspects, with different broiler rearing systems gaining increasing importance. Along with the fact that the modern poultry meat market is dominated solely by price competitiveness, this has been radically transformed the market into one equally dominated by both price and quality competitiveness (Bogosavljević-Bošković et al., Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, PAVLOVSKI, PETROVIĆ, DOSKOVIĆ and RAKONJAC2010b). Pavlovski et al. (Reference PAVLOVSKI, ŠKRBIĆ, LUKIĆ, PETRIĆEVIĆ and TRENKOVSKI2009) reported increased consumer demands for poultry meat products resulting from less intensive production systems that involve comfortable rearing conditions for broilers. According to a survey by Fanatico et al. (Reference FANATICO, PILLAI, CAVIT, MELLUENT, EMERT and OWENS2006), consumers highlighted higher health benefits of meat produced under these conditions compared to that from indoor reared broilers. Along with a range of laws adopted in many countries regarding broiler welfare, as well as on environmental protection, the above situation has stimulated interest in non-intensive poultry rearing systems that satisfy all of the above requirements. Bancos (Reference BANCOS2010) outlined major advantages of organic poultry production over intensive production, including a lower risk for human health, higher welfare and better conditions for birds, environmental preservation and cost-effectiveness of the use of this rearing system in rural environments.
The most widely practised and most commonly investigated non-intensive poultry rearing systems include extensive indoor, free-range, traditional free-range and organic systems, commonly characterised by reduced stocking density in the poultry house and pasture availability. Furthermore, age at slaughter is higher in these systems compared to intensive rearing, due to which the use of slow-growing genotypes (Castelini et al., Reference CASTELINI, MUGNAI and DAL BOSCO2002; Nielsen et al., Reference NIELSEN, THOMSEN, SORENSEN. and JOUNG2003; Mench, Reference MENCH2004; Sirri et al., Reference SIRRI, CASTELINI, BIANCHI, PETRACCI, MELUZZI and FRANCHINI2010b) and a modified diet plan is required in order to obtain optimum broiler weight at slaughter as required by the market.
There is a need to define alternative poultry meat production as a rearing method rather than as a new method of minor importance or as a substitute to conventional practises (Bogosavljević-Bošković et al., Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, MITROVIĆ S., RADOVIĆ and DOSKOVIĆ2007; Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, ĐOKOVIĆ, RADOVIĆ, PETROVIĆ and DOSKOVIĆ2009).
The results obtained by a number of authors indicate a high popularity of non-intensive broiler rearing systems in the scientific research field. This has been further stimulated by legal regulations adopted in many European countries as well as by product quality and poultry welfare standards.
Non-intensive rearing systems have been analysed in terms of the effect of a variety of factors. Most authors agree that chicken growth and slaughter traits in such rearing systems are dependent upon genetic factors, broiler hybrid, broiler sex and length of fattening period (Bogosavljević-Bošković et al., Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, PETROVIĆ, DOSKOVIĆ and RAKONJAC2010c).
Given the above, the subject matter of this study involved a comparative overview of the literature on the effect of rearing system on productive traits and meat quality in broiler chickens. Key meat quality traits included dressing percentage, yield, proportion of primal carcass cuts and chemical composition of muscle tissue. The objective of the study was to use this review to determine the effect of rearing system on the above traits as a tool in determining the future direction of research in this field as well as an indicator of its practical application.
Body weight and feed conversion ratio
The effect of rearing system on final body weight has been extensively studied by many authors, most of whom have suggested that free-range rearing results in lower body weight of broilers as compared to indoor rearing. However it appears that the main factor affecting body weight of broilers is genotype, regardless of production system used. Ristić (Reference RISTIĆ2003) suggested a fattening requirement of 10 to 32 more days for slow-growing hybrids to reach the same body weight attained by fast-growing hybrids in 42 days.
In the USA, fast-growing hybrids specifically bred for intensive production are primarily used in free-range and organic production systems, whereas European producers tend to use slow-growing genotypes that reach slaughter weight after 81 days, as prescribed by European Union regulations (Ponte, Reference PONTE2008). This prolonged production period requires the use of slow-growing genotypes, inevitably leading to lower body weights at the end of fattening (Ristić, Reference RISTIĆ2003). Castelini et al. (Reference CASTELINI, MUGNAI and DAL BOSCO2002) stated that fast-growing hybrids are used in organic production systems primarily for economic reasons, although they are not bred for this type of production, which leads to frequent health and welfare problems. Although fast-growing hybrids attain higher body weight under free-range rearing conditions as compared to slow- and moderately fast-growing hybrids (Fanatico et al., Reference FANATICO, PILLAI, CAVIT, OWENS and EMERT2005), most authors give priority to the use of slow-growing hybrids for free-range rearing primarily due to their better adaptation to rearing conditions and higher resistance (Castelini et al., Reference CASTELINI, MUGNAI and DAL BOSCO2002; Nielsen et al., Reference NIELSEN, THOMSEN, SORENSEN. and JOUNG2003; Sirri et al., Reference SIRRI, CASTELINI, BIANCHI, PETRACCI, MELUZZI and FRANCHINI2010b). Ponte (Reference PONTE2008) suggested that the selection of fast-growing hybrids for high productive traits has resulted in their modified behaviour, lower mobility and reduced foraging activity, as confirmed previously by Weeks et al. (Reference WEEKS, NICOL, SHERWIN and KESTIN1994), who observed that fast-growing hybrids spend more time inside or immediately outside the indoor facility than on the range, most likely due to poor leg development and excessive weight. Conversely, Lewis et al. (Reference LEWIS, PERRY, FARMER and PATTERSON1997) reported that slow-growing hybrids spend more time on the range, with higher mobility and better forage feeding. Castelini et al. (Reference CASTELINI, MUGNAI and DAL BOSCO2002) report that fast-growing organic hybrids decrease their growth potential by as much as 25% as compared to intensively reared birds, whereas the decrease in slow-growing hybrids is only 8%, serving as an additional reason to favour less intensive genotypes in alternative broiler rearing systems.
Broilers reared under non-intensive conditions generally have a lower body weight as compared to intensively reared birds or those that are not provided outdoor access. Lewis et al. (Reference LEWIS, PERRY, FARMER and PATTERSON1997) compared the Label Rouge (high welfare) and intensive rearing systems with two different strains of broiler - ISA and Ross 1, and determined a higher daily weight gain, feed consumption and a improved feed conversion ratio in intensively reared birds. Similar conclusions were drawn by Tolon and Yalcin (Reference TOLON and YALCIN1997) and Grashorn (Reference GRASHORN2004). Sekeroglu et al. (Reference SEKEROGLU, DEMIR, SARICA and ULUTAS2009) showed lower body weights and feed consumption values in free-range Ross 308 broilers fed standard broiler feed containing 22, 20 and 18% crude protein in three phases compared to indoor-reared broilers. There were no statistically significant differences between the two systems for feed conversion ratio. A comparison between Ross 308 broilers reared for 42 days with and without outdoor access was performed by Poltowicz and Doktor (Reference POLTOWICZ and DOKTOR2011) who found significant differences in their respective body weights (1.650 kg vs. 1.710 kg) but no difference in feed conversion ratio (1.99 vs.1.94). All chickens were fed ad libitum the same complete diets in the same three phase feeding system. Similar results were obtained by Pavlovski et al. (Reference PAVLOVSKI, ŠKRBIĆ, LUKIĆ, PETRIĆEVIĆ and TRENKOVSKI2009) in a study on the effect of rearing system on Redbro and Arbor Acres, showing that free-range and indoor-reared broilers gave a body weight of 1.667 kg and 1.820 kg, respectively. Lima and Naas (Reference LIMA and NAAS2005) found significant differences in the feed conversion ratio (2.98 vs. 1.97) between chickens reared with and without outdoor access. Significant differences were observed in daily gains (25 g/d vs. 56 g/d) as well as in the time required to reach slaughter weight (80 vs. 45 days). The chickens raised for 45 days were fed in four phases: pre-starter (1-7- day old), starter (8- 21 d), grower (22-38 d) and finisher phase (39-45 d), and those raised for 80 days were fed pre-starter (1-7 d), starter (8-31 d), grower (32-50 d); and finisher phase (51-80 d) using standard feed recommendations for Ross and Hybro hybrids. When interpreting these results, consideration should be given to the fact that different hybrids (Cobb and Hybro) were used in the comparison of rearing systems at different farm locations. Dou et al. (Reference DOU, SHI S.R., SUN and WANG2009) performed experiments on slow-growing Gushi chickens in indoor and free-range systems and determined significant differences in body weights after 112 days (1.610 kg vs. 1.419 kg) in favour of the indoor reared birds. Again, all the birds were fed the same diet. These indoor chickens showed a more favourable feed conversion ratio of 3.95 (versus 4.41). In addition, free-range rearing was found by Baeza et al. (Reference BAEZA, LESSIRE, BERRI and YUIN2001) to reduce body weight.
Castelini et al. (Reference CASTELINI, MUGNAI and DAL BOSCO2002) compared organic and conventional rearing systems and confirmed previous findings. After a fattening period of 56 days, organic Ross broilers had a body weight of 2.861 kg, which was considerably lower than the 3.219 kg recorded for conventionally reared broilers. The chicks were fed the same diets ad libitum. The weight differences at the legally prescribed minimum slaughter age of 81 days were even higher at 3.614 kg vs. 4.368 kg. Feed conversion ratio was found to be lower in Label Rouge organic broilers (2.75 vs. 2.31 after 56 days, and 3.29 vs. 2.89 after 81 days). Filho et al. (Reference FILHO, MENTEN, NEVES DA SILVA, COELHO and SAVINO2003) ascribed higher mobility and lower feed intake in the free-range broilers to their body weight differences. It took more than two days on average for free-range broilers in their study to reach the final body weight of 2.30 kg as compared to intensively reared birds. Likewise, Buchanan et al. (Reference BUCHANAN, HOTT, KIMBLER and MORITZ2007) did not associate free-range rearing with a reduction in feed conversion efficiency, but rather ascribed reduced body weight to lower feed intake.
Santos et al. (Reference SANTOS, SAKOMURA, FREITAS, FORTES and CARRILHO2005) studied Paredo Pedres and ISA Label hybrids under two rearing systems (with and without free-range access) and obtained results that were in complete disagreement with the findings of the already cited authors. They determined that free-range broilers reached a higher body weight at a low feed consumption rate, leading to a more favourable feed conversion ratio at each control point measurement performed on days 21, 49, 77 and 105. The broilers were fed an ad libitum diet containing 20.7% crude protein (1-21 days), 18.7% crude protein (22-49 days), and 17.4% crude protein until the end of the rearing period). The authors attributed these results to better welfare provided under free-range conditions. Barbos Filho et al. (2005) observed that birds’ comfort significantly affected productive performance in different rearing systems. They used four strains of broiler chickens: two experimental strains (Caipirinha and Carijó) and two commercial lines (Embrapa and Paraíso Pedrês) fed a standard two-phase diet. One group of broilers, reared under a semi-intensive system was provided with a sheltered outdoor area for thermal comfort purposes, whereas the other broilers were kept on an open range. The results clearly showed statistically significantly higher final body weights in broilers reared under sheltered free-range conditions. Ponte (Reference PONTE2008) obtained higher body weights and feed consumption in free-range birds owing to increased pasture coverage by high-quality clover that was consumed by the chickens. However, feed conversion ratio did not show statistically significant differences as compared to the non-free-range system.
Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, PAVLOVSKI, PETROVIĆ, DOSKOVIĆ and RAKONJAC2011) found no significant differences in body weights of Cobb 500 broilers reared under two different non-intensive systems, including extensive indoor and free-range, after a fattening period of 56 and 63 days. Similar results were reported by Milošević et al. (2005) who compared Hybro PN broilers reared under extensive systems with and without free-range and found a numerically higher feed conversion ratio in free-range birds, and body weight in indoor-reared broilers (P>0.05).
Mortality
Analysis of mortality as an important parameter in poultry production requires consideration of overall conditions both in the poultry house and on the range, due to the fact that bird death can be caused by a number of factors. Mortality in indoor rearing systems is induced by the following: low temperatures during the first few days of life, high temperatures at later stages, water and feed supply problems, inadequate stocking density, etc. (Sorensen et al., Reference SORENSEN, SU and KESTIN2000; Heier et al., Reference HEIER, HOGASEN and JARP2002). An additional problem in free-range systems is the range itself. Sossidou et al. (Reference SOSSIDOU, DAL BOSCO, ELSON and FONTES2011) reported the main cause of death under these production systems to be higher exposure to diseases and parasites in the outdoor environment, along with the separate problem of predation-induced mortality. Some of the main reasons for the transfer of poultry production to indoor facilities include diseases, parasites and predation (Fanatico, Reference FANATICO2000). Hegelund et al. (Reference HEGELUND, SORENSEN and HERMANSEN2006) reported a predator-induced mortality rate of 6.4% in birds reared under the organic systems in Denmark. Therefore, this problem must be adequately addressed. A potential method to reduce predation in this production system is the use of genotypes that do not have white feathers, as these are less conspicuous to potential predators (Van de Weerd et al., Reference VAN DE WEERD, KEATINGE and RODERICK2009). Moreover, the behaviour of birds coming out to the range immediately after a rain shower to drink rainwater from puddles can result in the spread of different infectious diseases and increase mortality rate (Gordon and Charles, Reference GORDON and CHARLES2002, Jones et al., Reference JONES, FEBER, HEMERY, COOK, JAMES, LAMBERTH and DAWKINS2007). Another reason for the frequently high mortality rate under free-range systems is the use of conventional genotypes showing a daily weight gain of up to over 60 g, which, along with the prolongation of the growing period, leads to high body weights that tend to be unsupported by the cardiovascular and locomotor systems (Bassler, Reference BASSLER2005; Ponte, Reference PONTE2008). To illustrate this, Julian (Reference JULIAN2004) determined a mortality rate of 2-4% caused by sudden heart attacks in male birds showing metabolic disorders. Additionally, mortality can be largely affected by temperature (St. Piere et al., 2003; Vale et al., Reference VALE, MOURA, NAAS and PEREIRA2010) which is almost uncontrollable under non-intensive systems. Poltowicz and Doktor (Reference POLTOWICZ and DOKTOR2011) detected no mortality in indoor reared Ross 308 birds, and a mortality rate of 4.2% in free-range birds. Phelps (Reference PHELPS1991) reported a mortality rate of as high as 15% in free-range broilers as compared to the 4% under indoor rearing conditions, while Filho et al. (Reference FILHO, MENTEN, NEVES DA SILVA, COELHO and SAVINO2003) found no significant differences in the mortality rate between birds reared intensively and those reared under semi-intensive systems in their comparison of four different genotypes. Conversely, Lima and Naas (Reference LIMA and NAAS2005) determined 1.3% mortality in free-range birds and as high as 5.3% in non-free-range systems. However, as these results were obtained on different farms with two hybrids (Ross and Hybro), they must not be taken for granted. Still, these findings contributed to the suggestion that farm conditions have a crucial impact on the mortality rate in broilers.
Dressing percentage and proportion of primal carcass cuts
Research conducted by Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, PAVLOVSKI, PETROVIĆ, DOSKOVIĆ and RAKONJAC2011) showed no significant effect of rearing system on dressing percentage of Cobb 500 broilers. These results agree with those of Poltowicz and Doktor (Reference POLTOWICZ and DOKTOR2011) who found that free-range Ross 308 birds showed only a 0.64% higher dressing percentage in free-range broilers than in indoor-reared broilers. Dou et al. (Reference DOU, SHI S.R., SUN and WANG2009) compared dressing percentage of broilers from indoor or free-range systems and obtained the results 69.90% vs. 69.88% in favour of the indoor floor system. These differences were not statistically significant, as confirmed by Wang et al. (Reference WANG, SHI, DOU and SUN2009). Pavlovski et al. (Reference PAVLOVSKI, ŠKRBIĆ, LUKIĆ, PETRIĆEVIĆ and TRENKOVSKI2009) reported no statistically significant differences in conventional dressing percentage between free-range and indoor reared Arbor Acres and Redbro broilers, although the parameter was 0.40% higher in the latter system. However, a comparison between the dressing percentage of ready-to-roast carcass (76.56%:75.56%) and ready-to-grill carcass (67.13%:65.45%) suggested a higher proportion in outdoor reared birds. Skomorucha et al. (Reference SKOMORUCHA, MUCHACKA, SOSNÓWKA-CZAJKA and HERBUT E.2008) determined a higher dressing percentage in 42-day old Cobb broilers reared without access to outdoors as compared to free-range broilers.
Many authors have stated that slaughter traits and dressing percentage can be affected not only by genetic factors but also by a number of non-genetic factors. Smith and Teeter (Reference SMITH and TEETER1987) found that if the outside air temperature exceeds the optimum range, this can induce a decrease in dressing percentage. This observation is of particular importance for free-range rearing systems where air temperature cannot be controlled. Ponte (Reference PONTE2008) observed differences in the dressing percentage of 67.1:65.4% in favour of the pasture-fed birds whereby high-quality clover was used as an additional source of feed for the broilers.
The proportion of primal carcass cuts was found to be variously affected by free-range rearing systems in different studies. Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, MITROVIĆ, DJOKOVIĆ, DOSKOVIĆ and DJERMANOVIĆ2010a, Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, PAVLOVSKI, PETROVIĆ, DOSKOVIĆ and RAKONJAC2011) observed no effect of the free-range system on percentage of primal carcass cuts, including breast, thighs and drumsticks as compared to indoor-reared broilers, although different hybrids (Hybro G and Cobb 500) were used in the two experiments. Similar findings were obtained by Santos et al. (Reference SANTOS, SAKOMURA, FREITAS, FORTES and CARRILHO2005). A comparison between indoor and free-range systems in a study by Dou et al. (Reference DOU, SHI S.R., SUN and WANG2009) revealed differences in the breast (20.17:17.44%) and thigh (27.75:26.68%) yield in Gushi broilers in favour of the free-range system. Castellini et al. (2002) found differences of 23.2%:22% in breast (P<0.05) between conventional and organic rearing systems in favour of the organic system after 56 days of fattening. Even larger effects seem to occur with increasing length of fattening period up to the organic production standard of 81 days – 25.2%:23.2% and 15.5%:15.0% in breast and drumstick proportion, respectively (Ross). Comparable results were obtained by Lei and Van Beck (1997), who associated their own findings with the fact that ranging activities lead to an increased proportion of both breast and, most notably, leg muscles. The breast, thigh and drumstick proportion in a study by Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, KURĆUBIĆ, PETROVIĆ and DOSKOVIĆ2006b) showed significant differences in free-range Hybro G broilers as compared to intensively reared birds, with meat yield in free-range birds accounting for a higher proportion of the carcass. Contrarily, Poltowicz and Doktor (Reference POLTOWICZ and DOKTOR2011) report a numeric 2.1% increase in the breast, thigh and drumstick proportion in conventional indoor-reared Ross 308 birds compared to free-range broilers (P>0.05). Abdominal fat content was generally lower in free-range broilers, potentially induced by increased locomotor activity in these broilers (Lewis et al., Reference LEWIS, PERRY, FARMER and PATTERSON1997; Castelini et al., Reference CASTELINI, MUGNAI and DAL BOSCO2002; Dou et al., Reference DOU, SHI S.R., SUN and WANG2009). Castelini et al. (Reference CASTELINI, MUGNAI and DAL BOSCO2002) reported differences of 0.9% vs. 1.9% after 56 days and 1% vs. 2.9% after 81 days of fattening, with organically raised Ross broilers showing a considerably lower fat compared to conventionally reared birds. The free-range and intensive indoor Hybro G broilers used in a study by Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, KURĆUBIĆ, PETROVIĆ and RADOVIĆ2006a) were found to have 1.88% and 1.98% of abdominal fat, respectively. Dou et al. (Reference DOU, SHI S.R., SUN and WANG2009) reported 3.0% abdominal fat in free-range chickens and as high as 6.5% in indoor-raised Gushi broilers. Contrary results were published by Poltowicz and Doktor (Reference POLTOWICZ and DOKTOR2011) whereby free-range and conventionally reared Ross 308 broilers had 1.52% and 0.72% abdominal fat, respectively.
Protein and fat content
The chemical composition of chicken meat, primarily the protein and fat content and fatty acid profile, shows strong links with rearing systems. Nevertheless, Castelini et al. (Reference CASTELINI, BERRI, LE BIHAN DUVAL and MARTINO2008) and Sirri et al. (Reference SIRRI, CASTELINI, RONCARATI, FRANCHINI and MELUZZI2010a) insisted that genotype was the major factor affecting quality and chemical composition of meat in non-intensive rearing systems, which is another reason for attention to be paid to the choice of breed or hybrid. Apart from genotype, Castelini (Reference CASTELINI2005) showed that major factors affecting the quality and chemical composition of meat in free-range rearing systems included age at slaughter, physical activity and foraging availability. The choice of rearing system is a highly important parameter of meat characteristics (Meluzzi et al., Reference MELUZZI, SIRRI, CASTELINI, RONCARATI, MELLOTI and FRANCHINI2009), with organic production giving better quality meat (Bancos, Reference BANCOS2010). Outdoor access also appears to improve sensory meat characteristics (Ponte, Reference PONTE2008).
However, bearing these bold statements in mind, there are many studies that have observed no significant effects of rearing system on the composition of broiler meat. Holcman et al. (Reference HOLCMAN, VADNJAL, ŽLENDER and STIBILJ2003) found that fattening practised according to the European Union regulations under indoor and free-range rearing systems did not affect the chemical composition of the breast and leg muscle in broilers aged 56 days. Dou et al. (Reference DOU, SHI S.R., SUN and WANG2009) examined slow-growing Gushi chickens under free-range and indoor rearing systems and found no differences in protein (24.49% vs. 24.46%) or fat content (0.54% vs. 0.86%) in the meat produced. Likewise, no differences in the protein and fat content of breast and thigh muscle was found between Cobb broilers fattened over a period of 45 days under intensive rearing conditions and Master and Paraiso Pedres strains fattened over a length of 85 days in a free-range system. However, these results came from the study of different genotypes over varying lengths of fattening; the two parameters most likely play a role in determining meat quality over all other considerations.
Protein content is much less affected by rearing system than the fat content in some major primal cuts, as suggested by a number of studies. Latif et al. (1996) found no differences in the protein content of breast and leg muscles in Transylvanic Naked Necked strains reared under both extensive and intensive rearing systems, whereas the fat content in the dark meat was significantly lower (1.8% vs. 3.9%) in extensively reared chicks. Fat content of breast was numerically 0.2% lower (P>0.05) in broilers that had outdoor access. Similar results were reported by Castelini et al. (2002) who found no significant differences, even after 81 days of fattening, in the protein content of breast and drumsticks between organic and intensively reared Ross broilers aged 56 days. Contrarily, the fat content was considerably higher in intensively reared chicks: in both breast (0.72% vs. 1.46% after 56 days; 0.74% vs. 2.37% after 81 days) and drumsticks (2.83% vs. 5.01% on day 56; 2.47% vs. 4.46% on day 81).
Apart from the Transylvanian Naked Necked breed, Latif et al. (Reference LATIF, DWORSCHAK, LUGASI, BARNA, GERGELY, CZUCZY, HOVARI, KONTRASZTI, NESLEZLENYI and BODO1996) studied commercial broilers reared under both extensive and intensive systems. Extensively reared broilers had a 1.8%-2.7% and 1.3% higher protein content in leg and breast muscles, respectively. The ratio regarding fat content was similar as in previous studies – being twofold or more lower in chicks that were provided outdoor access (3.40%-7.40% in legs; 1.45%-3.35% in breast). Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, KURĆUBIĆ, PETROVIĆ and DOSKOVIĆ2006b) found 0.71%, 0.59% and 1.05% higher protein content in the breast, drumstick and thighs, respectively in free-range birds compared to indoor reared Hybro G chicks. As for the fat percentage, it was found to be lower by 0.37% in breast, 1.10% in thighs and up to 1.39% in drumsticks during the summer season. Similar results were obtained by Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, MITROVIĆ, DJOKOVIĆ, DOSKOVIĆ and DJERMANOVIĆ2010a) in an experiment with the same genotype (0.18% and 0.52% higher protein content in the breast and legs respectively in the free-range system; the fat content in the same system was 0.56% and 0.66% lower in the breast and legs respectively than in the indoor system), as well as by Ristić (Reference RISTIĆ2003) and Lepold Centre researchers (2006).
The differences in chemical composition of broiler meat in most cases can be attributed to broiler foraging activity (Latif et al., Reference LATIF, DWORSCHAK, LUGASI, BARNA, GERGELY, CZUCZY, HOVARI, KONTRASZTI, NESLEZLENYI and BODO1996; Castelini et al., Reference CASTELINI, MUGNAI and DAL BOSCO2002, Castelini, Reference CASTELINI2005; Bogosavljević- Bošković et al., 2006b). Bogosavljević-Bošković et al. (Reference BOGOSAVLJEVIĆ-BOŠKOVIĆ, PAVLOVSKI, PETROVIĆ, DOSKOVIĆ and RAKONJAC2010b) associated the effect of rearing system on the chemical composition of chicken meat with the fact that indoor and outdoor free-range rearing under natural ambient conditions (fresh air, sunlight) led to differences in the structural manifestations of both tissues and organs, as well as affecting metabolic biochemical processes.
Content and ratio of individual fatty acids
Apart from the protein and fat content, the ratio of individual fatty acids is an important parameter of the quality of broiler meat and meat in general. The unfavourable ratio of polyunsaturated fatty acids to saturated fatty acids, along with a high fat percentage in feed, is the major causative agent of cardiovascular disease which is the leading cause of death in human populations from developed Western European countries. The content and ratio of individual fatty acids in poultry meat can be affected by genotype and production systems, but nutrition plays a dominant role in defining these parameters, hence primary attention should be given to the level of pasture availability and fresh plants for consumption by chicks. Given that broilers are non-ruminants, the ratio of individual fatty acids in their meat can be controlled through diet as fatty acids are not being utilised by rumen microflora (Ponte, Reference PONTE2008). However, the same author observed that the use of clover pasture for broiler rearing did not have a significant effect on the content and ratio of individual fatty acids in broiler meat, due to low feed consumption, as compared to broilers not provided with access to grass.
A high level of saturated fatty acids, along with cholesterol, is considered a proven cause of cardiovascular disease. A preventative measure recommended by Simopoulos (Reference SIMOPOULOS2002) includes the necessity to reduce the intake of saturated fatty acids and cholesterol, and increase the consumption of unsaturated fatty acids, n-3 fatty acids in particular. In this respect, chicken meat has high health benefits for the human population and is a major source of unsaturated fatty acids, n-3 fatty acids in particular (Sioen et al., Reference SIOEN, PYNEART, MATTHYS, BACKER, CAMP and HENAUW2006; Howe et al., Reference HOWE, MEYER, RECORD and BAGHURST2006).
Enser et al. (Reference ENSER, HALLET, HEWEET, FURSEY, WOOD and HARRINGTON1998) attributed the high level of unsaturated fatty acids in lamb and calf meat to a high percentage of grass consumed, whereas Ponte et al. (Reference PONTE, FERREIRA, SOARES, AGUIAR, LEMOS, MENDES and FONTES2004) underlined that many active substances found in fresh grass can improve meat quality. However, the same author observed that the increased intake of this tissue-rich feed can worsen the feed conversion efficiency and reduce weight gain. The content of fatty acids in Ross broiler breasts was not found to be significantly affected by rearing system in a study by Sekeroglu et al. (Reference SEKEROGLU, DEMIR, SARICA and ULUTAS2009).
Leopold Centre researchers (2006) compared three chicken rearing systems, including organic, free-range and conventional systems and suggested the following: organic broilers had a lower proportion of saturated (30.14%) and monounsaturated (31.67%) fatty acids as compared to free-range systems (32.46% and 38.82%) and conventional systems (32.31% and 39.13%). Organically produced meat, on the other hand, had significantly higher (38.19%) levels of polyunsaturated fatty acids as compared to the free-range (28.72%) and conventional systems (28.57%). The content of n-3 and n-6 fatty acids (3.92% and 34.28%, respectively) was higher in organic birds as compared to free-range chicks (2.92% and 25.79%) and conventionally reared broilers (1.92% and 26.64%).
Castelini et al. (Reference CASTELINI, MUGNAI and DAL BOSCO2002) determined similar patterns with respect to mono-(MUFA) and polyunsaturated fatty acids (PUFA) in their study on conventional and organic production systems. Saturated fatty acids in the breast and drumsticks from their research were higher in organically reared Ross broilers as compared to non-free-range broilers. The ratios of individual fatty acids in breast meat after 81 days of fattening in the organic system were SFA:MUFA:PUFA=37.89%:29.72%:32.38%, whereas those in the intensive system were SFA:MUFA:PUFA=35.87%:32.96%:31.15%. Similar ratios were found in the drumsticks: organic system: SFA:MUFA:PUFA=36.18%:31.69%:32.13% as compared to intensively reared broilers: SFA:MUFA:PUFA=34.56%:38.79%:27.55%. The results obtained after 56 days of fattening were almost identical. Similar findings were reported by Husak et al. (Reference HUSAK, SEBRANEK and BREGENDHAL2008), whereas Latif et al. (Reference LATIF, DWORSCHAK, LUGASI, BARNA, GERGELY, CZUCZY, HOVARI, KONTRASZTI, NESLEZLENYI and BODO1996) observed that the content of palmitic acid (as the main saturated fatty acid in the broiler body) was considerably lower in the legs (22.7%) and breast (23.7%) of extensively reared broilers than in intensively reared birds (25.7 and 26.6%, respectively).
Ponte (Reference PONTE2008) compared two different genotypes (Ross and RedBro Cou Nu X RedBro M) under two different production systems imposing different slaughter ages i.e. Ross broilers reared in intensive conditions, slaughtered after 56 days, and slow-growing hybrids reared under a free-range system for 81 days. The results were somewhat confusing, as they called into question the belief that slow-growing hybrids reared under free-range conditions can be proven to give nutritionally better quality meat. Intensively reared broilers had considerably higher (36.9%) polyunsaturated fatty acids as compared to Label broilers (31.1%), and, hence, a considerably higher content of n-3 (2.93%:2.47%) and n-6 (33.9%:28.6%) fatty acids. However, the content of saturated and monounsaturated fatty acids was higher in Label broilers (38.8 and 27.2%) than in intensively reared Ross birds (35.6% and 24.3%, respectively), with the PSFA:SFA ratio of 1.04 being more favourable in intensively reared chicks as compared to that of 0.81 in Label broilers (P<0.05).
Conclusions
Analysis of the literature suggests that rearing systems, especially non-intensive and organic systems, are an essential part of modern chicken meat production technology. They are important factors in governing both productive traits (growth, feed conversion and mortality) and meat quality traits (dressing percentage, yield and percentage of primal carcass cuts, sensory traits and chemical composition of meat).
Most authors report lower weigh gain and poorer feed conversion efficiency in free-range systems as compared to intensive rearing systems. Conversely, meat quality traits, primarily the chemical composition of the meat produced, are more favourable in non-intensive and organic broilers. Moreover, the analysis of different findings showed that differences in these traits can also be found within the same rearing system, being induced by a range of factors, including the use of different broiler genotypes, lengths of fattening period, diet formulations, levels of outdoor access and pasture quality. The wide range of factors governing rearing systems and affecting productive and meat quality traits in broilers have led to extensive research findings and opened up many issues that will, without doubt, provoke further research and new dilemmas.
Over the last few years, poultry rearing systems have gained importance not only in terms of scientific research but also in terms of continuous identification of possibilities for their practical implementation. Better quality products, poultry welfare and environmental protection are all valid reasons (covered by legal standards and regulations in many countries) to encourage interest in innovation of rearing systems and the introduction of new ones.
Acknowledgement
This study is part of the Project No. 31033 titled ‘Sustainable Conventional and Revitalized Traditional Production of Value-Added Poultry Meat and Eggs’ funded by the Ministry of Science and Technological Development of the Republic of Serbia.
