Introduction
Of all domesticated poultry species, ducks can be easily reared and need less care than chickens (Modak, Reference MODAK1996). The welfare of ducks is based on their health, performance, and some behavioural and physiological factors. They are very susceptible to stress, and under stressful conditions they easily become sick, demanding more care and drugs, which may be needed to maintain health. This excessive use of drugs results in increased drug residues in animal products, directly threatening public health. Under stressful conditions, rapid but temporary changes occur in the body initially. If the stress persists for long enough, these changes become permanent and irreversible, eventually resulting in a decline in yield, lower meat quality and susceptibility to diseases. In order to meet the optimum requirements for animal welfare and food safety, appropriate production systems must be developed (Onbaşilar and Aksoy, Reference ONBAŞILAR and AKSOY2005; Chen et al., Reference CHEN, AORIGELE, YAN, LI, CHENG and QI2015). Different rearing systems are used for ducks, which, depending on various factors, affect animal welfare, fattening performance, meat quality as well as the price of the products. The type of floor, stocking density, access to water for swimming, outdoor activity, drinker systems, light wavelength and photoperiod are rearing factors involving the welfare of Pekin ducks.
Duck meat has been popular in South-East Asia and it has become increasingly popular in European Union countries (Graczyk et al., Reference GRACZYK, GORNOWICZ, MUCHA, LISOWSKI, GRAJEWSKI, RADZISZEWSKA, PIETRZAK and SZWACZKOWSKİ2016). Pekin ducks are reared commonly for meat production (El-Edel et al., Reference EL-EDEL, EL-KHOLYA and ABOU-ISMAIL2015). Duck has higher red muscle fibre in breast compared to chicken and is considered to be red meat (Smith et al., Reference SMITH, FLETCHER, BUHR and BEYER1993; Ali et al., Reference ALI, KANG, YANG, JEONG, HWANG, PARK and JOO2007). Broiler ducks are raised to seven or eight weeks of age and slaughtered at a body weight of proximately 3.5 kg (Fan et al., Reference FAN, XIE, WANG, HOU and HUANG2008). The average body and carcass weight of Pekin ducks increases 16-fold and 23-fold between one and eight weeks of age, respectively (Murawska, Reference MURAWSKA2012). Feed conversion measurement has become more important for all kinds of animal production, in response to rising feed costs, awareness of limited resources, and environmental issues (Thiele, Reference THIELE2016). Baltić et al. (Reference BALTIĆ, STARČEVIĆ, BAŠIĆ, ZENUNOVIĆ, IVANOVIĆ, MARKOVIĆ, JANJIC and MAHMUTOVIĆ2015) stated that Pekin duck breast muscle contained 76% moisture, 21% protein, 2% lipid and 1% ash. Ali et al. (Reference ALI, KANG, YANG, JEONG, HWANG, PARK and JOO2007) reported that the ratio of polyunsaturated fatty acids to saturated fatty acids is 0.8. The three most important quality attributes for poultry meat are appearance, texture and chemical composition. Appearance is critical for both the consumers' initial selection of the product as well as for final product satisfaction. Texture is the single most important sensory property affecting final quality assessment (Fletcher, Reference FLETCHER2002). Meat composition is important to assess, as it dictates the nutritional value of the final product for consumers (Wołoszyn et al., Reference WOŁOSZYN, KSIĄŻKIEWICZ, ORKUSZ, SKRABKA-BLOTNICKA, BIERNAT and KISIEL2006).
A comparative overview of the literature on the effect of rearing systems on fattening performance and meat quality of Pekin ducks was conducted for this paper to determine the effects of the rearing systems on the above features to determine the future direction of research in this field as well as a tool of its practical application.
Floor type and stocking density
Pekin ducks are reared in different systems, such as in automatic brooders during the first week of life; on deep litter or plastic slatted system; or in a cage system with different stocking densities for growing ducks. The first week of a duck's life is of great significance as, during this time, growth rate rapidly increases (Pingel, Reference PINGEL1999). Erdem et al. (Reference ERDEM, ONBAŞILAR, KOCAKAYA and YALCIN2015a) compared two rearing systems: the automatic brooder and deep litter. In their study, ducklings were reared in a brooder for one week and then transferred to the deep litter system until slaughter. In the other system ducklings were reared on the deep litter throughout the experiment (from 1 to 42 days old). The results showed that the total body weight gain of ducks in the litter group was significantly higher than the brooder group. During the first week, the body weight gain of ducks reared in both systems was found to be similar, but at the end of experiment the total weight gain of the ducks reared with the litter system was significantly higher than the brooder group. Faure et al. (Reference FAURE, VAL-LAILLET, GUY, BERNADET and GUÉMENÉ2003) stated that Pekin ducks have a higher susceptibility to environmental stress, as affected by rearing environment. Hot carcass yield, breast percentage and the fat contents of breast and thigh meat were also significantly higher in the litter group than the brooder group (Erdem et al., Reference ERDEM, ONBAŞILAR, KOCAKAYA and YALCIN2015a). Maruyama et al. (Reference MARUYAMA, AKBAR and TURK1999) reported that breast muscle grows faster than the rest of the body during the fattening period, and breast meat builds in carina sterni and the pectoralis major due to activity when the bird beats it wings (Baumel, Reference BAUMEL1993; Welch and Altshuler Reference WELCH and ALTSHULER2009). Thus, regular movement of wings might have contributed to the development of breast muscle in the litter group as their breast percentage values were higher than the brooder group (Erdem et al., Reference ERDEM, ONBAŞILAR, KOCAKAYA and YALCIN2015a).
Sarı et al. (2013) studied the effects of cage and deep litter floor systems on some carcass traits. They found that body weight gain, slaughter weights and the feed conversion ratio of ducks on the deep litter floor were higher than the cage system. They explained their results by the differences in some activities in both systems such as feeding, lying, walking and pecking. These findings suggest that these activities are important in duck performance parameters. Karcher et al. (Reference KARCHER, MAKAGON, FRALEY, FRALEY and LILBURN2013) and Fraley et al. (Reference FRALEY, FRALEY, KARCHER, MAKAGON and LILBURN2013) examined the impact of floor type (litter and plastic slatted floor) on the condition of ducks and their production parameters. They evaluated the interaction of two different seasons (winter and summer) with the flooring type. Although higher weight gain was observed in ducks raised on wood shavings in summer (Fraley et al., Reference FRALEY, FRALEY, KARCHER, MAKAGON and LILBURN2013), the results from both studies conducted in different seasons were not conclusive enough to show whether one flooring type was more profitable than another.
Stocking density is an important factor affecting duck production. Higher economic returns can be obtained with a higher number of ducks per unit area. However, higher stocking density reduces the bird performance, health and welfare (Xie et al., Reference XIE, JIANG, TANG, WEN, HUANG and HOU2014). Xie et al. (Reference XIE, JIANG, TANG, WEN, HUANG and HOU2014) conducted two experiments evaluating the effects of stocking rate on Pekin duck performance. In both experiments, all ducks were reared on plastic wire-floor pens. In the first experiment (from one to 14 days old), ducks were reared according to the stocking densities of 13, 15, 17, 19 and 21 birds/m2, while in the second experiment (from 14 to 42 days old) they were reared according to the stocking densities of 5, 6, 7, 8 and 9 ducks/m2. The results showed that increasing stocking density had a negative effect on the growth performance of Pekin ducks. The final body weight and weight gain of ducks were reduced significantly as stocking density increased from 17 to 21 ducks/m2 in the first experiment and when the stocking density was 9 ducks/m2 in the second experiment. According to Xie et al. (Reference XIE, JIANG, TANG, WEN, HUANG and HOU2014), the maximum stocking densities for ducks in the first and second experiment should not exceed 19 and 8 ducks/m2 respectively. The study also showed that stocking densities had no significant effect on the carcass yield, breast meat, leg meat or abdominal fat.
Ahaotu and Agbasu (Reference AHAOTU and AGBASU2015) examined the effects of stocking rate on performance in Pekin ducks. They raised male ducklings on two different rearing systems with stocking densities of 2, 4, 6 and 8 birds/m2 on the floor or 4, 8, 12 and 16 birds/m2 in cages. On the floor system, increasing densities above 4 birds/m2 at six weeks old and above 2 birds/m2 at eight weeks old significantly decreased body weight, while rearing ducklings in cages at a stocking rate above 8 birds/m2 at four weeks old significantly decreased body weight. It was proposed that, at six and eight weeks, the stocking rate should not exceed 4 birds/m2. In addition, at eight weeks old, increasing the density above 2 birds/m2 on the floor and 4 birds/m2 in the cages, significantly decreased feed intake. An improvement in cumulative feed conversion was observed when the stocking rate was increased from 2 to 4 birds/m2 of the floor area at four, eight and 10 weeks old, while in the cages such improvement was only significant when the stocking density exceeded 4 and 8 birds/m2 of cage area at four and eight weeks respectively. It was suggested that at higher stocking densities, the improvement in feed conversion may have resulted from decreased energy requirements due to reduced birds’ activity and heat conservation (Sharma, Reference SHARMA2001; Buckland and Guy, Reference BUCKLAND and GUY2002). The results showed that with the increasing density on floor, the pH value of breast meat was increased progressively, but this was only noteworthy (P<0.05) when the density exceeded 6 birds/m2 of the floor area. This could be explained by lower glycogen reserves due to the limited feed intake or increased susceptibility of birds to stress caused by overcrowding. The stocking rate on the floor or in cages caused no significant changes in the colour of breast and thigh muscles. Only the water holding capacity of breast meat was significantly affected by stocking rate on the floor (P<0.01).
Water holding capacity is an important packaging and cooking quality characteristic of meat and dictates whether there is liquid exudate in packaged meat (which is unappealing to purchasers) and has an impact on juiciness and tenderness of the final cooked product. The water that the meat contains is retained by proteins. The water holding capacity of thigh meat was numerically highest when the density was 2 birds/m2 of the floor area.
Access to an outdoor area and open water
For hygienic, labour and economic reasons, ducks have been reared mostly in closed poultry houses without access to open water. However, with the evidence that domestic ducks kept for fattening purposes still show preferences for open water (Knierim et al., Reference KNIERIM, BULHELLER, KUHNT, BRIESE and HARTUNG2004), production has changed to incorporate other systems. Rodenburg et al. (Reference RODENBURG, BRACKE, BERK, COOPER, FAURE, GUÉMENÉ, GUY, HARLANDER, JONES, KNİERİM, KUHNT, PİNGEL, REİTER, SERVİERE and RUIS2005) stated that access to open water increases the opportunity to exhibit natural behaviour Erisir et al. (Reference ERISIR, POYRAZ, ONBAŞILAR, ERDEM and KANDEMIR2009a) conducted an experiment with Pekin ducks reared in four different intensive systems (two intensive systems (IS), with and without water access; two intensive systems + an outside area (IOS), with and without water access) until they were six weeks old. They reported that both outside activity and access to water increased body weight. In the IOS system with water access, the final body weight of ducks was higher than that of those reared without water access. In the IS system, water access negatively affected the final body weight of ducks, which may have resulted from the dirty environmental conditions of the inside area caused by wet birds and faecal material. In another study by Erisir et al. (Reference ERISIR, ONBAŞILAR, ERDEM and ÖKSÜZTEPE2009b), water access had no significant effect on the percentages of cold carcass and leg meat composition of ducks.
El-Edel et al. (Reference EL-EDEL, EL-KHOLYA and ABOU-ISMAIL2015) compared the outdoor and indoor housing systems for ducks. During the first four weeks, ducks housed indoors had higher body weights and weight gains than those in outdoor systems. Damaziak et al. (Reference DAMAZIAK, MICHALCZUK, ADAMEK, CZAPLIŃSKI, NIEMIEC, GORYL and PIETRZAK2014) found that intensive and outdoor systems had a greater impact on the growth of Pekin ducks. Ducks reared in outdoor systems had higher body weights and feed efficiency than ducks reared in the indoor system (P<0.05). These results indicated that providing birds with more space for free movement decreased feed intake and improved feed conversion efficiency. However, they didn't find any significant effect of the intensive and outdoor systems on carcass yield, the weight of breast and leg muscles and the diameter of small muscle fibres.
Michalczuk et al. (Reference MICHALCZUK, DAMAZIAK, PIETRZAK, MARZEC, CHMIEL, ADAMCZAK and FLOROWSKI2016) studied the effect of intensive and outdoor systems on some meat quality characteristics in Pekin ducks. Ducklings were reared in the intensive system from hatching to three weeks old. Half of the flock were then kept under free range conditions, while the other half were reared under the same intensive conditions throughout the experiment (to seven weeks of age). The housing system did not affect the moisture, protein, fat, ash, pH, shear force or colour of breast muscles. Colour is important from the consumer's point of view as well as for technological application of meat as a raw material (Wołoszyn et al., Reference WOŁOSZYN, HARAF, KSIAZKIEWICZ and OKRUSZEK2009; Mikulski et al., Reference MIKULSKI, CELEJ, JANKOWSKI, MAJEWSKA and MIKULSKA2011). Breast meat from ducks reared in the outdoor system was brighter also had better scores for tenderness and juiciness. Under laboratory conditions, tenderness is determined by the shear force test. In the study of Michalczuk et al. (Reference MICHALCZUK, DAMAZIAK, PIETRZAK, MARZEC, CHMIEL, ADAMCZAK and FLOROWSKI2016), lower shear force of meat was reported from ducks kept under the outdoor system, which may be related to muscle fibre characteristics. The lower shear force may be associated with less collagen content and, therefore, fewer muscle fibres from Pekin ducks kept in outdoor systems.
Drinkers
The drinkers used and their design are important for ducks to ensure a quality supply of water, as ducks beaks are broader than chickens. The Council of Europe (1999) recommends that, where accesses to an outside area and water for bathing are not possible, ducks must be provided with water facilities sufficient in number, and designed to allow water to cover the head and be taken up by the beak so that the duck can shake water over the body without difficulty.
Ducks prefer bell drinkers as it allows them to dabble their beaks as well as to dip their heads (Cooper et al., Reference COOPER, MCAFEE and SKINN2002; Rodenburg et al., Reference RODENBURG, BRACKE, BERK, COOPER, FAURE, GUÉMENÉ, GUY, HARLANDER, JONES, KNİERİM, KUHNT, PİNGEL, REİTER, SERVİERE and RUIS2005). Onbaşılar et al. (2013) compared bell drinkers of different width and depth for ducks. Two types of drinkers were used (narrow bell drinkers with a depth of 47 mm and a width of 48 mm, and wide bell drinkers with a depth of 78 mm and a width of 90 mm) from one to 42 days old. The type of bell drinkers affected the body weight and feed conversion, slaughter weight, and percentages of breast, wings and subcutaneous fat. Ducks reared using wide bell drinkers had higher body weight gain, better feed efficiency, higher slaughter weight, lower percentage of skin and subcutaneous fat and higher percentages of breast and wings. It was recommended that if a swimming pool is not available for duck production, wide bell drinkers should be used.
Schenk et al. (Reference SCHENK, PORTER, ALENCIKS, FRAZIER, BEST, FRALEY and FRALEY2016) compared the two different styles of water provision systems; water troughs and pin-metered water lines. There were no differences in body weight and feed conversion between the groups. Ducks housed with water troughs appeared to eat significantly less (P<0.05) feed at 28 and 33 days old, and mortality in ducks housed with water troughs was higher (P<0.05) from weeks three to five.
Photoperiod and light wavelength
Photoperiod and light wavelength are important rearing factors. The Council of Europe (1999) recommends that photoperiod should follow a 24-hour rhythm and include a sufficient uninterrupted period of dark, which, as a guideline, means approximately a third of the day being dark. Hart et al. (Reference HART, PARTRIDGE and CUTHILL1999) reported that ducks have a predominance of short wavelength sensitive (blue-sensitive) cones and relatively few long wavelength sensitive (red-sensitive) cones. Campbell et al. (Reference CAMPBELL, COLTON, HAAS, RICE, PORTER, SCHENK, MEELKER, FRALEY and FRALEY2015) evaluated the effects of different light wavelengths on the body weight and carcass yields of ducks raised until 35 days old. Red light (approximately 625 nm), blue light (approximately 425 nm) and white light were used in their study. The body weights of ducks reared under white and red lights were higher than of those reared under blue lights. The percentages of carcass yield of ducks were similar for different light groups; however, the percentages of breast skin with fat, breast meat and total carcass fat were lower in ducks reared under blue light. It was reported that blue light had a detrimental effect on the growth and carcass quality.
Erdem et al. (Reference ERDEM, ONBAŞILAR and HACAN2015b) examined the effect of different photoperiods on fattening performance and meat composition of ducks. In one group, ducklings were reared under the 24L:0D photoperiod while in the other group 16L:8D photoperiod was applied until six weeks old. Ducklings reared under the continuous light consumed more feed and gained more weight from one to 42 days old. However, the feed conversion was significantly higher only from one to 21 days old in the group exposed to continuous light. Ducks reared under continuous light had higher percentages of breast and wing at slaughter. Photoperiod affected the fat and pH of the thigh meat of ducks, whereby reared under continuous light had a higher percentage of fat. Lipids are an important component of duck meat, and intramuscular lipid levels varied with genotype, age, nutrition and photoperiod (Baéza, Reference BAÉZA2005; Erdem et al., Reference ERDEM, ONBAŞILAR and HACAN2015b).
Conclusions
The duck meat industry provides consumers with a high-quality protein source by taking advantage of intensive rearing systems that minimise input costs and maximise the efficiency of these inputs. However, Pekin ducks should be kept in a suitable rearing system to enable their genetic growth potential and to allow natural behaviour. Only a few studies addressing the effects of different rearing systems on the fattening performance of Pekin ducks have been published, despite the fact that it is a breed which is often grown for meat production. Based on the results of the studies discussed in this article, it can be concluded that stocking density, water access, type of drinkers, rearing method in the first week, colour of light and photoperiod should all be taken into account as these affect fattening performance. It should not be forgotten that the first week of a duckling's life is important and stress during this period negatively affects the fattening performance. Therefore, no changes to the rearing system should be made in the first week of life. Pekin ducks reared on the deep litter system with 4 ducks/m2 at six weeks old had enhanced performance and better percentages of breast and wing. The wide bell drinker is the most appropriate water system, because it allows the ducks to cover their head with water and shake it about their body. Water access important for fattening performance but it should be placed outdoors to limit hygiene problems within the house. Blue light and a 16L:8D photoperiod have negative effects on body weight and the percentages of breast and wings, and fat percentages of meat. According to published data, breast meat of ducks reared in the outdoor system was brighter in colour and had a better score for tenderness and juiciness. Water holding capacity of breast meat was positively affected by decreasing the stocking density on the floor. Due to the limited published data and that work done on other poultry species cannot be generalised to Pekin ducks as a waterfowl, more detailed studies are needed, especially to confirm the non-significant trends seen by some researchers.
