Research Progress on Effects of Stocking Density on Performance Health and Welfare of Duck

Chaowu YANG Chunlin YU Zengrong ZHANG Mohan QIU Huarui DU Qingyun LI Xiaosong JIANG Xia XIONG

Abstract Stocking density is an important factor for animal production performance. It appears particularly urgent and important to research systematically on stocking density for the dual restriction of environmental protection and animal welfare. This review dealt with the influence of stocking density on ducks productivity and health through production performance， animal behavior and animal welfare， and analyzed the possible mechanism of high stocking density reducing production performance， resulting in abnormal behavior and stress， causing welfare and health problems. We consider that it might be associated with heat stress， more spatial competition （to gather the food， drinking water， sports area， etc.）， as well as the quality deterioration of litter and house air caused by high stocking density. Finally， we put forward some suggestions on the study of stocking densitys effects on ducks， and proposed the future work that need to be studied and problems that need to be solved imminently.

Key words Duck Production; Stocking Density; Animal Welfare; Performance

China is the largest country of duck production and consumption in the world. However， for thousands of years， duck production in China has been carried out in traditional waters （semi grazing）， which not only causes pollution of rivers and water sources， but also causes some severe infectious diseases to spread with the flow of water， seriously endangers the living environment of human beings and animals， and produces serious public health and safety problems[1]. In recent years， in order to meet the needs of environmental protection and save costs， Chinese duck production has shown a trend of intensive， large-scale and high-density in-house feeding. The way of raising ducks in the shed mainly includes the ground flat raising （padding， fermentation bed） and net bed raising in meat ducks production， cage raising and ground flat raising in laying ducks production.

In the production of duck in the shed， the main environmental factors affecting duck production include light， temperature， humidity， harmful gas content， dust， ground type and padding quality， etc. The main group environmental factors affecting duck production include stocking density and group size. High density feeding can produce more economic benefits to some extent， but it also brings many negative impacts and great challenges to duck production performance， disease prevention and control， product quality and animal welfare. Stocking density has always been one of the key issues in the aquaculture industry. Many researchers also try to study the effects of stocking density on animal production and health welfare from various angles. Although studies on the effects of stocking density on duck health and production performance are being carried out one after another， the studies are not systematic and there is no uniform parameter for the range of suitable stocking density for large-scale breeding. In this paper， the effects of stocking density on the performance and health of barn ducks were summarized， and the possible mechanism of the effects and the research situation of suitable stocking density range were discussed， which could provide reference for further study in the stocking density on duck.

Overview of Stocking Density on Duck Production

Stocking density on production performance

Production performance is the most basic and intuitive index in animal experiments. Relevant studies show that high-density stocking will reduce duck growth performance， feed intake， feed reward and survival rate， and affect slaughter performance and meat quality[2-6]. However， the negative effects of high-density feeding on duck performance are not all simultaneous， that is， high-density feeding significantly reduces the slaughter weight and weight gain of ducks， but the effects of high-density feeding on feed intake， survival rate， feed remuneration， slaughter performance and meat quality are inconsistent in different studies[2，4，6]， which may be caused by different duck breeds， house types and feeding management level in different studies. It is generally believed that the decline in production performance caused by high-density feeding may be related to increased competition for food intake. Xie et al.[6] reported significant reductions in feed intake， body weight and daily gain at high density and believed that the growth inhibition effect of high density from 1 to 2 weeks old is related to increased competition in feed intake and drinking water space. However， the significant decrease of body weight and weight gain from 3 to 6 weeks old is not accompanied by the decrease of food intake， which is partially the same as the results of Osman[2] and Baeza et al.[7]. Baeza et al.[7] found that high density stocking significantly reduced the feed intake of American ducks at the end of feeding period and the feed intake of Pekin ducks at the whole stage， and speculated that the mechanism of high density may be related to the duck breeds. The results of Jones and dawkins study showed that at high density， the increase of mortality and the decrease of growth speed were related to the high temperature in the duck house[8]. In addition， some scholars believe that the poor ventilation caused by high density stocking and the increase of dust， microorganism and harmful gas content in the house are the reasons for the slow growth of ducks and the decline of survival rate[5]. High-Density feeding will also accelerate the deterioration of bedding and increase the air humidity and ammonium content. Rodenburg et al.[9] believe that the quality of bedding has a greater impact on animals than the stocking density itself. If the temperature， humidity， ammonium level and bedding quality in the house are closely monitored， many production problems can be avoided.

Some scholars hold different opinions that high density stocking cannot cause significant decline in production performance. There was no significant difference in growth performance and survival rate of ducks between different experimental density groups， and the effect of stocking density on duck production performance was not significant[10-12]. However， the growth performance and carcass quality of American ducks in the high density group are the best[7]. The results of different studies on the effect of stocking density on production performance are quite different， which may be related to the differences in duck breeds， feeding methods， feeding conditions， population number and feeding management level in different studies. However， the results of different studies on the suitable stocking density of ducks are different. Osman[2] suggested that the stocking density of Pekin ducks should be 8 ducks per square meter for 2-4 weeks old and should not exceed 4 ducks per square meter for 5-10 weeks old. Xie et al.[6] suggested that the stocking density of Pekin ducks should be 19 per square meter for 1-2 weeks old and 8 per square meter for 3-6 weeks old. The difference in stocking density between the two may be due to Osmans small experimental population （80-160）， while Xie et als research was carried out under the modern large-scale production system （population 3150-10200）. Europe[9] suggests a fattening density of 6-8 Pekin ducks per square meter; and FASS[13] recommends feeding densities of 43.1 birds per square meter， 22.8 birds per square meter， 15.4 birds per square meter， 10.3 birds per square meter， 8.4 birds per square meter， 7.1 birds per square meter and 6.2 birds per square meter for 1-7 weeks old respectively.  Britain also suggests that the commercial stocking density of meat ducks for 3-8 weeks old should be 7 birds per square meter[14]. However， it is believed that when birds per square meter are raised， bedding and feces will quickly cause high pollution of the water pool and affect the health of ducks， and when the stocking density is 4.1 ducks/m2， as long as the appropriate open water source is provided， the growth and feather health of ducks can be improved[14].

In addition， Xie et al.[6] showed that the population number of each density treatment group increases with the increase of density， and it is not clear whether the negative effects of stocking density on duck production traits interact with the population size. Relevant studies show that the larger the population， the more neurotic and fearful birds will be and the mortality rate will increase[9]. Therefore， research shows that high stocking density will cause the decline of duck production performance， but the management level and the environment in the house may have greater influence on the production performance， and the population scale may also participate in it.

Stocking density on animal behavior

At the behavioral level， most studies showed that high stocking density will reduce the available space and exercise amount of ducks， increase the frequency of abnormal behaviors such as stress convulsion and pecking addiction， which may affect feather quality and walking ability and lead to skin damage. Generally， the leg health and walking ability of ducks are evaluated by quality scores of gait abnormalities and foot pad inflammation. The high density group of American ducks had severe feather pecking behavior[15]， and 80%-90% of the individuals in the high density group of cherry valley ducks had feather pecking behavior， which was attributed to the increase of stocking density， the decrease of duck activity space， and the increase of contact and interference between individuals[5]. Some studies also showed that the low density group of American ducks is more prone to feather pecking behavior at 4 weeks of age， while the high density group has the most severe feather pecking behavior at 8-12 weeks of age， but high density feeding has no significant effect on the feather score of Pekin ducks[7]. Jones and Dawkins[8] reported that with the increase of day age， duck individuals increased and stocking density increased significantly. Compared with 23-day-old， inflammation and abnormal gait of 41-day-old Pekin duck foot pad increased. Gait deteriorated with the increase of indoor temperature and padding humidity， and was significantly related to the increase of padding humidity and ammonium content caused by the increase of density. Jones and Dawkins[16] also found that the behavior of commercial rearing Pekin ducks is more affected by environment， age and physical condition. It is believed that with the increase of age， the raising density increases significantly， and the behaviors related to thermal comfort （heat dissipation and panting） and feather condition （dry and wet combing） increase greatly. These behaviors also increase with the increase of temperature， humidity and ammonium content. The walking ability of ducks decreases and is related to increased panting frequency， decreased drinking water behavior and increased rest behavior.

But there are also different research reports. Xie et al.[6] showed that the stocking density has no significant correlation with the occurrence rate of foot pad inflammation in Pekin ducks， nor does it affect walking ability， and that the occurrence of foot pad inflammation is related to the type or quality of the ground. However， Karcher and Makagon reported that regardless of the ground type， Pekin duck had foot pad inflammation in winter[17] or summer[18]. Skin damage also increased with increasing age and decreasing usable area of a duck， but the overall gait score improved with the age increasing.

High density stocking will cause heat stress and respiratory tract irritation， which will stress poultry and affect animal behavior. Studies showed that laying ducks are cold-resistant and heat-resistant， and heat stress will affect their egg-laying behavior and egg-laying performance[19]， which may be due to increased fighting behavior， resulting in increased activity and stress response[20]. Therefore， lack of individual space and environmental deterioration caused by high stocking density may be the causes of duck stress and abnormal behavior.

Although the results of different studies are not completely consistent， the stocking density does significantly affect the behavior of ducks fed in the house， resulting in health problems， which may be related to the decrease of individual activity space and deterioration of the raising environment caused by high density stocking.

Stocking density on animal welfare

High density stocking limits the moving space and distance of ducks， which not only increases the blood corticosterone level of ducks and reduces antioxidant activity and immunity[6]， but also intensifies padding pollution， increases pecking behavior， leads to foot pad inflammation and feathering， and seriously causes gait abnormality， skin damage， feather quality deterioration， mortality increase， and affects animal welfare. Jones and Dawkins[8，16] invented a quality evaluation system for visual evaluation of individual duck condition， which was based on the evaluation of eyes， nostrils， feather cleanliness， feather quality， movement ability and foot pad condition. The evaluation system has been adopted by relevant researches on ducks[6，8，14，16-18] to evaluate the health and welfare of ducks.

Osman[2] reported that the feather coverage of Pekin ducks became significantly worse when the stocking density increased to over 4 birds per square meter. Foot pad injury is a common welfare parameter in poultry production， and severe foot pad injury can develop into ulcer swelling （avian palmitis）. Compared with chicken and turkey， ducks sole skin is thinner and duck is more prone to foot pad inflammation. Karcher and Makagon et al.[17，18] reported foot pad inflammation and skin injury in Pekin ducks， and the skin injury increased with increasing of the age and raising density. Jones and Dawkins[8] reported that compared with the 41-day-old and 23-day-old the physical condition of Pekin ducks deteriorated， the foot pad inflammation became more serious， gait abnormality intensified， and the incidence of foot pad inflammation was significantly related to the increase of padding humidity and ammonium content. However， Xie et al.[6] reported that high-density feeding had no significant effect on the incidence of foot pad inflammation and skin damage. In broilers， it is generally believed that foot pad inflammation is mainly caused by the dirty and humid mat materials caused by high-density feeding. However， Xie et al.[6] used metal net flat feeding in their research， without mat materials， so they speculated that this can explain that there is no mat inflammation and stocking density in metal net flat feeding ducks， and the occurrence of foot pad inflammation is related to the ground type or pad material quality. Karcher and Makagon found that no matter in winter[17] or in summer[18]， Pekin duck would suffer from foot pad inflammation and skin injury regardless of ground type. Jones and Dawkins[8] believed that an effective ventilation system， high quality padding and access to some open water sources are very important to the health and welfare of ducks. Therefore， it is very important to control the environmental factor parameters （air temperature， humidity， etc.， padding humidity， ammonium content， etc.） in duck houses.

Although the results of different studies are not completely consistent， high density stocking does cause duck health and welfare problems， which is also related to the deterioration of indoor environment[12，15].

Chaowu YANG et al. Research Progress on Effects of Stocking Density on Performance， Health and Welfare of Duck

Conclusion

Most studies have provided evidence for the negative effects of excessive increase in stocking density. High density stocking not only affects ducks production performance and leads to stress and abnormal behavior， but also as an important reference index for health and welfare， it will，  ① have negative effects on duck feather quality， foot pad inflammation and gait score， ② reduces duck feed intake， weight gain and slaughter weight， increases mortality rate and affects slaughter performance， ③increases the frequency of abnormal behaviors such as pecking addiction and aggressive behaviors， and ④ increases plasma corticosterone level， physiological stress， pressure， and reduces duck immunity.

The mechanism of the effect of stocking density on duck production performance and health has not yet been determined. Some scholars believe that high density stocking limits the animals activity space and intensifies the competition for food intake， drinking water and rest space. Some people also think that high density feeding does not affect the feeding behavior and time of animals， but the appetite becomes worse after the mucous membrane and nose and mouth are stimulated due to environmental deterioration caused by high density. Some scholars also found that the deterioration of the indoor climate environment （temperature， humidity， NH3， H2S， CO2 content， etc.） and the quality of bedding caused by high density have greater impact on duck production performance and health than the stocking density itself. We consider that under the condition of limited equipment or poor management， there is interaction between stocking density and environmental factors， and that the stocking density may have a strong negative impact on environmental factors， worsen the environment in the house， cause stress， affect food intake， increase the occurrence rate of pecking and other abnormal behaviors， and eventually lead to duck health and welfare problems and decreased production performance.

In short， in modern duck production， stocking density may cause production， health and welfare problems， but its influence mechanism may be related to the deterioration of indoor climate environment and bedding quality caused by high density. Therefore， in modern high density production， ducks may be more affected by feeding management level and environmental factors in the house. Having a good ventilation system and controlling the climate environment and padding quality in the duck house may be more important to ensure duck production performance and health.

Suggestions for Future Research

Stocking density is closely related to the main concerns of duck producers （reducing production costs as much as possible， doing a good job in prevention and control， and ensuring product quality）. However， However， there are few studies on the effect of stocking density on duck performance in the world[6]， let alone systematic， and the research results are quite different， and the studies on the effects on egg laying stage are even less[19]. Therefore， on the basis of the existing research results， it is necessary to continue to carry out in-depth research on duck stocking density， and strive to establish a feasible standard， which can not only affect animal welfare， but also maximize animal genetic potential， improve breeding efficiency and meet market demand. Based on previous research experience in other livestock， we consider that the following issues should be paid attention to when carrying out research on duck stocking density：

① The suitable stocking density of poultry is related to breed， age， purpose， nutrition level， feeding environment and management level. The main breeds of ducks in various countries and regions have different levels of feeding management and different standards of stocking density. It is advisable to first select mainstream duck breeds， conduct relevant research with current or possible future main feeding methods， and formulate appropriate stocking density standards.

② The negative effect of high density stocking may be related to competition for food intake. Whether the stocking density will affect the nutritional needs of ducks is a lack of research and should be carried out simultaneously.

③ The lack of systematic research on stocking density has increased the difficulty of comparison and comprehensive application of research results and greatly reduced the value of existing results. Although there may be great differences between the results of small-scale research and large-scale production， we should try our best to unify the conditions of all aspects of the test in order to facilitate the comparison and continuous research between the studies.

In order to improve the level of poultry production and protect the health and welfare of animals， it is necessary to conduct in-depth research on the stocking density， so as to reduce the breeding cost， ensure the quality of poultry products， improve the breeding efficiency and reduce environmental pollution. In addition， for the following research， we should increase the evaluation of stocking density  and population size factors on the performance impact in egg laying stage， and try to determine whether there is interaction between stocking density and population size on the influence of ducks. Meanwhile， we should monitor the climate and environmental factors in the duck house， and try to distinguish whether changes in air temperature， humidity or padding humidity or ammonium content cause duck health and welfare problems. Furthermore， it is necessary to continue to carry out visual quality evaluation of duck health welfare， evaluate the representative value of the evaluation system， and make improvements. As well， we should try to determine the range of suitable stocking density that can give consideration to both production performance and duck health and welfare， and try to build a bridge for duck industry producers and consumers to effectively communicate on ethical conflicts of animal welfare.

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